1. Foraging and Nesting Habits of Woodpeckers in a Subalpine Forest
Alex Goldsmith
Winter Ecology – Spring 2016
CU Mountain Research Station
Alex Goldsmith
Winter Ecology – Spring 2016
CU Mountain Research Station
Size: 19.6 mb
Keywords: woodpeckers, forest, foraging, nesting, cavities, beetles, insects, bark insects, snags, DBH, height, trees, birds
Photo of male Downy Woodpecker sourced from:

2. Background Woodpeckers are keystone species
Create homes
Insect control
Relationship between woodpeckers and bark insects
Snag selection
Snags with large DBH and height preferred
Woodpeckers are one of only a few species capable of excavating cavities in snags. Cavities provide habitats for many species of cavity-nesting birds which rely on woodpeckers and other excavators to create their homes. Woodpeckers are unique amongst cavity nesters in that they get to choose where they create cavities (Conner et al; Ripper et al.). Therefore important to understand and preserve woodpecker habitat.
Bark insects are not only food for woodpeckers but also create snags, allowing woodpeckers to excavate cavities. Woodpecker populations have been documented to increase with insect outbreaks (Koplin 1969). Insect populations have also been documented to decrease with woodpecker activity (Yeager 1955).
Multiple studies from the pacific northwest showed woodpeckers were found to prefer snags with large DBH and height (Ripper et al. 2007; Ohmann et al. 1994)

3. The Question
Are certain criteria influencing snags woodpeckers utilize for cavity construction and foraging?
Insect presence
DBH
Height
Expected results
Woodpeckers are most active amongst snags with insect presence.
Woodpecker activity will be influenced by snag height and DBH.
I expected more activity amongst snags with insect presence because insects are woodpeckers’ primary food source
I expected DBH to be important because at least a certain diameter is necessary to build a cavity
I expected height to be important mostly as it is closely linked to DBH

4. Study area Niwot Ridge Biosphere Reserve Species involved
3,140 m elevation (10,300’)
Species involved
Downy Woodpecker (Picoides pubescens)
Hairy Woodpecker (Picoides villosus)
Subalpine Fir (Abies lasiocarpa)
Engelmann Spruce (Picea engelmannii)
Subalpine forest in winter time. A harsh environment.
Forest dominated by Subalpine Fir. Some Engelmann Spruce as well. No other tree species observed.
Photo: Alex Goldsmith

5. Methods 50m x 50m plot 5 transects Snags recorded and measured
DBH
Height
Woodpecker presence and activity
Insect presence
DBH and Height grouped into categories for Chi2
Elected my study area based off observed physical presence of Hairy woodpecker
Additional measurements of tree species, whether snag is broken or whole, and needle presence also recorded but not utilized in analysis.
Results may be skewed and/or inconclusive due to small sample size. I believe that with a larger sample area or perhaps additional subalpine plots trends may be found.
Categories were selected by what I though may be critical points in woodpecker snag selection.
Photos: Alex Goldsmith

6. Results 92% of snags with woodpecker activity had insect boreholes
12 with woodpecker foraging
2 with woodpecker cavities
24 with bark insects
92% of snags with woodpecker activity had insect boreholes
Worth noting that the 2 snags that had woodpecker cavities had multiple cavities
24/32 = 75%
Snags without recorded insect presence were all very old and lacking all bark. It is most likely that these snags were home to bark insects at one point but evidence was gone by the time I arrived.
1 live tree with bark scraping found. No presence of insects noticed at base but it is very possible that tree was in early stage of infestation. Bark scraping activity took place well above eye level. Was impossible to see if insects were present at location of bark scraping.
Photo: Alex Goldsmith

7. Hypotheses Does snag DBH affect woodpecker cavity presence?
H0: Snag DBH does not affect woodpecker cavity presence
Ha: Snag DBH does affect woodpecker cavity presence
Does snag height affect woodpecker cavity presence?
H0: Snag height does not affect woodpecker cavity presence
Ha: Snag height does affect woodpecker cavity presence
Does snag DBH affect woodpecker foraging behavior?
H0: Snag DBH does not affect woodpecker foraging behavior
Ha: Snag DBH does affect woodpecker foraging behavior
Does snag height affect woodpecker foraging behavior?

8. P=0.44
P=0.53
Results of Chi-square test
No significant results found. Null hypothesis of no significance accepted in all cases.

9. Yates P-value used for foraging data because when Chi-squared was run 20% of expected frequencies were less than 5.
After adjusting, results for DBH are not significant. Null hypothesis accepted.
Foraging p-value inaccurate due to low expected frequencies. Needs to be corrected with more sampling.

10. Discussion
Height and DBH not significant factors in snags woodpeckers use for foraging and cavities
Lack of differentiation between Downy and Hairy woodpecker
Patterns suggest need for more samples
Most foraging in tall snags
Most foraging in snags with large DBH
Most cavities in snags with large DBH
Overall forest composition more important than individual snag?
Downy and Hairy Woodpeckers have different nesting and foraging habits. Conner et al. (1977) found Hairy Woodpeckers to have much more diverse nesting habits than Downy Woodpeckers.
By looking at the two species as one group I may have created a scenario that neglects their differences. This could have skewed my data by creating conflicting scenarios.
By simply looking at the patterns in my data it lines up with that found in previous studies with the one exception being cavities and snag height.
In woodpecker foraging habits it is important to note that before the p-value was adjusted it was significant. This represents that with the same pattern and a larger sample size my results would have been significant.
Ripper et al. (2007) suggest that by looking only at snag composition one is obscuring the importance of overall forest composition in the creation of woodpecker habitats. Factors such as stand age and patch size may play a more important role.

11. Summary Insect presence most important factor
Snag height and DBH not significant indicators of woodpecker cavities or foraging
Larger sample
Forest composition
Woodpeckers important as keystone species
One snag without boreholes possibly still had insect presence higher on tree.
Woody Woodpecker Photo:

12. Acknowledgements
First and foremost, Tim Kittel for his continued support and for equipping me with the knowledge and resources to pull this off
Arvind Punjabi for helping me identify different signs of woodpeckers
Derek Sweeney as a constant source of humor and his encyclopedic knowledge of everything ecology
All of YOU for being awesome and making this class worth every moment!

13. Works Cited
Conner, R. and C. Adkisson Principal Component Analysis of Woodpecker Nesting Habitat. The Wilson Bulletin. 89: URL:
Koplin, J The Numerical Response of Woodpeckers to Insect Prey in a Subalpine Forest in Colorado. The Condor 71: 436–438. DOI: /
Ohmann, J., McComb, W., and A. Zumrawi Snag Abundance for Primary Cavity-Nesting Birds on Nonfederal Forest Lands in Oregon and Washington. Wildlife Society Bulletin. 22: URL:
Ripper, D., Bednarz, J., and D. Varland Lanscape Use by Hairy Woodpeckers in Managed Forests of Northwestern Washington. Journal of Wildlife Management. 71: DOI: /
Yeager, L Two Woodpecker Populations in Relation to Environmental Change. The Condor 57: 148–153. DOI: /