The Decline of Aerial Insectivores
The term “aerial insectivore” refers to the guild of birds that feed on insects while flying. They have amazing flying abilities, and help us out by controlling insect populations. In recent decades, these birds have been gaining attention in the scientific community because their populations have been mysteriously declining.
Aerial insectivores feed in open areas like ponds, lakes, fields, or those corridors under hydro lines - often at habitat edges (e.g. beside a forest). They need space to dive through the air while pursuing moths, mosquitoes, and other insects. While these acrobatic birds are a fairly common sight for bird-watchers, their populations are nowhere near where they were fifty years ago. On average, aerial insectivore species have declined by 59% since the 1970s. This loss is greater than in any other group of birds.
Aerial Insectivore species at risk in The Land Between
- Barn Swallow (Threatened)
- Bank Swallow (Threatened)
- Chimney Swift (Threatened)
- Common Nighthawk (Threatened)
- Eastern Whip-poor-will (Threatened)
- Olive-sided Flycatcher (Threatened)
- Eastern Wood-pewee (Special Concern)
- Acadian Flycatcher (Endangered)
Trends in Aerial Insectivore Research:
In general, birds who migrate longer distances are experiencing higher rates of population decline than short-distance migrants. Why?
- Increased energy demands could make them more vulnerable to any other changes (e.g. in food or habitat quality).
- Because they rely on various areas to stop over to rest and refuel along their journey, they are more susceptible to habitat loss or changes. If they lose any key stopping grounds (e.g. marsh lands, estuaries) throughout Central America or the US, they may not even make it to Canada.
The decline is more pronounced in Canada than in the US, and is especially concentrated in the NE corner of North America. Migratory Aerial insectivores in Central Ontario have been shown to be one of the greatest categories at risk. Why
- Canadian populations must migrate longer-distances to get to the tropics, so there could be a compounding effect.
- Could be connected with the huge industrial activity in these areas. Lots of manufacturing, smelting operations, means high concentrations of pollution in the air and water.
Declines began in the 1980s, and have continued ever since. Why?
- In short, we don’t really know why - more research is needed!
- Around this time acid rain was notably affecting waterways due to sulfur releases. Lakes around Sudbury and Central Ontario became dead zones, and forests were defoliated.
- Years of successful lobbying by scientists and environmental groups helped ensure legislation that significantly reduces sulfur emissions from manufacturing plants.
- While acid rain became largely under control in the 1990s, the populations of aerial insectivores have continued to decline since that time. Clearly, there is more going on here.
Further Resources:
Brochure outlining Aerial insectivore declines: (final) Aerial Insectivore brochure)
Research Article: Spiller and Dettmers, 2019. Evidence for multiple drivers of aerial insectivore decline.
Research Article: Nebel, S, Mills, A, McKracken, JD, and Taylor, PD. 2010. Declines of aerial insectivores in North America follow a geographic gradient.
The Land Between hosted a public webinar on Aerial Insectivores in July, 2020. Check back here soon for a video recording of that event.