By Robyn Bailey, NestWatch Project Leader
Birds have evolved to time their nesting season to coincide with the peak abundance of food in the environment, a strategy that is especially noticeable in highly seasonal environments like the northeastern United States. The food peak should ideally occur just as young are hatching and need lots of nutrients to grow up quickly. For birds that eat insects, one key nutrient the growing young need is long-chain omega-3 polyunsaturated fatty acids—usually referred to as “omega-3 fatty acids” for simplicity. Omega-3 fatty acids are essential nutrients and must be obtained from the diet for many species.
Aquatic insects have 4 to 34 times more omega-3 fatty acids than terrestrial insects. For this reason, aquatic insects (i.e., those whose larvae mature in the water) are nutritionally superior to terrestrial insects, and many birds cannot simply replace them by eating terrestrial insects without sacrificing key nutrients like omega-3s, which are critical to development and reproduction. Therefore, it is important that researchers not just count how many insects are available, but what kinds of insects and their nutritional quality.
This was the focus of a recent study which combined long-term data on both insect abundance and the timing of nesting for birds that rely on insects (Shipley et al. 2022). The scientists were able to look at both aquatic and terrestrial insect biomass from the same site in Ithaca, New York, over a 25-year period (1989-2014). They found that both aquatic and terrestrial insects have substantially advanced their emergence in association with warming early spring temperatures, by 3-12 days. The researchers wanted to know if the birds were keeping pace—that is, advancing their nesting dates to keep up with the insects.
They used data from NestWatch and Project MartinWatch (11,970 total nest records within 150 km of Ithaca, NY where insects were sampled) to investigate the relationship between shifts in egg-laying dates and those of insect emergences. They included Barn Swallows, Eastern Phoebes, Purple Martins, Eastern Bluebirds, Tree Swallows, and House Wrens, all of which primarily feed insects to their young.
Unfortunately, they found that the insectivorous birds were not keeping pace; although they advanced their lay dates to track their prey, they still lagged behind the pace of insects. The study found no annual decrease in overall insect abundance, but rather a change in when the insects were available. The insects were emerging earlier and over a shorter period, creating a punctuated pulse of insect food available to birds. Although terrestrial insects still increase throughout the season, aquatic insects peaked early and declined after May 15th. This could have negative consequences for birds that breed later in the season like Purple Martins and Tree Swallows.
The lead author Dr. Ryan Shipley said of the study that “It highlights the importance of having long-term data on insect communities and knowledge of whether their larval stage is aquatic or terrestrial, because they vary in their nutritional quality in terms of omega-3 fatty acids. Aerial insectivores like swallows are declining across much of North America and Europe, and we know they are chasing a fast moving and elusive target to time reproduction. Long-term nesting data, like those provided by citizen scientists, are critical to understanding these declines.”
What can people do to help?
- Protect ponds, lakes, estuaries, and wetlands (aka irreplaceable “nutrient hotspots”) from disturbances like urbanization, agricultural pesticides, and fertilizers.
- If you have a backyard or campus pond, don’t mow right up to the edge. Leave a vegetation buffer around it.
- Support policies that reduce greenhouse gas emissions.
- Encourage insects in your spheres of influence (e.g., turn off the bug zapper, put down the insecticide sprayer, and use protective clothing and bug spray to keep yourself protected).
- Shipley, J. R., C. W. Twining, M. Mathieu-Resuge, T. P. Parmar, M. Kainz, D. Martin-Creuzburg, C. Weber, D. W. Winkler, C. H. Graham, and B. Matthews. 2022. Climate change shifts the timing of nutritional flux from aquatic insects. Current Biology 32(6):1342-1349.e3. DOI: https://doi.org/10.1016/j.cub.2022.01.057