Home for the Holidays: Nesting Tree Preferences of Fox Squirrels at Grinnell College
Taylor Durak & Rowan Hutchinson
Grinnell College is known for its large fox squirrel population, with squirrels becoming Grinnell’s unofficial mascot. Grinnell students run into squirrels everywhere they walk on campus, making us wonder how large the squirrel population really is. In 2009, 2021, and 2023, Grinnell ecology students conducted surveys in the efforts to investigate the number of squirrels on our college campus. To assess the squirrel population, students collected data on squirrels’ leaf nests within the trees on campus. The data that was recorded included the location of the tree, number of nests within the tree, and the type of tree. The 2021 researchers focused on comparing the number of squirrel nests present in 2009 and 2021, particularly to examine the effects of the 2021 derecho that damaged and destroyed many trees on campus. In 2023, we performed this data collection and wanted to focus more in depth on cross examining the squirrels’ preferred location and tree type on campus from 2021 to 2023. To understand a squirrel's preferred home for winter, we turned to previous studies to understand their favored tree type, size, and environment when it comes to choosing where to build their leaf nests. The article “Nest Characteristics of Sympatric Populations of Fox and Gray Squirrels” examines squirrel preferences through their research of nesting behaviors and interactions of co-existing (sympatric) fox squirrel and gray squirrel populations. Their previously found favorable locations helped us predict where we may find squirrel’s homes for the holidays on campus this year.
The previous research on nest characteristics of fox and gray squirrels was performed by John W. Edwards and David C. Guynn, Jr. at the Piedmont National Wildlife Refuge. Their goal was to look at the characteristics of cavity and leaf nests inhabited by gray squirrels and fox squirrels across seasons to determine if these squirrels engage in interspecific competition. It was the first study ever to focus on sympatric populations of fox and gray squirrels. To study this co-existing population, Edwards and Guynn, Jr. used telemetry to find the cavity and leaf nests. They recorded what species of squirrel occupied the nests, as well as the current season, tree species, tree height, nest height, and position of the nest within the tree. Therefore, this data showed us what fox squirrels see as prime real estate for their winter homes so we can compare these trends to our own findings.
Edwards and Guynn, Jr. found that gray squirrels chose cavities as a home more often than fox squirrels, as seen in Figure 1. Although cavity nests can trap in more heat than leaf nests during the winter, it seemed that fox squirrel populations remained successful in the colder months with leaf nests as a home. Yet, it makes us wonder if fox squirrels may not use cavity nests because of their popularity with gray squirrels in sympatric populations. Fortunately, on Grinnell College’s campus, there are only fox squirrels, meaning that we can evaluate the fox squirrel population solely by visually examining trees for leaf nests, and can understand their popularity with fox squirrels independent of other squirrel species.
As for fox squirrel preference, fox squirrel leaf nests were found more commonly in relatively taller trees with wide diameters of about 20-60cm. The researchers also found that fox squirrels preferred trees without vines and occupied pines more than expected with other trees such as hickory and elm less than expected. We could therefore expect to find nests in trees commonly occupied by squirrels from this study, such as taller trees with wider diameters. This research also found that gray squirrels preferred oak trees; knowing Grinnell’s campus has many oaks, this leads us to wonder if fox squirrels will make their homes in oaks when gray squirrels are not around.
In our research, we evaluated the fox squirrel population by locating and counting the leaf nests on Grinnell College’s campus in fall of 2023. Overall, we found that there were less nests present in 2023 than in 2021, with the most present in 2009. What could have caused this decrease in squirrel nest abundance? Given this article explaining the difference between cavities and nests, it could be that fox squirrels have adapted to the cold winters by using more cavities instead of building leaf nests. Although this research found that fox squirrels were less likely to use cavity nests, we did not survey cavity nests and instead relied only on visually spotting leaf nests in every tree on campus, meaning we cannot know how many cavity nests are used by squirrels at Grinnell.
Additionally, Edwards and Guynn, Jr.’s performed their research in the 1990s in a pine forest in rural Georgia, meaning temperatures and plant communities differ from those in Iowa. Unlike these researchers, we did not find any squirrels’ nests within pine trees and found that they were more common in elms in 2021 and 2023 (Table 1). Fox squirrels, however, did commonly make their homes in oak trees in both 2021 and 2023 at Grinnell, meaning they may prefer oak trees when not interacting with other squirrel species (Table 1). Overall, while Grinnell prides itself on being a home to fox squirrels, data shows that these fuzzy friends have been making their homes less and less on the Grinnell campus. How can we help our squirrel population survive and thrive? That is a question for further research, but we have some ideas. Knowing what trees squirrels like, such as oaks, basswoods, and chestnuts, we can figure out what trees we can plant to both replace damaged derecho trees and encourage squirrels to make their homes at Grinnell. We hope future ecology students will continue to study the squirrels on campus, and that squirrels will choose Grinnell to make their homes more in future years.
Table 1. Number of trees on the Grinnell College Campus with squirrels’ nests by tree type in 2021 and 2023.
|
Tree Type |
Number Inhabited (2021) |
Number Inhabited (2023) |
|
Alder |
1 |
1 |
|
Basswood |
4 |
3 |
|
Birch |
1 |
2 |
|
Cherry |
0 |
1 |
|
Chestnut |
1 |
3 |
|
Cottonwood |
2 |
3 |
|
Cypress |
1 |
0 |
|
Crabapple |
0 |
1 |
|
Elm |
6 |
2 |
|
Gingko |
1 |
1 |
|
Hemlock |
1 |
0 |
|
Hickory |
0 |
2 |
|
Honey locust |
3 |
0 |
|
Kentucky coffeetree |
0 |
1 |
|
Maple |
4 |
0 |
|
Oak |
13 |
12 |
|
Pine |
2 |
0 |
|
Poplar |
1 |
0 |
|
Sycamore |
4 |
3 |
|
Tulip |
0 |
1 |
|
Walnut |
0 |
2 |
|
Total |
45 |
38 |
Work Cited
Edwards,
J. W., & Guynn, D. C. (1995). Nest characteristics of sympatric populations
of fox and
gray squirrels. The Journal of Wildlife Management, 59(1),
103–110. https://doi.org/10.2307/3809122.
