Why are big, insect-eating birds disappearing? Maybe we’re running low on bugs.

Desdemona Despair

By Brandon Keim
14 March 2018

(Anthropocene) – No guild of North American birds is declining so rapidly as aerial insectivores: acrobatic marvels whose maneuvers make our hearts soar, and who provide a vital ecosystem service. Why are their numbers plummeting? A leading explanation is a widespread decline in insect populations — a troubling possibility, hinted at by many studies but also one difficult to pin down. There are few records of historical insect numbers against which to compare our own.

In a study published in the journal Frontiers in Ecology and Evolution, researchers led by biologist Philina English of Simon Fraser University describe their ingenious workaround: they used museum specimens of whip-poor-wills, a medium-sized insect-eater whose populations are falling by 3.5 percent each year, as biological time machines. By contrasting the chemical composition of their bodies with the composition of living whip-poor-wills, the researchers could extrapolate how the birds used to eat.

“These results are consistent with the hypothesis that aerial insectivore populations are declining due to changes in abundance of higher trophic-level prey,” wrote the researchers. To translate that into normal speak: the birds are suffering because there seems to be a lot fewer big bugs than there used to be.

If insect populations are indeed falling, surmised the researchers, it stands to reason that bugs higher up the food chain, who consume insects smaller than themselves, would be declining even faster. This would push whip-poor-wills, who prefer to eat larger, more nutrient-rich insects, to eat smaller and less-sustaining fare.

As it happens, smaller bugs and their insect predators accumulate subtly different forms of nitrogen in their tissues. The presence of these nitrogen signatures in bird bodies thus becomes a record of their own diet. When English and colleagues compared the chemistry of claw and feather samples from whip-poor-wills caught recently in Ontario to specimens collected for Ontario museums during the late 19th and 20th centuries, they found a “significant decline” in chemical traces of larger insects.

The pattern “is consistent with contemporary whip-poor-will populations feeding lower in the food web than in the past,” wrote the researchers. While English cautioned that more testing is necessary to be absolutely certain that something else wasn’t responsible for the chemical shift, the findings fit with other observations of insect decline. [more]

Why are big, insect-eating birds disappearing? Maybe we’re running low on bugs.

ABSTRACT: Identifying the mechanisms of ecological change is challenging in the absence of long-term data, but stable isotope ratios of museum specimen tissues may provide a record of diet and habitat change through time. Aerial insectivores are experiencing the steepest population declines of any avian guild in North America and one hypothesis for these population declines is a reduction in the availability of prey. If reduced prey availability is due to an overall reduction in insect abundance, we might also expect populations of higher trophic level insects to have declined most due to their greater sensitivity to a variety of disturbance types. Because nitrogen isotope ratios (δ¹⁵N) tend to increase with trophic-level, while δ¹³C generally increases with agricultural intensification, we used δ¹⁵N and δ¹³C values of bird tissues grown in winter (claw) and during breeding (feathers) from museum specimens spanning 1880–2005, and contemporary samples from breeding birds (2011–2013) to test for diet change in a migratory nocturnal aerial insectivore, Eastern Whip-poor-will (Antrostomus vociferus) breeding in Ontario, Canada. To test if environmental baselines have changed as a result of synthetic N fertilizer use, habitat conversion or climate, we also sampled δ¹⁵N values of three potential prey species collected from across the same geographic region and time period. Over the past 100 years, we found a significant decline in δ¹⁵N in tissues grown on both the breeding and wintering grounds. Prey species did not show a corresponding temporal trend in δ¹⁵N values, but our power to detect such a trend was limited due to higher sample variance. Amongst contemporary bird samples, δ¹⁵N values did not vary with sex or breeding site, but nestlings had lower δ¹⁵N values than adults. These results are consistent with the hypothesis that aerial insectivore populations are declining due to changes in abundance of higher trophic-level prey, but we caution that museum-based stable isotope studies of terrestrial food chains will require new approaches to assessing baseline change. Once addressed, an ability to decode the historical record locked inside museum collections could enhance our understanding of ecological change and inform conservation decisions.

Stable Isotopes from Museum Specimens May Provide Evidence of Long-Term Change in the Trophic Ecology of a Migratory Aerial Insectivore