Authors
  • English, Philina A.
  • Nocera, Joseph J.
  • Green, David J.
Universities
  • Simon Fraser University
  • University of New Brunswick

Summary

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 (δ15 N) tend to increase with trophic-level, while δ13 C generally increases with agricultural intensification, we used δ15 N and δ13 C 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 δ15 N 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 δ15 N in tissues grown on both the breeding and wintering grounds. Prey species did not show a corresponding temporal trend in δ15 N values, but our power to detect such a trend was limited due to higher sample variance. Amongst contemporary bird samples, δ15 N values did not vary with sex or breeding site, but nestlings had lower δ15 N 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.

Methodology

We focused our sample collection on the province of Ontario, Canada due to evidence of a ∼50% decrease in the probability of detecting whip-poor-wills over recent decades (Cadman et al., 2007). To establish how contemporary carbon and nitrogen isotope values vary with the age and sex of birds, we sampled primary coverts and claw tissue from individuals captured between 5 May and 25 July in 2011–2013 at three breeding sites situated across the species’ range in Ontario: Rainy River district (48◦ 52′ N 94◦ 01-08′ W), Norfolk County (42◦ 42′ N 80◦ 21-28′ W), in Frontenac County (44◦ 28-34′ N 76◦ 2025′ W). The Rainy River sites consisted of a 40,000-hectare mosaic of agriculture, poplar (Populus sp.), coniferous forests, logged areas, and wetlands. The Norfolk County site was St. Williams Conservation Reserve, which consists of two forest patches totaling 1035 hectares of pine-oak sand barrens and pine reforestation in a zone of intensive agriculture. The Frontenac County site was Queen’s University Biological Station (QUBS), which consists of over 3200 hectares of deciduous forest and abandoned farmland in various stages of succession, both with scattered small rock barrens. Across the whole province both afforestation of abandoned farmland and deforestation for timber harvesting, agriculture and urban development have continued throughout the twentieth century that together result in an overall loss of unimproved farmland and an increase in woodland (Cadman et al., 2007). A concurrent geolocator study established that winter locations of whip-poor-wills from Rainy River and Frontenac overlap throughout southern Mexico and Central America, but that Norfolk birds may winter further north in Mexico or the southern United States (English et al., 2017a). Due to this low connectivity between breeding and wintering locations, we do not assess spatial variation in winter grown tissues because all samples were collected on the breeding grounds.

Location