Birds that feed entirely on aerial insects, like swallows and martins, are experiencing widespread and worrisome declines across North America. Population trends estimated from the continent-wide Breeding Bird Survey (BBS), conducted since 1966 by volunteers and coordinated by VCE in Vermont, suggest that these specialized birds are declining at significantly steeper rates than other bird guilds. It appears that declines are most prevalent in the Northeast, where they became especially apparent in the mid-1980s.
One of these aerial insectivores, the Bank Swallow, may be particularly vulnerable in the Green Mountain State. According to BBS data from Vermont, populations have declined nearly 3% annually over the last 45 years. However, this decline has not been steady. From 1966 to 1979 Bank Swallow numbers remained relatively stable. In the 1980s populations began to turn downward with annual declines reaching nearly 3.5% across the state.
The first Vermont Breeding Bird Atlas (1976-1981) was conducted just before declines became noticeable. Volunteer atlasers found Bank Swallows on 108 survey blocks scattered across the state. It was a common colonial breeder just about everywhere. Fast-forward to the second-generation atlas, just completed in 2007, when breeding colonies were found on only 58 blocks statewide, a decline of almost 50%. Other breeding bird atlases in the Northeast have found similar results: New York -28%, Pennsylvania -35%, Ontario -45%. As for other swallow species, reasons for long-term declines remain unclear, but in the case of Bank Swallows could include gravel pit management and reclamation practices, riverbank erosion control, climate change, reduced insect prey, and pesticides.
Bank Swallows historically nested in exposed banks along rivers, but now also use open gravel and sand pits. They build nests in colonies containing from just a few up to 2,000 nests. These colonies are usually located near waterways, in areas of fairly loose soils that are easily excavated for burrows. Males begin to dig a burrow into the bank prior to pairing with a female; females then hover in front of burrows to inspect males and their nest sites before selecting a mate. Nests are usually located in the upper third of a bank to avoid ground predators.
In most years a few Bank Swallows begin to arrive in mid- to late April, but at many colony sites in Vermont, birds do not appear until early May. Breeding behavior commences within a week of arrival. Although Bank Swallows dig their own burrows, returning birds often reuse burrows from previous years. Burrows are 15 to 48 inches deep; the terminal nest is made of grasses, leaves and rootlets with a lining of feathers. Females lay 2-6 eggs. There are only 7 known dates for egg laying in Vermont, ranging from May 18 to July 10. The incubation period lasts 15 days with fledging 22 to 23 days later. Fledglings are quick learners, remaining dependent on their parents for food for only about 5 days after leaving the nest.
Because this species is both strongly colonial and restricted in habitat, a comprehensive survey of existing colonies is entirely feasible with an army of citizen scientists like you. Participating in the mission is simple. Volunteers locate Bank Swallow colonies, record how many birds and burrows are present and gather basic habitat information, and submit the data to Vermont eBird. Multiple visits to sites are highly recommended, if possible.
1. Scour the state during their breeding season in May and June looking for eroding banks that may contain from a handful to perhaps many hundreds of nests. Riverbanks, sand pits, lakesides, reservoirs, roadsides and other places near waterways are great places to look.
2. Estimate the number of birds in the colony and the number of active and inactive nests.
3. Enter your stationary count of the colony into Vermont eBird placing the point that you surveyed as accurately as possible on the map so that it is centered on the actual colony. This is important for us to know exactly where the colony is located. Put your total count of Bank Swallows in the checklist, and any other birds noted.
4. Click on the button that says Show Details next to Bank Swallow on the checklist after you enter the total amount of birds you counted. This will give you a box to add comments and a chance to select a breeding code. In the comment box, please enter the total number of nesting holes you counted and the total number of active nests you detected. Also add a describe of the type of bank that you found: active sand pit, old sand pit, eroding riverbank, etc. etc.
5. Try to revisit the colony a few times during the breeding season and repeat your counts of adults and active nest holes, entering each visit as a checklist on Vermont eBird as described above.
That’s all there is to it! It’s an easy and fun treasure hunt. Thanks for helping us learn more about these amazing insectivores!
Resources and Information
Vermont Breeding Bird Atlas - Bank Swallow Account
All About Birds - Bank Swallow
Plight of the Bug Eaters - Ontario Nature Magazine