Wednesday, June 19, 2019

Bluebirds and Swallows React to Temperature and Rainfall Variations

How will future generations of nesting Tree Swallows be affected by warmer or wetter springs?

Bluebirds and Tree Swallows among the most likely birds to utilize nest boxes in the United States and southern Canada. In fact, birders have been instrumental in providing needed artificial nesting cavities in the form of nest boxes, which have enabled these birds to increase their populations and expand their nesting range in many areas of the continent. In America’s heartland, many farmers enlist birds to help suppress agricultural pests by installing nest boxes around their farms.

Several recent research studies suggest that climate change is predictably causing birds to start nesting earlier in the spring as temperatures warm, which may throw insectivorous birds out of sync with hatches of their insect prey. In an effort to add information one way or the other to this premise, researchers tested which climate variables most correlated with the onset of egg laying by Eastern Bluebirds and Tree Swallows in Ohio.

In a new publication in the American Midland Naturalist, Jesse Sockman and Jason Courter of Malone University analyzed 4,417 Ohio nest records submitted from 2000 to 2015 to NestWatch, a program spearheaded by the Cornell Lab of Ornithology.

It should be noted that Eastern Bluebirds in Ohio, as in most parts of their range, re-nest after their initial nesting attempt, so only their first nesting attempt of the year was used in this study. Tree Swallows only nest once per year, therefore all Tree Swallow nesting data could be utilized.

Sunny Springs or Rain Delays

As expected, both species started nesting earlier in the southern portion of the state, but latitude alone did not explain the variation for when birds started to lay eggs in Ohio. The study concluded that Eastern Bluebird first-egg-dates were earlier during warmer springs, but later during wetter springs. Tree Swallows, on the other hand, did not seem to be impacted by spring temperature as much; but, like bluebirds, they initiated egg laying later during wetter springs.

The timing of egg laying and the nesting period for both species, fortunately, overlapped with insect life cycles in ways that would help the birds control the effects of insect pests. Birds want to be nesting when the most prey is available to feed their nestlings and fledglings, and it appears that they are still nesting within this time range in Ohio.

Tree Swallow nesting activity seemed to be more closely tracking a measure of accumulated heat known as “growing degree days,” whereas Eastern Bluebird egg laying did not coincide with this measure of cumulative warmth. Instead of using the calendar day, “growing degree days” is a temperature-based measure familiar to most farmers because it provides a measure of seasonal crop progress and a guide for timing farming activities such as pesticide and herbicide applications.

Insect pests such as eastern tent caterpillars, gypsy moths, inkberry leafminers, and lilac borers, among others, are significant agricultural pests that impact farmers economically, and the emergence of these insects and their larvae coincides well with the growing degree day measurement. When birds can act as biological controls that suppress crop pests, farmers may benefit from increased avian populations on their property.

However, climate models are predicting warmer annual temperatures and wetter springs in Ohio during the next century, further complicating the interpretation of these results. For example, could temperature-related advances be offset by precipitation-related delays in nesting? Or are Eastern Bluebirds and Tree Swallows actually tracking some unmeasured variable such as insect abundance?

One of the study’s authors, Dr. Jason Courter, explained that it’s very difficult to predict how a changing climate will affect birds. He did, however, express his gratitude to NestWatch participants for their help in this study, saying “We are grateful for the countless volunteers who have faithfully submitted nesting observations over these 16 years through NestWatch and made a project of this magnitude possible.” Indeed, thank you NestWatchers for providing information about thousands of nesting attempts by birds each year to support studies like this!

For more information about NestWatch and the original article authored by Robyn Bailey, NestWatch Project Leader at https://nestwatch.org/connect/blog/buckeye-birds-track-temperature-and-precipitation/