Hematological Health State Indices Predict Local Survival in a Small Passerine Bird, the Great Tit (Parus major)

Division of Comparative Physiology and Biochemistry, Society for Integrative andComparative Biology

Hematological Health State Indices Predict Local Survival in a Small Passerine Bird, the GreatTit ( Parus major )Author(s): Priit Kilgas, Vallo Tilgar, and Raivo MändSource: Physiological and Biochemical Zoology, Vol. 79, No. 3 (May/June 2006), pp. 565-572Published by: The University of Chicago Press. Sponsored by the Division of ComparativePhysiology and Biochemistry, Society for Integrative and Comparative BiologyStable URL: http://www.jstor.org/stable/10.1086/502817 .

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565

Hematological Health State Indices Predict Local Survival in a

Small Passerine Bird, the Great Tit (Parus major)

Priit Kilgas*Vallo TilgarRaivo MandInstitute of Zoology and Hydrobiology, University of Tartu,Vanemuise 46, Tartu 51014, Estonia

Accepted 8/11/2005; Electronically Published 4/7/2006

ABSTRACT

In birds, it has been shown that reproductive effort may impairparental condition, while the relation of different conditionindices to subsequent survival is still poorly understood. In thisstudy, we measured body mass and various hematological con-dition indices in breeding great tits in relation to local survival.Number and quality of nestlings and the occurrence of secondbroods, potentially reflecting parents’ breeding effort, were alsoconsidered in analyses. The great tits, both male and female,that returned the following year had had a higher albumin/globulin ratio, lower plasma globulin concentration, and alower heterophile/lymphocyte ratio during breeding in the pre-ceding year, compared to those who did not return. Survivingmales (but not females) also had had a higher level of circulatinglymphocytes, compared to nonsurvivors. There was no cor-relation between breeding effort and survival. We conclude thatbetter immunological state and lower stress in great tits duringbreeding were positively related to their survival probability.

Introduction

According to the life-history theory (Stearns 1992), physiolog-ical trade-offs arise when animals have limited resources toallocate between competing life-history traits. However, it is acomplicated task to find the appropriate traits that are currentlybeing traded off or the currency in which the cost is paid(Stearns 1992). It has been recognized that physical conditionhas an important role in mediating the life-history trade-offs(Stearns 1992; Sanz et al. 2002). However, we still largely lack

* E-mail: [email protected].

Physiological and Biochemical Zoology 79(3):565–572. 2006. � 2006 by TheUniversity of Chicago. All rights reserved. 1522-2152/2006/7903-5007$15.00

knowledge on how different aspects of condition should bemeasured (Gustafsson et al. 1994; Horak et al. 2002).

It has been proposed that reproductive costs can be reflectedin an individual’s immune system because it interacts with thegeneral health state of an organism and competes for the re-sources that can be allocated to reproduction (Gustafsson etal. 1994; Sheldon and Verhulst 1996; Zuk and Stoehr 2002).For example, immunological processes interact with an indi-vidual’s nutritional condition (Lochmiller et al. 1993; Saino etal. 1997b), reproductive effort (Deerenberg et al. 1997; Ilmonenet al. 2000), and survival (Saino et al. 1997a; Møller and Erritzøe2000; Ardia et al. 2003; Hanssen et al. 2003, 2004; see reviewin Møller and Saino 2004).

Recently, the use of several hematological parameters (e.g.,leukocyte counts, hematocrit, and plasma protein concentra-tions) as indicators of an individual’s condition and immunefunction has become increasingly popular in avian ecology (e.g.,Gustafsson et al. 1994; Horak et al. 1998b; Gonzalez et al. 1999).However, sometimes the variation in the values of conditionand health indices in wild animals is difficult to interpret (Readand Allen 2000; Horak et al. 2002; Adamo 2004), because themeaningfulness of the changes in particular indices is largelyunknown. Therefore, it is important to know how differentcondition indices are related to fitness components such assurvival. Still, so far only a few studies have examined con-nections between widely used hematological condition indicesand survival in adult birds (Horak and Ots 1998; Gonzalez etal. 1999; Hanssen et al. 2003; Verhulst et al. 2004).

Sexes can also differ considerably in susceptibility to diseases.Males tend to suffer more from parasitic infections and havereduced immune responses, compared to females (Klein 2000;Barbosa and Moreno 2002; Zuk and Stoehr 2002; but seeMcCurdy et al. 1998; Ots et al. 1998). However, knowledgeabout sex differences in the ways that individual conditionrelates to survival in natural populations is still poor.

According to life-history theory, reproducing parents thatraise a relatively large number of high-quality offspring arepredicted to allocate relatively large amounts of time and energyto parental activities, thereby suffering larger mortality or re-duced future reproductive success, compared to parents witha small number of low-quality offspring (reviewed in Lindenand Møller 1989). Although some studies in birds have indeedfound deleterious effects of reproductive effort on parental sur-vival (Nur 1984; Golet et al. 1998; Morton et al. 2004; Stjern-man et al. 2004), in other studies such a relationship was not



566 P. Kilgas, V. Tilgar, and R. Mand

revealed (Blondel et al. 1998; see also Golet et al. 1998 forreview).

The first objective of this study was to explore the possiblerelations of different condition indices to the local survival ofreproducing great tits (Parus major L.). The indices used in-cluded body mass and several widely used hematological pa-rameters: albumin, globulin, and triglyceride concentrations,hematocrit, albumin/globulin ratio, lymphocyte and hetero-phile concentrations, and heterophile/lymphocyte ratio. Ofthese variables, body mass (Verhulst et al. 2004), lymphocyteconcentration (Hanssen et al. 2003), and hematocrit (Verhulstet al. 2004) have previously been found to be positively asso-ciated and globulin content (Gonzalez et al. 1999) negativelyassociated with adult survival in birds. The second objectivewas to examine the presumed existence of a negative relation-ship between parental survival and putative indicators of theirreproductive effort: brood size, fledging success, offspring qual-ity, and occurrence of second broods.

Since physiological trade-offs often appear only under foodstress (Stearns 1992; Moret and Schmid-Hempel 2000; Hanssenet al. 2004), we conducted our study in two contrasting habitats,deciduous and coniferous woods, which differ from each othersignificantly by food availability as well as by reproductive out-put of great tits breeding there (Mand et al. 2005).

Material and Methods

Study Site and Field Methods

The study was conducted in the surroundings of Kilingi-Nomme (58�07�N, 25�05�E), southwestern Estonia, in 2000–2002 (with the main study carried out during the first twoyears; in the last year, only local survival was recorded). Variousforest habitats in the 50-km2 study area can be divided intotwo contrasting types—deciduous and coniferous woods. Fordetailed description of the habitats and the scheme of the studyarea, see Magi and Mand (2004).

Great tits bred in nest boxes mounted on tree trunks at aheight of about 1.5–2.0 m. The internal size of the nest boxeswas cm, and the diameter of the entrance11 cm # 11 cm # 30hole was 3.5–4.0 cm. The distance between neighboring boxeswas 50–60 m. In total, we had 1,079 nest boxes. The mean nestbox occupancy by great tits was 12.8% during the first breedingattempt (123–156 pairs annually). Old nest material was re-moved every year before the start of nest building. The nestboxes were checked regularly to record basic breeding param-eters, such as the number of hatchlings and fledglings. To assessthe offspring quality, all fledglings were weighed with a Pesolaspring balance to a precision of 0.1 g, and their tarsi weremeasured with sliding calipers to the nearest 0.1 mm on day15 posthatch. Adult great tits were captured on their nests whennestlings were at least 8 d old. They were also weighed, andtheir tarsi were measured. Sex was determined by brood patch.

The occurrence of a second brood was determined by capturingparents during their second breeding attempt.

All captured adults were banded with individual rings, andthose not recorded as breeders in the subsequent year wereconsidered to be nonsurvivors. The great tit is characterized byhigh breeding-site fidelity (Kluijver 1951), and breeding dis-persal, when it occurs, is very limited in distance (50–200 m;Harvey et al. 1979; Paradis et al. 1998). Besides, great tits clearlyprefer nest boxes to natural cavities, even when these are plen-tiful (Lohmus and Remm 2005). Thus, given that our studyarea is as large as 50 km2 and that there is a surplus of suitablenest boxes (Mand et al. 2005), we have good reason to believethat our recapture data indeed reflect local survival rather thandispersal to outside the study area or to natural holes that wedo not control. The same approach to estimating local survivalhas been used previously in several works with different Parusspecies (e.g., Blondel et al. 1998; Stjernman et al. 2004), in-cluding the great tit (e.g., Verhulst 1998; Tinbergen and Verhulst2000; Horak 2003).

Hematological Parameters Studied

Hematocrit measures the relative amount of red blood cells intotal blood volume. It reflects the intensity of oxygen transportinto tissues. Its low values (anemia) are indicative of bacterialor parasite infections but also accompany starvation and reflectthe scarcity of some micronutrients, such as iron, copper, andvitamin B12 (Coles 1997). Hematocrit value can rise with hardphysical exercise (Horak et al. 1998b). Therefore, the variationof hematocrit in natural populations can be hard to interpret.

The lymphocyte concentration in peripheral blood is some-times used as an indirect measure of cell-mediated immunity.However, sometimes its values are difficult to interpret (e.g.,Read and Allen 2000). Decreased lymphocyte concentrationsmay signal stress-induced immunosuppression (Horak et al.1998b) but may also indicate a lack of parasite infections (Otsand Horak 1998). A high heterophile/lymphocyte (H/L) ratiois indicative of stress (Gross and Siegel 1983; Ots and Horak1996; Moreno et al. 2002; Laaksonen et al. 2004).

A decrease in albumin concentration in the blood plasmaaccompanies almost any disease and also malnutrition (e.g.,Kawai 1973). An increase in globulin concentration is expectedafter chronic or acute infections and with inflammatory pro-cesses, and healthier individuals have higher albumin/globulin(Alb/Glo) ratios (Kawai 1973).

Triglyceride concentration in blood plasma reflects the in-tensity of lipid transport into adipose tissues and can be con-sidered as an estimate of recent (some hours before bloodsampling) nutritional changes. Underfed individuals generallyexhibit lower concentrations of triglycerides (Jenni-Eiermannand Jenni 1997).



Health State and Survival in Great Tits 567

Blood Sampling and Laboratory Analyses

Blood samples (ca. 100–150 mL) for hematological measure-ments were taken during first broods from the tarsal or brachialveins in adults. For measuring hematocrit, plasma proteins, andtriglycerides, blood samples were collected into heparinizedcapillary tubes. Plasma was separated from blood cells after 10min of centrifugation at 10,000 rpm (relative centrifugation

g) and stored at �20�C until analyzed.force p 10,621Hematocrit was measured with sliding calipers to the nearest

0.1 mm. Standard agarose gel electrophoresis with the REPsystem (Helena Laboratories) was used for detection of majorprotein groups. Gels were stained with Ponceau S stain usingan REP gel processor and densitometrically scanned at a wave-length of 525 nm. Because of difficulties in separating the pre-albumin fraction from albumin, we used the summed concen-tration and termed it the albumin concentration. The sameapproach was used by Horak et al. (1998a) and Ots et al. (1998).Concentration of triglycerides was determined using a quan-titative enzymatic test (Sigma Diagnostics).

For identification of leukocytes, a drop of blood was smearedon a microscope slide, air-dried, fixed in absolute methanol,and stained with azure-eosin. In the analyses, only data forlymphocytes and heterophiles, as the most numerous immunecells, were used. The proportion of heterophiles and lympho-cytes was assessed under 1,000# magnification on the basis ofan examination of a total of 100 leukocytes in oil immersion.Differential leukocyte concentrations were obtained by multi-plying their proportions by total white blood cell count per10,000 erythrocytes. The person examining the blood smearshad no information about the individual birds except the ringnumber. The same methods for estimating leukocyte concen-trations have been used by, for example, Horak et al. (1998a)and Ots et al. (1998). Plasma chemistry values (triglyceride,albumin, and globulin concentrations and Alb/Glo ratio) wereavailable only for the year 2000.

Statistical Analyses

Statistical analyses were performed using the SAS 8.1 statisticalpackage (SAS Institute 2000). In order to normalize the dis-tribution of explanatory variables, all traits except hematocritand mass were ln transformed. The effect of each conditionindex on adults’ survival probability was tested with logisticregression (CATMOD procedure). Main effects of sex, habitat,year (where data for two years were available), and health stateindicator were included as explanatory factors in all models.Interactions between health state indices and categorical factorswere included in final models only if significant. To control forstructural size when analyzing the effect of adult body mass onthe recapture probability, the cubed tarsus length was includedas a covariate in the model (Garcıa-Berthou 2001). The fit ofeach model to the data was checked with the likelihood ratio

test. In all cases, the P value was greater than 0.05, suggestinga reasonable fit of the model. For nestlings’ body mass andtarsus length, the mean values for the brood were used inanalyses. Only data from first clutches were included, and dep-redated or deserted nests were omitted from all analyses. Theeffect of having a second brood on parental survival was cal-culated for females only, because too few males were caughtduring the second breeding attempt.

Results

The average return rate of the breeders in the subsequent yearwas 22.8% in females and 21.7% in males. There were nosignificant differences in local survival of adults either betweenhabitats (x2 test; females: , ; males:2x ! 0.01 P p 0.971, 183

, ) or between sexes ( ,2 2x p 1.34 P p 0.25 x p 0.05 P p1, 138 1, 321

). The effects of the studied condition indices on the local0.82survival of great tits are presented in Table 1, and the meanvalues of the indices by sexes and survival status are presentedin Table 2. Great tits with higher Alb/Glo ratios during breedinghad enhanced survival probabilities (Fig. 1; Tables 1, 2). Thisdifference was caused by lower globulin concentration in sur-viving individuals, while the plasma albumin content showedno difference in this respect (Tables 1, 2). The surviving in-dividuals also had significantly lower H/L ratios than those notreturning in the next year (Tables 1, 2). Although the maineffect of lymphocyte hemoconcentration did not affect localsurvival probability, a significant interactiontrait # sexemerged (Fig. 2; Tables 1, 2). In males, the surviving birds hadhigher lymphocyte concentrations than nonsurvivors (lym-phocytes: , ; habitat:Wald p 5.14 P p 0.023 Wald p1, 115 1, 115

, ; year: , ), while in0.10 P p 0.748 Wald p 0.89 P p 0.3471, 115

females, lymphocyte concentration did not affect local survival(lymphocytes: , ; habitat:Wald p 0.69 P p 0.4061, 150

, ; year: , ).Wald p 0.01 P p 0.927 Wald ! 0.01 P p 0.9831, 150 1, 150

All the above results were unchanged when we omitted insig-nificant categorical factors from the models. They also remainedwhen body mass was entered into the models as a covariate.However, the effect of body mass itself remained nonsignificantin all models. Other studied parameters (hematocrit, residualbody mass, and triglyceride and heterophile concentrations)had no effect on adult survival (Tables 1, 2). Also, all otherinteractions between studied condition indices and categoricalfactors (sex, habitat, year) were not significant and thereforewere removed from final models. We did not find any asso-ciations with adult survival in the traits reflecting parental re-productive success and/or effort: number of nestlings, fledgingsuccess (percentage of hatched young that reached 15 d of age),nestling body mass and tarsus length, and occurrence of secondbrood (all ).P 1 0.10




Table 1: Effects of different condition indices on local survival of adult great tits

Index, Measure Habitat Sex Year Trait Trait # Sex Likelihood Ratio

Albumin/globulin ratio:Walddf 3.101, 78 2.091, 78 7.951, 78 82.2677

P .078 .148 .005 .320Albumin:

Walddf 1.331, 78 .081, 78 .271, 78 87.0171

P .248 .777 .601 .095Globulin:

Walddf 2.521, 78 .731, 78 6.331, 78 81.1467

P .112 .393 .012 .115Hematocrit:

Walddf .731, 295 .061, 295 2.311, 295 1.001, 295 309.57292

P .395 .802 .129 .317 .230Triglycerides:

Walddf 1.471, 78 .191, 78 .371, 78 86.9272

P .225 .664 .543 .111Body mass:

Walddf .171, 314 .121, 314 1.551, 314 1.281, 314 322.91299

P .682 .733 .213 .258 .164Heterophile/lymphocyte ratio:

Walddf .191, 268 .761, 268 .061, 268 4.981, 268 259.41247

P .664 .384 .804 .026 .281Heterophiles:

Walddf .291, 268 .571, 268 .571, 268 1.191, 268 284.83261

P .588 .449 .451 .275 .149Lymphocytes:

Walddf .091, 267 6.841, 267 .371, 267 1.611, 267 6.751, 267 278.09265

P .761 .009 .542 .205 .009 .278

Note. Only final models are shown, and interaction terms are included only if significant. For albumin/globulin ratio and albumin,

globulin, and triglyceride concentrations, data were available for only the year 2000. For body mass, the cubed tarsus length was included

in the model to control for structural size ( , ). Probability of a likelihood ratio greater than 0.05 indicates thatWald p 0.03 P p 0.8711,314

the model fits the data reasonably well.

Discussion

We found that the survival of adult great tits (both males andfemales) was related positively to their plasma Alb/Glo ratioand negatively to their globulin concentration and H/L ratio.Low values of Alb/Glo ratio are known to reflect various kindsof infections and/or symptoms of malnutrition (Kawai 1973;Coles 1997). An increase in plasma globulin concentration isexpected after injury or pathogenic challenge and indicates theactivation of the humoral immune system (Kawai 1973; Otsand Horak 1998; Guemuessoy et al. 2004). H/L ratio is a stressindicator that is known to increase in the presence of variousstressors, such as infectious diseases or starvation (Gross andSiegel 1983). In great tits, it has also been shown that parentswho make intense reproductive effort have high H/L values(Ots and Horak 1996; Horak et al. 1998b). Thus, our resultsindicate that nonsurviving individuals may experience higherthan average pathogenic challenge and are more stressed at the

end of the breeding session, compared to surviving birds. Un-fortunately, we have no data about infection rates withpathogens.

Local survival of male great tits was also positively relatedto lymphocyte concentration, while the value of this trait didnot contribute significantly to survival of females. Lymphocytenumbers in the peripheral blood of birds are known to decreasein response to different stressors, such as food scarcity (e.g.,Totzke et al. 1999), and after intense physical activity. For ex-ample, Horak et al. (1998b) found that experimental enlargingof brood size in great tits resulted in lowered levels of circulatinglymphocytes in adults. Although we do not know exactly whatmechanisms are behind the observed sex difference in the re-lationship between lymphocyte concentration and survival, theanswer probably lies in differing endocrine and immune systeminteractions in males and females (Folstad and Karter 1992;Klein 2000; Barbosa and Moreno 2002). Male birds have gen-




Table 2: Condition indices of adult great tits in relation to sex and local survival

Index, Measure

Males Females

Survivors Nonsurvivors Survivors Nonsurvivors

Albumin/globulin ratio:Mean 2.23 1.77 1.69 1.56SD .56 .38 .27 .27N 9 27 14 32

Albumin (g/L):Mean 17.8 19.5 18.8 18.2SD 1.67 2.86 3.88 2.94N 9 27 14 32

Globulin (g/L):Mean 8.4 11.6 11.4 12.2SD 1.83 3.23 3.31 3.71N 9 27 14 32

Hematocrit:Mean .47 .47 .51 .50SD .03 .04 .04 .03N 29 103 38 130

Triglycerides (mg/dL):Mean 155.2 179.4 204.3 186.7SD 62.6 47.5 87.8 60.3N 9 27 14 32

Body mass (g):Mean 18.3 18.2 17.8 17.8SD .98 .83 .71 .80N 30 108 41 141

Heterophile/lymphocyte ratio:Mean .51 .67 .68 .78SD .29 .36 .49 .52N 25 94 37 117

Heterophiles:Mean 11.9 10.5 13.3 14.9SD 10.0 7.1 15.7 11.4N 25 94 37 117

Lymphocytes:Mean 23.2 17.4 20.1 21.1SD 11.7 11.2 14.2 13.0N 25 94 37 117

Note. Data are pooled over both habitats. Presented are untransformed means, standard deviations, and

sample sizes. Concentrations of heterophiles and lymphocytes are presented per 10,000 erythrocytes.

erally higher concentrations of testosterone during breeding,compared to females, and high concentrations of testosteronecan sometimes suppress immunity (Verhulst et al. 1999; Peters2000). Our result is partly supported by another study on greattits, in which infection with Haemoproteus blood parasite re-sulted in elevated lymphocyte concentrations only in males andnot in females (Ots and Horak 1998).

Results similar to those of this study were previously foundonly in a few studies. Gonzalez et al. (1999), in an experimental

study with captive house sparrows (Passer domesticus L.), foundthat plasma immunoglobulin concentration was negatively re-lated to survival. In female common eiders (Somateria molis-sima L.), it has been shown that the return rate among brood-abandoning females was positively related to lymphocyteconcentration in their blood (Hanssen et al. 2003). In contrastto our study, Horak and Ots (1998) did not find that adults’hematological parameters were related to local survival of greattits, although the adults in their study showed approximately




Figure 1. Relationship between local survival and albumin/globulinratio in the blood of adult great tits during breeding in 2000. Data arepresented as (boxes). Error bars denote 95% confidencemean � SElimits. Numbers in all figures denote sample sizes.

Figure 2. Relationship between local survival and lymphocyte concen-tration in the blood of adult great tits during breeding in the years2000–2001. Symbols as in Figure 1.

the same return rate (ca. 20%) and similar mean values of thephysiological parameters as they did in our study. Unfortu-nately, these authors did not consider the effect of sex in theiranalyses. Also, they conducted their study in habitats quitedifferent from those in our study (they worked in an urbanarea and mixed woods). Perhaps the inconsistencies betweendifferent studies are caused by some population or year dif-ferences, since great tit individuals, even in adjacent popula-tions, can differ considerably in body condition (Ots et al. 1998)and in reproductive tactics and success (Mand et al. 2005).

A positive association between aspects of immune functionand survival need not imply that immunological state per seinfluences fitness, because it is possible that both traits arecondition dependent and are affected by condition simulta-neously (Sheldon and Verhulst 1996). However, in our study,local survival of adults was not influenced by general bodycondition (residual body mass) or triglyceride and albuminconcentrations, the traits reflecting mainly the nutritional statusof an individual (Coles 1997; Jenni-Eiermann and Jenni 1997).Moreover, inclusion of body mass in models simultaneouslywith hematological variables did not alter the results. Therefore,despite the correlational nature of our study, we believe thatthe observed relationships indicate a causal link between someaspects of immunological condition during breeding and futuresurvival. Our results are indirectly supported by the study ofHorak et al. (2002) with captive greenfinches (Carduelis chlorisL.). In this species, both heterophile and lymphocyte concen-trations and H/L ratio showed high individual consistency overa 4-mo period, thus probably reflecting interindividual differ-ences in prevalence of chronic infections (Horak et al. 2002).

Various aspects of an individual’s health state and immunesystem can be affected by food quantity and quality (Lochmiller

et al. 1993; Totzke et al. 1999). Our study was carried out in twocontrasting habitats, deciduous and coniferous forests. As pre-viously shown, both food and natural calcium sources are sig-nificantly more abundant in the deciduous than in the coniferoushabitat of our study area (Tilgar et al. 2002; Mand et al. 2005).Since physiological trade-offs in animals often appear only un-der food stress (Stearns 1992; Moret and Schmid-Hempel 2000;Hanssen et al. 2004), one may assume that the possible effectof parental condition on their future survival would be stronger,or would be revealed only, in the poorer habitat. However, wedid not find any significant habitat effect on great tits’ survival.Also, the relationship between condition indices and local sur-vival did not differ between contrasting habitats. However, wehave previously found that great tits breeding in coniferoushabitat tend to be in better condition and immunologically lesschallenged than those breeding in deciduous habitat (Kilgas etal., forthcoming) and also that the number of nestlings andnestling body mass are relatively higher in coniferous habitat(Mand et al. 2005). It is possible that the potential advantageof food richness in the deciduous forest was outbalanced byhigher population density in this habitat, compared to the co-niferous forest (Mand et al. 2005).

As in some experimental studies with different tit species(Blondel et al. 1998; see also Golet et al. 1998 for review), wedid not detect any significant association between adults’ sur-vival and parental breeding success or breeding effort (but seeNur 1984; Stjernman et al. 2004). Therefore, it is likely thatthe higher subsequent mortality of the birds with lower healthstate during breeding is not caused by a trade-off betweenreproduction and survival (see “Introduction”), and instead,other explanations are needed. For example, it has been shownthat great tits, as well as many other bird species, experienceincreased mortality during winter (reviewed in Newton 1998).




Thus, it may be that the lower health state of an adult bird,which is detectable already during the breeding period, provesnot to be really fateful until winter. However, because of thecorrelational nature of the present study, we cannot entirelyrule out that a causal link still exists between reproduction andsurvival of great tits.

In conclusion, the results of this study indicate that hema-tological traits reflecting the immunological state and stress ofadult great tits when caring for young are related to their sur-vival probability. This effect seems to be more pronounced inmales. This is one of the few studies that has detected a rela-tionship between hematological health state indices and survivalof adult birds in the wild.

Acknowledgments

We thank Marko Magi, Agu Leivits, and Ulle Mustonen fortheir assistance in the field. We also thank Peeter Horak andtwo anonymous reviewers for providing constructive criticismon the manuscript and Indrek Ots for plasma protein analyses.This study was supported by the Estonian Science Foundationgrants 5489 and 5716 to R.M. and V.T.

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Hematological Health State Indices Predict Local Survival in a Small Passerine Bird, the Great Tit (Parus major)