If You Build It Will They Come? Associations Between Birds and Vegetation Structure During

Ten Years After ThinningSveta Yegorova, Drs. Matt Betts , Joan Hagar* and Klaus Puettmann

Department of Forest Ecosystems and Society, Oregon State University*U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, OR

Corvallis, Oregon 97330 USAUSGS, Corvallis, Oregon

Background

Methods

Results

Discussion

Hypotheses

I. Vegetation effects varied among years and were inconsistent in size

II. In some years vegetation models performed well (e.g., MacGillvray’s Warbler and Golden-crowned Kinglet, see figures 1 and 2 below)

III. Changing population sizes may explain varying model fits

Figure 1. MacGillivray’s Warbler: Occupancy by Shrub Cover

Figure 2. Golden-crowned Kinglet: Occupancy by Conifer Density

Quantifying Variability of Vegetation/Treatment Effect on OccurrenceWe used the ratio of variable estimate to its

standard error (GOF=Estimate/SE(Estimate)) to quantify the variability of the size of the effect. GOF appears to be highly variable even for species associated with vegetation structures (as in Fig. 1 and 2) in some years. See figures 3-6 for illustration.

• Forest thinning encourages understory and overstory development in young Douglas-fir forests

• Effects of thinning on birds have been shown to vary by species and with time since thinning

• These effects are assumed to be mediated by the residual vegetation structure but this assumption is rarely tested

• Alternatively, habitat selection may be driven by other (non-vegetation) factors, including previous experience, social information, and density of conspecifics

• Therefore disturbance (thinning) may cause a lag in bird-vegetation association

• Large population size may result in poor fit between bird occurrence and vegetation structure due to population spillover into low quality habitat

I. If vegetation drives bird occurrence, thena. There will be strong associations between bird occurrence and vegetation structure immediately after thinningb. These associations will be consistent over time

II. If other factors are contributing to habitat selection there may be high year-to-year variability in bird-vegetation associations

• 30-50 yr old Douglas-fir stands• Mid-elevation (500-900ft) forests on west slopes of the central Cascade Mountains in Oregon

• Detailed understory and overstory vegetation measurements taken four times since thinning

• Breeding season point count surveys conducted six timesAnalysis

We selected best performing model, using Akaike Information Criterion (AIC), from a priori set pool of models that reflects the hypotheses stated above.

Bird-vegetation associations appear to be highly variable in this long-term (10 years) study. Single-season bird surveys are unlikely to properly reflect an underlying relationship between bird occurrence and vegetation structure. Our results suggest that population size, for abundant species, and possibly social factors interact with vegetation structure to produce observed bird occurrence patterns.

Treatment Density (trees per hectare)

Heavy 125Light 250-300Light with Gaps 250-300 with 0.2 ha gaps

Control 650

Occurrence of eight species was modeled as a function of vegetation structure or treatment (fixed effects) and block and stand (random effects) using mixed-effects logistic regression.

Analysis continued

MacGillivray’s Warbler

Swainson’s Thrush

Golden-crowned Kinglet

Hammond’s Flycatcher

150 250 350

12

34

56

GOF for Abundant Species

Abundance

GO

F

50 100 150

0.0

1.0

2.0

3.0

GOF Less Abundant Species

Abundance

GO

F

Cumulative GOF Graphs

Figure 6

1998 2000 2002 2004 2006

1.0

1.5

2.0

2.5

3.0

3.5

Treatment GOF vs Year

Year

GO

F

30 40 50 60

1.0

1.5

2.0

2.5

3.0

3.5

Treatment GOF vs Abundance

Abundance

GO

F

Hammond’s Flycatcher

Figure 5

1998 2002 2006

0.5

1.5

Shrub GOF by Year

Year

GO

F

50 70 90 110

0.5

1.5

Shrub GOF vs Abundance

Abundance

GO

F

Swainson’s Thrush

Figure 4

1998 2002 2006

0.0

0.5

1.0

1.5

Shrub GOF by Year

Year

GO

F

20 40 60 80 100

0.0

0.5

1.0

1.5

Shrub GOF vs Abundance

Abundance

GO

F

MacGillvray’s Warbler

Figure 3

When pooled across species, GOF is associated negatively with abundance for relatively abundant species (Fig 6). GOF did not correlate with time since thinning (data not shown).

Hammond’s Flycatcher

Golden-crowned Kinglet

0 500 1000 2000

0.0

0.2

0.4

0.6

0.8

1.0

1997

Occupancy

0 500 1000 2000

0.0

0.2

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1.0

Occupancy vs Shrub Cover

Shrub Cover

0 500 1000 2000

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0.4

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1.0

1999

0 500 1000 2000

0.0

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2001

Occupancy

0 500 1000 2000

0.0

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2006

Shrub Cover

psi

0 500 1000 2000

0.0

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2007

Figure 1

0 40 80 120

0.0

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1997

Occupancy

0 40 80 120

0.0

0.4

0.8

1998

Coniferous Stem Count

0 20 60 100

0.0

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1999

0 20 60 100

0.0

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2001

Occupancy

0 20 60 100

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2006

Coniferous Stem Count

0 20 60 100

0.0

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0.8

2007

Figure 2