Home. Holidays. Squirrels.

 

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.

 

Hot Takes on Cool Nests

 

Hot Takes on Cool Nests: How Paper Wasps Thermoregulate Their Nests

Ainsley Yuknis, Rachel Bottum, Henry Liu; BIO 368

Polistes is the most common and widespread genus of paper wasps. They are known for creating open, comb-shaped nests, often on human structures. They are a eusocial species, with a foundress laying all eggs within a brood while other members help care for the larvae (Jeanne & Suryanarayanan., 2011). These social adaptations give them an advantage when it comes to thermoregulation: paper wasps can work together to stay warm. In most social insects, thermoregulation in nests is achieved through a mix of metabolic and behavior strategies. This usually means crowding together and vibrating their wings to heat up their nests. However, paper wasps create open nests, meaning metabolic thermoregulation is not as  efficient, especially in cooler climates, as most heat generated is lost to the air. For polistes in cold areas, selection plays a larger role in nest location than with species who build nests capable of internal thermoregulation. In addition to nest site choice, ‘immediate’ behavioral means like cooling by fanning and distribution of water droplets on the nest for evaporative cooling allow control of the nest temperature (Stabentheiner et al., 2022).

“Effect of climate on strategies of nest and body temperature regulation in paper wasps” explores our questions about wasp habitat selection. This study aimed to determine how thermoregulatory and site-selection behavior differentiated between two species of Polistinae in different climates. Two species were compared; one from the Mediterranean (P. gallicus) and one from the Alps (P. biglumis). The two species differed in how they maintained suitable brood temperature.  This study analyzed both substrate temperature (material on which the nest was built) and the thermal environment within nests (brood and body temperature) over the course of the day. Since the Alps species resides in conditions resembling that of Iowa, we include more detail on P. Biglumis than P, Gallicus results to create the hypotheses for our experiment.

For Polistes Biglumis, nest and wasp temperature were low before sunrise (mean ~ 15 C).  The substrate that the nest was built on was higher (~ 20C).  The nest temperatures rose very quickly after sunrise.  After 13 minutes of radiation the nest temperature went from ~17 C to ~25 C.  After one hour, the temperature had risen 20 C from before sunrise.  At peak heat, the wasps fanned to cool the nest down, they also gathered water and used evaporative cooling.  At ~13:00, when the nest was shaded, the nest temperature decreased (to ~21 C after dusk).  The nest did not cool down once shaded nearly as quickly as it warmed up once exposed to sunshine (Fig 1 a,b).  Overally, the substrate was always approximately 5 degrees warmer than the nest, which slowed the nest from cooling down as the air temperature cooled down. Strategies of maintaining appropriate brood temperature differed between the two species.  In cool climates (P. Biglumis), wasps increased the brood temperature simply by choosing nest locations that were facing South East (Figure 2).  On the other hand, P. gallicus nests were found in locations with little direct sunshine.

Figure 1: Daily temperature changes of nests and wasps of P. biglumis (c,d) T_thorax =mean thorax surface temperature of up to five adult individuals per time of measurement; gray ribbon: total range of nest temperatures (T_max:T_min) with mean; T_substrate=temperature beside the nest; T_anest=ambient air temperature directly at the nest. T_a=ambient air temperature in shade 1–3 m away from nest; Radiation=global radiation hitting the nest; black bars=fanning events at the time of thermographic measurements.

 

Figure 2: Horizontal and vertical nest orientation of P. biglumis in Alpine climate, and of Polistes gallicus in Mediterranean climate. Mean values and Medians (thin bars) calculated according to the rules of circular statistics.

For our experiment, we did not look at individual species because we only analyzed one environment; Grinnell College. We hypothesized that paper wasps in Iowa would value warm and sunny areas (South East facing), much like P. biglumis. Iowa also has the factor of persistent winds due to the flat terrain. This may factor into paper wasps’ habitat selection, so we ranked the recession levels of windows and eaves hypothesizing that they would seek places protected from these winds. We took 5.5 meter transects of buildings on campus, noting whether nests were present, the floor they were on, the substrate temperature, and the building face.

            Since we analyzed campus buildings, the cardinal directions correspond exactly to the faces of buildings. This meant we did not have to factor in specific degrees in our measurements, simply going with the four categories of north, east, south, and west. We found that temperature was the largest factor in habitat selection. As hypothesized, wasps preferred warmer areas which also tended to be on the south and east faces of buildings. Wasps also preferred more recessed areas. This could both be due to the greater amount of space to build nests and protection from winds.

References

Jeanne, R. L., & Suryanarayanan, S. (2011). A new model for caste development in social wasps. Communicative & Integrative Biology, 4(4), 373-377.

Stabentheiner, A., Nagy, J. M., Kovac, H., Käfer, H., Petrocelli, I., & Turillazzi, S. (2022). Effect of climate on strategies of nest and body temperature regulation in paper wasps, Polistes biglumis and Polistes gallicus. Scientific Reports, 12(1), 3372.