How Birds Share Resources With Other Birds

Bird Foraging Activity in Ashuelot Park:
Determining forage similarities between bird species based on tree selection.
September, 2014
Jonathan Nathaniel Hayes
Ashuelot Park, Keene, New Hampshire

Foraging selectivity amongst insect syntopic gleaning birds has been an area of great interest to researchers for decades. It is deduced that niche foraging selection on various tree species- and on specific regions of the trees, reduces competition and allows for greater species diversification amongst the foraging birds of a given community. Some birds will favor certain types of trees. This preference is further divided by species in which portion of the tree the bird forages on. Trees in such studies are often divided up into foraging sectors. Some researchers will divide a tree into bole, inner branch and outer branch sectors, while other researchers divide the tree into vertical lower and canopy zones. Each of these has their merit.
On Sunday, September 6th, fellow Antioch University colleagues and I visited Ashuelot Park in Keene, New Hampshire. There we surveyed the bird species observed and what trees they foraged on. We also noted which portion of the tree each species favored in their foraging. This exercise was done to determine whether or not the birds of this park were specializing on foraging certain tree species, and certain zones of the trees, to reduce interspecies foraging competition.
In this report, I will focus on the 5 most reported birds in the park, and evaluate their foraging preferences on the 6 most commonly reported trees in the park to address hypothesis #2 posed by our class facilitator, Dr. Beth Kaplin, namely that various species do not differ in their foraging preferences. I will present a dendrogram similar to that used in other studies of this type (Airola, 1985) to visually display the results.

We were divided into groups, each with someone strong in bird identification and someone strong in tree identification. We noted all bird foraging activity and kept a tally of each tree species foraged on by each type of observed bird species. We also noted whether each foraging event took place on the bole (main trunk of the tree), the inner branch (the half of the branch closest to the tree) and the outer branch (the half of the branch furthers from the tree. While this data is extremely valuable, it is beyond the pale of this report and will therefore not be presented here. 
For the purposes of this report, I decided to limit my study to only the total of independent foraging events of the five most observed birds in the park. These five birds were the chickadee, the warbler, the nuthatch, the Gold Finch and the Downey Woodpecker. 
I broke down those bird species foraging event tallies by the six trees they most visited. These six most common trees in our study were the Red Maple, the White Pine, the Silver Maple, the Black Cherry, the Gray Birch and the Buckthorn. Although the buckthorn is not a proper tree- and has thus been excluded from other reports- I will include it here to show a further distinction between the foraging activity of the five birds under consideration.
I calculated the percentage of each bird’s foraging events per tree. I used these percentages to compare and contrast the tree foraging selection between the five birds. 
The following is a list of the each bird species’ independent foraging events broken down by tree species.
Chickadee: 122 observed individual foraging events
44.3% in White Pine
34.4% in Red Maple
12% in Silver Maple
8% in Black Cherry
Warbler: 56 observed individual foraging events
53.6% in Red Maple
28.5% in Black Cherry
8.9% in White Pince
8% in Bukthorn
Nuthatch: 86 observed individual foraging events
73.2% in White Pine
16.3% in Red Maple
5.8% in Black Cherry
1.2% in Silver Maple
Gold Finch: 40 observed individual foraging events
65% in Red Maple
12.5% in Black Cherry
12.5% in Buckthorn
10% in Gray Birch
Downey Woodpecker: 35 observed individual foraging events
85.7% in Red Maple
15% in White Pine

The ecological theory known as the competitive exclusion principle in resource and consumptions maintains that there must be a partitioning of resources in order for similar species to coexist. There are many variables that can effect the outcome of such a resource partitioning study. Sample size, the time of year, the time of day, the availability of the recourse, whether the composition of plant species and vegetation has been altered and migratory cycles are significant variables that can account for varied outcomes.
That being said, there are some strong inferences from the percentages given in this field report. It is obvious that some of these syntopic insectivorous birds are favoring some trees over others. The Nuthatch and Downey Woodpecker, for example, seem more focused on certain species, while the Goldfinch and Chickadee seem to be more diversified in the the tree species they forage on. 
We also can infer which where there is more similarity in foraging behavior and where there is the most opportunity for interspecific competition for resources.

Airola, A., and R.H. Barrett, 1985. Foraging and Habitat Relationships of Insect-Gleaning Birds in the Sierra Nevada Mixed Conifer Forest. Condor 87: 205-216.