The Condor 110(1):80–92c© The Cooper Ornithological Society 2008

CAVITY-NEST WEBS IN A LONGLEAF PINE ECOSYSTEM

LORI A. BLANC1 AND JEFFREY R. WALTERS

Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Abstract. Cavity-nesting communities can be viewed as interconnected webs that interact through the creationof and competition for cavities as nest sites. Using a web approach, we depicted the flow of cavity creation anduse in the cavity-nesting bird community of a Florida longleaf pine (Pinus palustris) ecosystem to examine therelationship between cavity-nesting bird abundance and cavity resources, and to identify species with potential torespond to cavity management for the endangered Red-cockaded Woodpecker (Picoides borealis). We identifiedtwo groups into which most cavity-nesting species could be placed: 1) six species associated with pine snags andRed-cockaded Woodpecker cavities, and 2) five species associated with hardwood snags. We found the majority ofnests (60%) in large pine snags. Red-cockaded Woodpeckers and Northern Flickers (Colaptes auratus) excavatedmost cavities used by other cavity nesting birds. The Northern Flicker was the primary creator of large nest cavities,through its excavation in snags and enlargement of cavities originally excavated by the Red-cockaded Woodpeckerin live pine. Large secondary cavity nesting birds were the primary users of Red-cockaded Woodpecker cavities,and we identified three cavity-nesting species with potential to respond to Red-cockaded Woodpecker cavitymanagement. The cavity-nesting web dynamics documented in this study, including the role of large pine snags,Red-cockaded Woodpecker cavities in live pine, and excavating species within the community, can serve as abaseline for comparison to other southern pine forests.

Key words: abundance web, cavity-nesting birds, community structure, nest web, Northern Flicker, Red-cockadedWoodpecker, snag.

Dinamica de Redes en la Nidificacion en Cavidades en un Ecosistema de Pinus palustris

Resumen. Las comunidades de aves que nidifican en cavidades pueden verse como redes interconectadasque interactuan mediante la creacion de cavidades para nidificacion y mediante la competencia por estas.Utilizando un enfoque basado en redes, representamos graficamente el flujo de creacion y uso de cavidades en unacomunidad de aves que nidifican en cavidades en un ecosistema de Pinus palustris de Florida para examinar larelacion entre la abundancia de estas aves y las cavidades como recurso. Ademas, identificamos las especies quepotencialmente podrıan responder al manejo de cavidades realizado para la especie amenazada Picoides borealis.Identificamos dos grupos en los que la mayorıa de las especies que nidifican en cavidades pueden ubicarse: 1) seisespecies asociadas con tocones de pino y cavidades de P. borealis y 2) cinco especies asociadas con tocones demaderas duras. Encontramos la mayorıa de los nidos (60%) en tocones de pino grandes. Las especies P. borealis yColaptes auratus excavaron la mayorıa de las cavidades utilizadas por otras aves. El principal creador de cavidadesde nidificacion grandes fue C. auratus, mediante la excavacion de cavidades en tocones y el agrandamientode cavidades excavadas originalmente por P. borealis en pinos vivos. Los usuarios principales de cavidades deP. borealis fueron aves grandes que nidifican en cavidades secundarias, e identificamos tres especies con potencialpara responder al manejo de cavidades realizado para P. borealis. La dinamica en red de la nidificacion encavidades documentada en este estudio, incluyendo el papel en la comunidad de los tocones de pino grandes, delas cavidades de P. borealis en pinos vivos y de las especies excavadoras, puede servir como base para realizarcomparaciones con otros bosques de pino del sur.

INTRODUCTION

Cavity-nesting species represent a consideral portion of forestbird communities and rely upon the availability of cavityresources for nesting and roosting. Cavities can occur naturallythrough fungal decay at broken tree limbs, or through the pro-cess of woodpecker cavity excavation. Woodpeckers (primaryexcavators) typically excavate new cavities each year for nest-ing, and abandoned woodpecker cavities are regularly used

Manuscript received 19 December 2006; accepted 12 November 2007.1Email: lblanc@vt.edu

The Condor, Vol. 110, Issue 1, pages 80–92. ISSN 0010-5422, electronic ISSN 1938-5422. c© 2008 by The Cooper Ornithological Society. All rights reserved. Please direct all requestsfor permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/reprintInfo.asp.DOI: 10.1525/cond.2008.110.1.80.

by nonexcavating (secondary cavity-nesting) species, whichrely upon existing cavities for nesting and roosting. Weakexcavators use existing cavities or excavate their own cavitiesin highly decayed wood. These cavity-nesting guilds interactthrough the creation of and competition for cavities as nestingand roosting sites, and interactions among species can havea strong influence on forest community structure (Martin andEadie 1999). Examining cavity-nesting birds at the community

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level can aid in the identification of important cavity-makingagents, tree resources, and interspecific interactions (Dobkinet al. 1995, Martin and Eadie 1999, Bednarz et al. 2004, Martinet al. 2004, Saab et al. 2004). One useful way of examiningconnections and interdependencies among cavity-nestingspecies is to visually depict the community as a hierarchy of in-terconnected tree resources, cavity excavators, and secondarycavity users (i.e., a nest web; Martin and Eadie 1999).

In this study, we apply the nest web concept to the long-leaf pine (Pinus palustris) ecosystem of the southeasternUnited States to identify cavity-nesting bird community struc-ture in relation to the federally endangered Red-cockadedWoodpecker (Picoides borealis). Fire-maintained longleafpine forests, which are dominated by living pine, can be arelatively cavity-poor environment due to a low availability ofsnags (i.e., dead trees) and hardwoods, and the low tree densityresulting from regularly occurring fire (Conner et al. 2001).The Red-cockaded Woodpecker is the only species in thisecosystem that excavates cavities in living pine trees, and doesso exclusively (Ligon 1970). Red-cockaded Woodpeckers livein family groups, and each group typically maintains multiplecavity trees (i.e., a cluster) within its territory (Walters 1990).

Cavity excavation in a living pine tree can take yearsto complete, as compared to a few weeks for cavities exca-vated in dead and decaying wood by other woodpecker species(Jackson et al. 1979, Conner and Rudolph 1995, Harding andWalters 2002, 2004). Because these cavities are excavated inliving pine, they provide a long-term nesting and roostingresource on the landscape, sometimes remaining in use forseveral decades (Conner and Rudolph 1995). In comparison,the persistence of cavities in southern pine snags can be highlyvariable, depending on tree species, diameter, cause of treedeath, and exposure to fire (Miller and Marion 1995, Moormanet al. 1999, Conner and Saenz 2005). Given the persistence ofRed-cockaded Woodpecker cavities on the landscape and thefact that 27 vertebrate species (including four of the six otherwoodpeckers that breed in longleaf pine forests) are knownto use Red-cockaded Woodpecker cavities, the Red-cockadedWoodpecker is often referred to as a keystone excavator withinthe fire-maintained longleaf pine ecosystem (U.S. Fish andWildlife Service 2003). Little is known, however, about howRed-cockaded Woodpecker cavity availability influences thepresence and abundance of other cavity-nesting birds, or howthe availability of snags and other excavating species influ-ences cavity-nesting bird community structure within the fire-maintained longleaf pine ecosystem.

The Red-cockaded Woodpecker is an endangered speciesthat is heavily managed throughout the Southeast. ManagingRed-cockaded Woodpecker habitat, which includes the use ofprescribed burning to maintain an open midstory, has improvedhabitat for other fauna and flora of the longleaf pine commu-nity (Litt et al. 2001, Provencher et al. 2001, 2002, Conner et al.2002), and the relationship among prescribed burning, habitat

condition (i.e., degree of hardwood midstory succession), andavian community structure within the longleaf pine ecosystemhas been studied (Provencher et al. 2002, Allen et al. 2006).Little is known, however, about the effects of Red-cockadedWoodpecker cavity management on other species (U.S. Fishand Wildlife Service 2003). Cavities are a limiting factor toRed-cockaded Woodpecker populations (Walters et al. 1992),and cavity management techniques include the provisioningof artificial cavities (Copeyon 1990, Allen 1991, Copeyonet al. 1991) and the use of metal restrictor plates to preventand repair enlargement of Red-cockaded Woodpecker cavityentrances by other woodpecker species (Carter et al. 1989).While there is potential for negative impacts of heterospecificcavity use on the endangered Red-cockaded Woodpecker(U.S. Fish and Wildlife Service 2003), there are also concernsabout how cavity management for the Red-cockaded Wood-pecker may affect other cavity-using species (U.S. Fish andWildlife Service 2003). Several studies have indicated theneed to integrate an understanding of community interactionsinto the management of Red-cockaded Woodpecker cavities(Kappes 1997, 2004, U.S. Fish and Wildlife Service 2003,Walters et al. 2004); however, the relationship between theRed-cockaded Woodpecker and other cavity-using species ispoorly understood (U.S. Fish and Wildlife Service 2003).

The goal of this study is to quantitatively describe thecavity-nesting bird community in a mature, fire-maintainedlongleaf pine ecosystem. We focus on 1) the flow of cavity cre-ation and use, 2) whether the abundance of other cavity-nestingbirds is positively related to Red-cockaded Woodpecker cavityand snag availability, and 3) identifying species with potentialto respond to Red-cockaded Woodpecker cavity management.

METHODS

STUDY AREA

We conducted fieldwork from April through July in 2002 and2003 on Eglin Air Force Base, located in northwest Florida(Fig. 1). Eglin is the largest forested military reservation in theU.S., encompassing 187 555 ha, and is the largest, least frag-mented longleaf pine ownership in the Southeast (McWhiteet al. 1999). Approximately 78% of the Eglin reservationconsists of longleaf pine sandhills (McWhite et al. 1999),characterized by scattered longleaf pine, with an open to sparsemidstory of turkey oak (Quercus laevis) and a ground coverof various fire-adapted forbs and grasses (Kindell et al. 1997).Red-cockaded Woodpeckers inhabit sandhills habitat over theentire reservation, with over 6000 Red-cockaded Woodpeckercavity trees and 300 occupied clusters in 2002 when this studycommenced (K. Gault, Eglin Air Force Base, pers. comm.).Thus, the Eglin reservation is well suited for large-scale studiesthat involve replicate plots within longleaf communities.

82 LORI A. BLANC AND JEFFREY R. WALTERS

FIGURE 1. The location of the 36 study plots used in the study of cavity nesting species on Eglin Air Force Base, Okaloosa, Santa Rosa andWalton counties, Florida (2002–2003).

Eglin contains a large amount of old-growth longleafpine (Varner and Kush 2001), over 1700 dead standingRed-cockaded Woodpecker cavity trees (Williams et al.2006), and an abundance of old-growth pine snags dueto excessive tree mortality after the reintroduction of firefollowing years of suppression (Varner et al. 2005). As partof their habitat restoration process, Eglin land managershave also conducted hardwood midstory reduction throughmechanical and herbicidal methods (McWhite et al. 1999).As a result, both hardwood and pine snags were abundant onparts of the reservation during the course of this study, but inother areas at Eglin, few snags were available, as is typical ofother fire-maintained southeastern pine forests. Additionally,Eglin land managers do not harvest dead trees, and thus large,old-growth snags remain on the landscape as a resource forcavity nesting birds. This provided us with the opportunityto observe heterospecific use of Red-cockaded Woodpeckercavities in areas of varying snag densities.

DATA COLLECTION

In April 2002, we established 36 research plots, which werestricted to areas within the longleaf pine sandhills habitatwith mature pine and a relatively open midstory (Fig. 1).We acquired the locations of all trees with Red-cockadedWoodpecker cavities from Eglin’s long-term managementdataset, which is updated annually through surveys andintensive year-round Red-cockaded Woodpecker popula-tion management. We then selected study plots to re-flect a broad range of snag densities and Red-cockadedWoodpecker cavity availability (between zero and 43Red-cockaded Woodpecker cavities per plot). We used thelargest plot size possible (800 × 600 m) to accommodatethe large territories of many cavity-nesting species in thestudy system, while minimizing the presence of roads andcreeks within the plots. We used the maximum numberof plots possible (n = 36) that met these selection cri-teria. The large scale of our study plots (48 ha each)

LONGLEAF PINE CAVITY-NEST WEBS 83

encompassed habitat patchiness inherent to this ecosystem,and each plot thus contained a combination of pine savannaand some degree of hardwood midstory development. In eachplot, we established a grid system to aid with distance es-timation and general navigation to nest locations. The gridsystem consisted of nine 600-m transects located 100 m apart,with each transect flagged at 50-m intervals for visibility. Weestablished 12 point-count sampling stations within each plot,with three stations located on each of four alternating transects.Point-count stations were located 200 m apart and were treatedas subsamples within each plot. The sampling unit was a plot.

We recorded the relative abundance (detections persurvey) of birds at all sampling stations within each plot twiceannually between 8 April and 21 June 2002 and 2003 using apoint-transect count sampling technique adapted for the openhabitat (Provencher et al. 2002, 2003). We sampled two plots(totaling 24 sampling stations) per morning within the first3 hr after sunrise. We conducted an 8-min point count at eachsampling station, during which we recorded species, timedetected, vocalization type, location, and movement within a100-m fixed radius of the point center (Hutto et al. 1986). Twoobservers sampled simultaneously at point-count stations onalternating transects situated 200 m apart and synchronizedstart times for the point counts. Birds detected by the twoobservers at the same time were not double-counted. The sametwo observers conducted the point counts during both years.In counts at each sampling station, we included birds detectedas the observers moved between adjacent stations. Thistransect-count addition to the point-count sampling enabledus to account for birds fleeing ahead of the observers and maybe appropriate for open habitat (Bibby et al. 1992). For eachspecies and each round of sampling, we calculated relativeabundance (detections per survey) by summing the totalnumber of individuals detected at all 12 sampling stations in aplot. We then used the mean number of detections for the plot(across the two sampling rounds) as the measure of abundancefor that season. In a previous study at Eglin, Provencher et al.(2002) found no substantial difference between a similar mod-ified point-transect count method and standard point-countmethodology, except for reduced variability in bird detectionrates using the former method. Because all study plots werein the same habitat type, observer bias was minimized byusing the same two observers both years and double countswere minimized, we believe that our sampling methodologyprovides a reliable measure of the relative abundance of birdsoccurring in all study plots. Nonetheless, we emphasize thatour measurement was used as an index of abundance ratherthan an attempt to quantify breeding densities.

We conducted two rounds of nest searching per year ineach plot (between 15 April and 30 June, 2002 and 2003).During each round, nest searching consisted of two separateefforts: one field crew that focused only on nests in Red-cockaded Woodpecker cavities and another field crew that

focused on cavity nests in snags. We located nests in Red-cockaded Woodpecker cavities by inspecting all such cavitieswithin each plot once during each round of nest searching.We examined cavity contents with a camera mounted atopa telescoping pole (Treetop Peeper, Sandpiper TechnologiesInc., Manteca, CA), which enabled us to access cavities up to15.3 m in height. In 2002, we conducted searches for snag nestsopportunistically. We located nests by walking along transectsand observing breeding bird behavior in each plot twice duringthe season and inspecting all snags for potential nest cavities.We examined the contents of potential nest cavities and markednest trees with uniquely numbered aluminum tags. In 2003, weused the same searching methodology and also checked cavitynest trees from the previous year for nesting activity. In ad-dition, we added a systematic time constraint to ensure equalamount of search effort on each plot. During each round of nestsearching, we conducted searches for snag nests on two plotsper day, with two observers simultaneously. Over the entire sea-son, each observer spent 6 hr nest searching in each plot, total-ing 12 person-hr per plot. In both 2002 and 2003, we also foundnests incidentally, while conducting point counts. In addition,we recorded nests found within 50 m of a plot. We determinedthe order in which plots were sampled and searched for nestsrandomly within each of the two rounds of sampling, and thenon the basis of obtaining access to restricted areas on Eglin.To minimize systematic detection biases, across plot visits, werotated transect starting order and observer assignment to tran-sect within a plot. We conducted point counts and nest search-ing under satisfactory weather conditions: good visibility, littleor no precipitation, and light winds (Martin et al. 1997).

To record cavity resource (snag) availability, we estab-lished a 25-m radius vegetation plot at each sampling station inJuly of each year, totaling 12 per study plot (total area sampledper plot = 23 562 m2). We defined snags as dead standingtrees ≥10.2 cm diameter at breast height and ≥1.4 m tall. Werecorded numbers of snags in each vegetation plot, classifiedby type (pine or hardwood) and qualitative decay class: 1)living tree, 2) recently dead tree, most bark and branches andtop of tree intact, 3) dead, >50% bark and branches intact,4) dead, <50% bark or branches intact, top usually broken,and 5) dead, no branches or bark, broken top, extensive decay.We calculated snag availability for each plot by summing thenumber of snags, by type, across all 12 sampling stations perplot, per year. In 2002, we recorded the number of old-growthlongleaf pine on each vegetation plot as a habitat variable.Old-growth pine trees were identified by their characteristicgnarled, flat top morphology (Harper et al. 1997).

A close association between habitat condition and cavityresource availability can confound our ability to detect rela-tionships between cavity-nesting species and cavity-resourcetypes. Because the avian community in the longleaf pineecosystem is known to change with fire suppression (i.e.,habitat condition; Allen et al. 2006), we wanted to ensure that

84 LORI A. BLANC AND JEFFREY R. WALTERS

our abundance data were not reflective of habitat conditionalone. To detect any confounding between cavity resourceavailability and habitat condition (Martin and Eadie 1999),we used an indirect measure of fire suppression by monitoringthe abundance of two sets of non-cavity-nesting birds that areassociated with either open pine savanna (i.e., fire-maintained)or a developing hardwood midstory (i.e., fire-suppressed).Specifically, we wanted to examine whether our indirect mea-sure of habitat condition was associated with the availabilityof each cavity resource type, including living Red-cockadedWoodpecker cavity trees, pine snags, and hardwood snags.Non-cavity-nesting species associated with an open midstorywere the Bachman’s Sparrow (Aimophila aestivalis), EasternMeadowlark (Sturnella magna), and the Loggerhead Shrike(Lanius ludovicianus). Non-cavity-nesting species associatedwith a developing hardwood midstory were the Blue-grayGnatcatcher (Polioptila caerulea), Carolina Wren (Thryotho-rus ludovicianus), and Eastern Towhee (Pipilo erythrophthal-mus). If a confounding of habitat condition and cavity-resourceavailability exists, we would expect to find significant relation-ships between the non-cavity-nesting bird groups and cavity-resource types. If all three types of cavity resources were avail-able regardless of habitat condition, then a positive associationbetween the abundance of a species and a particular cavityresource type would likely reflect a species’ relationship withthat resource type rather than the influence of habitat conditionalone.

STATISTICAL ANALYSES

Abundance and cavity resource data were nonnormallydistributed; therefore, we used nonparametric statistics for allanalyses. We conducted all statistical analyses using SAS, Ver-sion 9.1 (SAS Institute, Inc., Cary, North Carolina). We useda Mann-Whitney U test to determine if there was a significantdifference in bird detections between years (i.e., high levels ofannual variation). A significant difference occurred only forthe Eastern Bluebird (U = 2.4, P = 0.02, n = 36), and thus,we analyzed this species separately by year. We pooled theremaining data across both years. To test for confounding ofcavity resource availability with habitat condition, we corre-lated the abundance of each of the two non-cavity-nesting birdgroups with pine snags, hardwood snags, and Red-cockadedWoodpecker cavities in living pine. To determine if detectionsof cavity-nesting species within each plot were reflective ofnesting effort, we correlated the number of detections per plotwith the number of nests found per plot for each species. Toexamine the associations between cavity-nesting guilds, wegenerated correlations among the abundance of primary cavityexcavators, weak excavators, and secondary cavity nestingspecies. All correlations were Spearman’s rank analyses.

We created an abundance web diagram to visually depictthe correlations of each species to Red-cockaded Woodpeckercavities, pine snags, hardwood snags, and other cavity nesting

birds. Because old-growth pine was significantly correlatedwith both Red-cockaded Woodpeckers (r = 0.43, P < 0.001)and their cavities (r = 0.55, P < 0.001) and with pinesnags (r = −0.30, P = 0.01), we controlled for the effectsof old-growth pine when correlating bird abundance withcavity resources. Recently dead trees (decay class 2) werenot included as cavity resources; such sites were not usedfor nesting by any of the cavity-nesting species during thisstudy, presumably because the wood was not decayed enoughfor excavation (Mannan et al. 1980, Schreiber and deCalesta1992, Jackson and Jackson 2004). We grouped the remainingsnag decay classes (3–5) into pine and oak and correlatedthese cavity resource groups with the abundance of each birdspecies. We used the Benjamini and Hochberg (1995) falsediscovery rate method to control the rate of Type I errors whenconducting multiple statistical tests (alpha level = 0.05). Thismethod provides a less conservative approach to correctingfor multiple tests than Bonferroni and sequential Bonferronimethods (Verhoeven et al. 2005), and addresses some recentcriticisms of the use of Bonferroni corrections in ecologicalstudies (Garcia 2003, 2004, Moran 2003). We incorporatedall associations with a P-value < 0.05 into the abundanceweb diagram and noted the species that dropped out when afalse-discovery rate correction was used.

Finally, we incorporated nest data into two nest webdiagrams (Martin and Eadie 1999), which reflect the flowof cavity creation and use between levels of primary cavityexcavators, weak excavators, secondary cavity nesting birds,and trees (cavity resources). Nest webs included all nest cav-ities for which we could identify the excavator, either throughknown use of the cavity or reasonable estimation based oncavity characteristics. Nests used in the web include reuseof the same cavity across years but not renesting attemptsby the same species in the same cavity within the same year.Abundance webs and nest webs serve as alternative depictionsof cavity-nesting bird community structure at Eglin. Whensampling for bird abundance, it is possible to detect birdsthat are foraging but not nesting in the plot. Basing a web onnest data provides a more definitive measure of which speciesactually use the plot for nesting, although it is possible tomiss nests of species that are shy or stealthy or for whichour nest searching methodology is not well suited. Thus, theabundance and nest webs should complement one another.

RESULTS

We detected 14 cavity-nesting species in 2002 and 2003(Table 1). The abundance of nonexcavators was significantlyassociated with both weak excavator (r = 0.39, P < 0.001,n = 72 plots) and primary excavator abundance (r = 0.27,P = 0.02, n = 72 plots). Weak excavator abundance was alsosignificantly associated with primary excavator abundance(r = 0.46, P < 0.001, n = 72 plots). Significant associations

LONGLEAF PINE CAVITY-NEST WEBS 85

TABLE 1. Mean detections and nests found (± SE) per 48-ha plot, including the correlation between these two variables, based on datacollected from April to July, 2002–2003 at Eglin Air Force Base, Florida. Because Eastern Screech-Owls were only occasionally detectedusing our sampling methodology, and because we found no nests for the Carolina Chickadee, correlations could not be computed for thesetwo species (dashes indicate the data that were unable to be collected for these species). With the exception of the Pileated Woodpecker,Brown-headed Nuthatch, and Eastern Bluebird, all correlations remain significant after using the false discovery rate method to control therate of Type I errors when conducting multiple statistical tests (alpha level = 0.05; Benjamini and Hochberg 1995).

Species Common name Mean detections Mean nests Correlation coefficient P-value

Primary cavity excavatorsMelanerpes erythrocephalus Red-headed Woodpecker 5.3 ± 0.3 2.19 ± 0.21 0.49 <0.001Melanerpes carolinus Red-bellied Woodpecker 3.7 ± 0.2 0.57 ± 0.19 0.38 0.001Picoides pubescens Downy Woodpecker 0.6 ± 0.1 0.06 ± 0.03 0.33 0.004Picoides villosus Hairy Woodpecker 0.5 ± 0.1 0.03 ± 0.02 0.33 0.004Picoides borealis Red-cockaded Woodpecker 3.2 ± 0.2 0.88 ± 0.12 0.79 <0.001Colaptes auratus Northern Flicker 1.4 ± 0.1 0.60 ± 0.10 0.25 0.03Dryocopus pileatus Pileated Woodpecker 0.6 ± 0.1 0.04 ± 0.02 0.21 0.07

Weak excavatorsParus carolinensis Carolina Chickadee 1.6 ± 0.1 — — —Sitta pusilla Brown-headed Nuthatch 2.3 ± 0.2 0.03 ± 0.02 0.19 0.10

Secondary cavity-nestersFalco sparverius American Kestrel 1.9 ± 0.1 1.24 ± 0.14 0.65 <0.001Otus asio Eastern Screech-Owl — 0.29 ± 0.06 — —Myiarchus crinitus Great-crested Flycatcher 0.7 ± 0.1 0.03 ± 0.02 0.30 0.01Parus bicolor Tufted Titmouse 2.2 ± 0.2 0.06 ± 0.03 0.35 0.003Sialia sialis Eastern Bluebird 1.2 ± 0.1 0.01 ± 0.01 0.06 0.60

between individual species are depicted in the abundance web(Fig. 2), which indicates two groups into which most of thecavity-nesting species on Eglin can be placed. One groupcontains the Red-headed Woodpecker (Melanerpes erythro-cephalus), Red-cockaded Woodpecker, Northern Flicker (Co-laptes auratus), Brown-headed Nuthatch (Sitta pusilla), andAmerican Kestrel (Falco sparverius), which are associatedwith pine snags and Red-cockaded Woodpecker cavities inlive pines. The second group contains the Red-bellied Wood-pecker (Melanerpes carolinus), Downy Woodpecker (Picoidespubescens), Carolina Chickadee (Parus carolinensis), Great-crested Flycatcher (Myiarchus crinitus), and Tufted Titmouse(Parus bicolor), which are associated with hardwood snags.Over half of the Red-bellied Woodpecker nests were foundin pine snags, and because this species was associated with asubset of pine snags (decay class 2; r = 0.26, P = 0.03, n =72 plots), we added a link to the pine web to indicate this re-lationship. The Eastern Screech-Owl (Otus asio) was detectedonly rarely using our sampling methodology and thus was notincluded in the pine abundance web, although nest data indi-cated this owl may be a member of the pine group (see below).Eastern Bluebird (Sialia sialis) abundance was associatedwith Brown-headed Nuthatch abundance in 2002 (r = 0.51,P = 0.002, n = 36 plots) but had no significant associationswith any other bird species or cavity resources in 2002 or 2003.

We found 433 cavity nests for 13 of the 14 cavity-nestingspecies (Table 2). Naturally occurring cavities were rare,and almost all nests found (432 of 433; >99%) were inwoodpecker-excavated cavities. The majority of nests (60%,

excluding cavities in dead Red-cockaded Woodpecker cavitytrees, n = 261) were in pine snags, followed by living Red-cockaded Woodpecker cavity trees (27%, n = 116), hardwoodsnags (8%, n = 33), and then dead Red-cockaded Woodpeckercavity trees (5%, n = 23). Red-headed Woodpeckers, whichnested almost exclusively in large pine snags with very littlebark, accounted for 48% of all snag nests (Table 2).

A nest web illustrating cavity resource use at Eglin (Fig. 3)depicts the proportion of nests for a given species found in aparticular resource type; secondary cavity nesting birds areconnected to the species that excavated their nest cavities, andprimary cavity excavators are connected to either the tree re-source in which their cavities were excavated or to other speciesthat excavated their cavities. The nest web in Figure 4 showsthe proportion of cavities excavated by a particular species ineach tree type, regardless of which species actually nested inthe cavity at the time it was found. The Carolina Chickadee isnot shown in the nest web because we found no nests for thisspecies during the two years of this study. We found only oneEastern Bluebird nest (in 2003) but were unable to identifythe excavator of the cavity; therefore, we excluded this speciesfrom the nest web diagram.

The Red-cockaded Woodpecker and Northern Flickerprovided the greatest proportion of cavities used by othercavity-nesting birds in this system, relative to their nest-ing abundance (Fig. 3). Red-cockaded Woodpecker nestsaccounted for only 15% of all nests found, but 32% of allnests were found in cavities they had originally excavated.Northern Flicker nests accounted for only 10% of all nests

86 LORI A. BLANC AND JEFFREY R. WALTERS

FIGURE 2. Bird-abundance web based on correlations of sampling data from Eglin Air Force Base, 2002 and 2003, pooled across bothyears (n = 72 plots). Numbers shown represent Spearman’s partial correlation coefficients (r ), controlling for the number of mature pinetrees. All links within the web indicate a significant correlation (P < 0.05) between the connected nodes, and only species with a significantcorrelation with a nest resource type (hardwood snag, pine snag, or Red-cockaded Woodpecker cavity in living pine) are included. Dashedlines indicate those links that drop out of the web when the Benjamini and Hochberg (1995) procedure is used to control the false discoveryrate for multiple statistical tests.

TABLE 2. Cavity nests found between April and July, 2002–2003 on Eglin Air Force Base, Florida in pine andhardwood snags, and live and dead Red-cockaded Woodpecker cavity trees.

Red-cockaded WoodpeckerSnags cavity trees

Species Pine Hardwood Live Dead Nests found

Primary cavity excavatorsRed-headed Woodpecker 153 0 1 4 158Red-bellied Woodpecker 22 11 7 1 41Downy Woodpecker 0 4 0 0 4Hairy Woodpecker 0 2 0 0 2Red-cockaded Woodpecker 0 0 63 0 63Northern Flicker 28 6 6 3 43Pileated Woodpecker 1 0 2 0 3

Weak excavatorsCarolina Chickadee 0 0 0 0 0Brown-headed Nuthatch 0 2 0 0 2

Secondary cavity-nestersAmerican Kestrel 50 0 25 14 89Eastern Screech-Owl 5 3 12 1 21Great-crested Flycatcher 1 1 0 0 2Tufted Titmouse 1 3 0 0 4Eastern Bluebird 0 1 0 0 1

Total 261 33 116 23 433

LONGLEAF PINE CAVITY-NEST WEBS 87

FIGURE 3. Nest web diagram depicting nest resource use by the cavity-nesting bird community at Eglin Air Force Base, Florida, 2002 and2003. N indicates the number of nests found; E indicates the total number of all nest cavities excavated by that species. Links between thelevel of secondary cavity-nesting birds and the primary cavity-excavator level represent the proportion of nests of secondary cavity-nestingspecies found in a cavity excavated by the indicated excavating species. Links between the primary cavity excavator and tree levels reflect theproportion of that primary cavity-excavator’s nests found in the connected tree resource. The link between the Northern Flicker and Red-headedWoodpecker indicates that 2% of Northern Flicker nests were found in cavities originally excavated by a Red-headed Woodpecker.

found, but 17% of all nests found were in cavities they hadoriginally excavated. The Red-headed Woodpecker excavatedthe highest number of nest cavities (n = 153) at our study site;however, use of its cavities by other species was uncommon(n = 3; Figs. 3 and 4). The Northern Flicker was the primarycreator of large cavities used by other species at Eglin,through its enlargement of Red-cockaded Woodpecker cavityentrances and its excavation of new cavities in snags (Figs. 3and 4).

Most species detections were reflective of nesting effortwithin each plot; for 11 of the 14 species, the abundanceof a species was positively correlated with the number ofnests found for that species (Table 1). Correlations couldnot be computed for two species: the Carolina Chickadeewas often detected, but we found no nests, whereas we

found many Eastern Screech-Owl nests, but rarely detectedthem in point counts. Neither of the non-cavity-nesting birdgroups (i.e., the open savanna group or developing hardwoodmidstory group) was significantly associated with live Red-cockaded Woodpecker cavity trees (r = 0.03, P = 0.79 andr = −0.17, P = 0.16, respectively; n = 72 plots), hardwoodsnags (r = −0.16, P = 0.17 and r = 0.02, P = 0.90, re-spectively; n = 72 plots), or pine snags (r = 0.09, P = 0.45and r = 0.10, P = 0.43, respectively; n = 72 plots),indicating that all three types of cavity resources wereavailable to cavity-nesters regardless of habitat condition.Therefore, the associations depicted in our abundance web(Fig. 2) likely reflect community structure as influencedby cavity resource type, rather than habitat conditionalone.

88 LORI A. BLANC AND JEFFREY R. WALTERS

FIGURE 4. Excavation web diagram of the cavity nesting bird community on Eglin Air Force Base, Florida, 2002 and 2003. N indicates thenumber of nests found; E indicates the total number of nesting cavities excavated by that species. Links between a primary cavity excavatorand the tree level reflect the proportion of all nest cavities excavated by that particular excavator in the tree resource.

DISCUSSION

Because tree cavity availability can limit the numbers ofcavity-nesting birds, it is generally assumed that that thereis a critical ecological relationship between excavating andnonexcavating cavity-nesting species (Newton 1994). Only afew studies, however, have empirically demonstrated a signifi-cant association between these cavity-nesting guilds, by eithercorrelating the abundance of excavating and nonexcavatingspecies (Martin and Eadie 1999, Bai 2005, this study), or bycorrelating abundance of secondary cavity nesting birds withwoodpecker-excavated cavities (Dobkin et al. 1995). Recentstudies have noted that the importance of cavity excavatorsmay vary with the availability of naturally occurring cavitiesin different forest types (Remm et al. 2006, Aitken and Martin2007). For example, in conifer-dominated forests, where natu-rally occurring tree cavities are uncommon, woodpecker cavityexcavation may play a particularly important role in creatingnesting habitat for nonexcavating, cavity-using species (Bullet al. 1997, Walter and Maguire 2005). Alternatively, somemature deciduous forests have a superabundance of naturalcavities, and the role of woodpecker cavity excavation maybe less critical to secondary cavity-nesting species (Aitkenand Martin 2007, Weso�lowski 2007). In the Eglin sandhills,naturally occurring cavities were rare, and almost all nestsoccurred in excavated cavities, indicating the importance ofexcavators within this conifer-dominated system. In addition,the proportion of excavating birds at Eglin was greater than

nonexcavating cavity users (64% and 46%, respectively), sim-ilar to proportions found in ponderosa pine (Pinus ponderosa)forests of the western U.S. (66% excavators; Saab et al.2004).

The degree to which the grouping of cavity-nestingspecies within our abundance web is due to habitat condi-tion or nesting resources is unclear. To an extent, birds in thepine and hardwood groups reflect habitat associations, as mostmembers of the pine web are associated with open habitat,and most members of the hardwood web are associated witha hardwood midstory component (Shackelford and Conner1997, Allen et al. 2006). The lack of a relationship betweenour indirect measure of habitat condition and the three cavityresource types indicates that all resource types were availableto cavity nesting species regardless of a hardwood midstorycomponent. The positive association between abundance ofcavity-nesting birds and number of cavity nests found withinthe study plots, as well as the nest web structure being gener-ally reflective of the abundance web, suggest that these abun-dance webs also reflect community structure as influenced bycavity-nest resource type, a pattern that has been documentedin other cavity-nesting communities (Martin and Eadie 1999,Bai 2005). These results are consistent with previous studiesconducted at Eglin (Provencher et al. 2002) and longleaf pinesandhills in North Carolina (Allen et al. 2006). Provencheret al. (2002) found no significant effect of hardwood midstoryremoval on the abundance of the Carolina Chickadee, Downy

LONGLEAF PINE CAVITY-NEST WEBS 89

Woodpecker, or Great-crested Flycatcher. Allen et al. (2006)classified the Carolina Chickadee, Great-crested Flycatcher,Northern Flicker and Red-bellied Woodpecker as generalistspecies (not strongly associated with either open habitat or ahardwood midstory component), and suggested that suitablecavity distribution may be a more important habitat feature tocavity nesting birds than vegetation structure alone.

The numerical dominance of the pine web group in thisstudy may simply reflect use of the resource that dominatesthe landscape; however many of the nests at Eglin occurred inlarge, mature pine snags, which may not be typical of younger,managed pine forests in the South. Old-growth pine snags havelarge diameters and persist over time (Miller and Marion 1995,Conner and Saenz 2005) and may have a strong influence onthe presence and abundance of cavity-nesting communities insouthern pine forests. For example, Miller and Marion (1995)found that longleaf pine stands in Florida containing large,old pine snags averaged twice the number of cavity-nestingbirds despite a snag density 40% lower than pine plantations.The hardwood group identified in this study may reflect a mi-nor group within the sandhills habitat at Eglin, as relativelyfew nests were found in hardwood snags, and hardwood snagswithin our plots often occurred as patchy remnants of a previ-ously existing hardwood midstory.

The abundance and nest webs, both of which show aroughly equal number of species associated with each ofthe three cavity resource types, indicate the potential forcommunity-level nest resource partitioning by cavity-nestingbirds at Eglin. Resource or niche partitioning may provide amechanism for species to minimize resource overlap with othercommunity members and avoid interspecific competition.Such partitioning of cavity resources has been documented inother studies of cavity-nesting communities (Bull et al. 1986,Martin et al. 2004) and may provide insight into how up to 14cavity-nesting bird species can coexist and breed within thefire-maintained longleaf pine ecosystem, a system which isoften described as a relatively cavity-poor environment (Con-ner et al. 2001, U.S. Fish and Wildlife Service 2003). Indeed,the extent to which differences in structural characteristicsand decay dynamics of living pine, pine snags, and hardwoodsnags can create sufficient niche space for cavity nestingspecies, and how the modification of cavity-resource availabil-ity can affect community interactions (such as heterospecificuse of Red-cockaded Woodpecker cavities) remains relativelyunexplored.

The identification of the Northern Flicker as the primarycreator of large cavities within the sandhills at Eglin was un-expected, given that Pileated Woodpeckers are generally con-sidered the keystone provider of large cavities in southern pineforests (Saenz et al. 1998, U.S. Fish and Wildlife Service 2003).That Pileated Woodpeckers provide large cavities via the en-largement of Red-cockaded Woodpecker cavity entrances iswell documented (Jackson 1978, Walters 1991, Saenz et al.

1998, U.S. Fish and Wildlife Service 2003). At Eglin, PileatedWoodpeckers do enlarge Red-cockaded Woodpecker cavityentrances, but only in a few areas on the reservation that havea strong mesic component (K. Gault, Eglin Air Force Base,pers. comm.). The Pileated Woodpecker is also described asthe key provider of cavities for large, secondary cavity-nestingbirds in the Pacific Northwest (Bonar 2000, Aubry and Raley2002), although this species produces few cavities relative toother woodpeckers (Harestad and Keisker 1989, Martin andEadie 1999, Bonar 2000). Our results indicate that the North-ern Flicker plays a greater role than the Pileated Woodpecker asa provider of large cavities for secondary cavity-users at Eglin.Other studies in North America have identified the NorthernFlicker as a significant provider of large cavities (Moore 1995,Martin and Eadie 1999, Martin et al. 2004, Saab et al. 2004);that this role extends to the longleaf pine community has beenpreviously unappreciated. Further research is needed to exam-ine the relative importance of Northern Flickers and PileatedWoodpeckers within southern pine forests with varying de-grees of xeric and mesic habitat.

Given the well-documented use of Red-cockaded Wood-pecker cavities in other southern pine forests (U.S. Fish andWildlife Service 2003, Kappes 2004, Walters 2004, Walterset al. 2004), we were surprised by how few nests of heterospe-cific woodpeckers (n = 16 of 250; 6%), small secondarycavity-nesting species (n = 0), and weak excavators (n = 0)were found in these cavities at Eglin. Indeed, only two species,the Eastern Screech-Owl and American Kestrel, accounted forthe majority of heterospecific nests found in Red-cockadedWoodpecker cavities. The heavy use of pine snags, despite theavailability of Red-cockaded Woodpecker cavities, suggeststhat cavities in snags may have advantages over Red-cockadedWoodpecker cavities in living pine, a possibility previouslysuggested by Kappes (2004). One possible explanation forthe predominant use of Red-cockaded Woodpecker cavitiesby the Eastern Screech-Owl and American Kestrel at Eglinis that opportunities for these large secondary cavity-nestingbirds to find existing cavities with sufficiently large entranceand chamber sizes are limited (Martin et al. 2004, Aitken andMartin 2007). To test whether large cavities are indeedlimiting at Eglin, additional study is needed to examineuse vs. availability of large cavity resources and potentialinterspecific competition between these two large, secondarycavity-nesting species.

Our nest data for three species, the Brown-headedNuthatch, Carolina Chickadee, and Eastern Bluebird, shouldbe interpreted with caution for several reasons. First, the phe-nology of these species may have been too early for our search-ing efforts to have produced a representative sample of nests(Mostrom et al. 2002, Withgott and Smith 1998, Gowaty andPlissner 1998). Second, given our large plot size and widelyspaced transects, our nest-searching methodology may nothave been well suited for locating nests in small, short snags,

90 LORI A. BLANC AND JEFFREY R. WALTERS

typical of these species. Therefore, we may have underesti-mated the nesting effort of these species.

CONSERVATION AND MANAGEMENT IMPLICATIONS

The abundance of cavity resources at Eglin, in particular old-growth pine snags and Red-cockaded Woodpecker cavities,may account for the abundance of several cavity-nestingspecies that are in decline elsewhere. The Northern Flickerranks first among declining cavity-nesting species in thesoutheastern U.S. (Sauer et al. 2005). The American Kestrelat Eglin (F. s. paulus) is a state-listed threatened subspecies(Wood 1996) limited to the southeastern U.S. and may bereliant upon Red-cockaded Woodpecker cavities for nestinghabitat (Gault et al. 2004). The Red-headed Woodpecker, themost prolific cavity-nesting bird in this study, is a species ofcontinental concern (Panjabi et al. 2005), and a growing bodyof literature indicates that longleaf savannah provides highquality habitat for this species (Shackelford and Conner 1997,Belson 1998, Smith et al. 2000, Allen et al. 2006).

Our results suggest that the Red-cockaded Woodpecker,through its cavity-excavation in living pine, may positively af-fect American Kestrel, Northern Flicker, and Eastern Screech-Owl abundance, and cavity restrictor plates have potential tonegatively affect these larger species. In addition, the frequentuse of pine snags by other species in this study, despite theabundance of Red-cockaded Woodpecker cavities, suggeststhat retaining snags may benefit Red-cockaded Woodpeckersby reducing heterospecific occupation of their cavities. Giventhe large amount of management directed toward preventingheterospecific use of Red-cockaded Woodpecker cavities, fu-ture experimental studies are needed to clarify the relationshipbetween snag availability and Red-cockaded Woodpeckernesting success (Kappes and Harris 1995, U.S. Fish andWildlife Service 2003).

The dynamics of Eglin’s cavity-nesting bird communitymay be used as a baseline for comparison to other southernpine forests, where the absence of these cavity resources mayimpose stress on relationships among species and possiblyincrease competition among them. Southern pine forests canvary widely in their availability of old, dead or decayingtrees due to different management practices (McComb et al.1986), and our results suggest that the availability of resourcessuitable for cavity excavation may have a strong influence onnest web community structure, complexity and connectivity.The webs presented here may provide the first steps necessaryin the development of tools for predicting cavity-nestingcommunity structure under varying conditions (Blanc andWalters 2007) and for generating predictions about speciesinteractions, which can be tested through experimental ma-nipulation (Blanc and Walters 2008). The generation of suchcommunity-level nest webs in other southern pine forests withvarying levels of snag densities, age classes, and management

is needed to develop a better understanding of mechanismsthat enable the healthy coexistence of cavity-nesting speciesin southern pine communities.

ACKNOWLEDGMENTS

We are grateful to personnel from Jackson Guard at Eglin Air ForceBase for their support and assistance with this study, including (butnot limited to) Kathy Gault, Bruce Hagedorn, Marlene Rodriguez, RonTaylor, Dennis Teague, James Furman, J. Kevin Hiers and Steve Lane.We thank the cavity-nester field crew who assisted with data collectionduring 2002 and 2003, including Robert Emerson, Melanie Colon,and Lynnette Brock. Thanks also to volunteer Beth Orning-Tschampl.Fieldwork and logistical support were also provided by Andy Butler, JimKowalsky and Lou Phillips of the Virginia Tech RCW Research Team.We thank Lynn Adler, Carola Haas, Bob Jones and Dean Stauffer fortheir input throughout this study. We also thank D. Dobkin, J. Bednarz,and J. M. Eadie for their helpful comments on earlier drafts of thismanuscript. Funding for this study was provided by the Department ofDefense, through Threatened and Endangered Species funds at Eglin AirForce Base. Additional funding support was provided by Sigma Xi anda Virginia Tech Graduate Research Development Program grant.

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