Thursday, October 1, 2009


Tree Swallow

Even without the swallow, you would guess this was a bird box of some sort. Such boxes, meant primarily for Tree Swallows, are commonplace here in parks and conservation areas.

What then is this riverside, multi-unit construction with the horizontal, rectangular openings? I came across it yesterday at the Alaksen National Wildlife Area in Delta BC. I had a hunch, and did an image search. I had guessed correctly--these were nest boxes meant for Purple Martins, North America‘s largest swallow.

Purple Martins have a very fragmented breeding distribution, and the geographically separate subspecies have different nesting habits. The eastern form long ago abandoned natural cavities in favour of hanging gourds and the well-known martin apartments provided by humans. The western birds retained the historical habit of nesting in loose colonies, using abandoned woodpecker holes or other natural cavities in trees, often in recently burned areas, usually near freshwater. Southwestern BC, including Vancouver Island, was the historical northern limit of the breeding range. Numbers of breeding pairs declined rapidly after European settlement due to habitat loss: fire suppression, deforestation, the spread of agriculture and urban development. By 1950, none bred in the Lower Mainland. By the early 1980s, there were fewer than 10 breeding pairs in the province.

In recent years, there has been a remarkable rebound, thanks to the installation of nest box clusters such as those above in both the Georgia Basin (southern mainland of BC plus eastern Vancouver Island) and Puget Sound, Washington, starting in the 1970s and 80s. By 2002 there were more than 200 nesting pairs in BC. Five years later there were about 650! This latter number is thought to have been due to exceptionally favourable early spring conditions (warm, with an early abundance of flying insects) in 2002 and 2003, that led to high mean clutch sizes and subadult recruitment. Weather in subsequent springs was not as cooperative, and the numbers have dipped somewhat - about 570 pairs in 2008; this year’s predicted number was 450-500. This sort of weather-related fluctuation is characteristic of swallow populations.

The clusters of boxes are meant to simulate the loose colonies of western Purple Martins. There are more boxes than are expected to be used, but enough that even under competitive pressure from Tree Swallows, martins can be induced to nest. From the writing on the boxes in the image, it appears this cluster was established in 2005. Recent (up to 2008) breeding records known to me do not include this site. Maybe next year.

Source: Cousens, B. 2009. Western Purple Martin recovery status in BC (and Puget Sound, WA) - 2008. Georgia Basin Ecological Assessment and Restoration Society. See also here.


Karen said...

I have seen gourds for the PMs up at Discovery Park here in Seattle, and am glad the birds are starting to recover from our awful habitat ruining. Hey, do you know any Master Birders down our way? My daughter's class is doing a year-long bird project this year and we are looking for resources. I have been looking for someone via your blogroll too. Thanks!

Hugh said...

Karen, Sorry, I don't know any Seattle area birders. There must be a Seattle area/Pacific NW naturalist club. They are the typical haunts of birders.