For some reason, purely by accident, it turns out that many
of the world's swallow species now nest primarily in structures built by
humans.
One of these species is the Violet-green Swallow (Tachycineta thalassina). Its breeding distribution
lies mostly in extensive mountain ranges of the West. Originally the birds must
have nested in natural crevices in trees and cliffs as well as holes excavated
by woodpeckers, chickadees or nuthatches. Some of the swallows still nest in
such places, and they remain common as montane birds.
However, fast forward to the early Twenty-first Century, and
one of the best places to find Violet-greens is in our cities and suburbs. When
I first moved to Seattle in 1968, there were nesting swallows all over the
city. Some of them nested in houses, even more in commercial buildings, for
example all over the University District. As I drove and walked around, I saw
them all over the place. Any little opening into a building often had a pair of
these beautiful birds nesting in it.
Nowadays they are still present in Seattle, but in reduced
numbers. I have heard of no place where they are increasing and many
neighborhoods from which they have disappeared. When I moved into a house in
Maple Leaf, a wooded section of town, in 1991, I could dependably see and hear Violet-greens
overhead on a daily basis each summer. A decade later, they were scarcely to be
seen, and by 2010, they had disappeared from former haunts in many parts of the
city.
A possible cause of this decline is the tidying up of our
human habitat. The crevices in buildings that are used by Violet-greens and
some other urban/suburban birds are presumably decreasing in number as people
find them and seal them up. Who could argue with someone who wants to keep rats
and mice out of their house!
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Perhaps more significant, all swallows are aerial
insectivores. In other words, their diet consists almost entirely of flying
insects. Because of habitat destruction and pollution, primarily the use of
insecticides on so many of our crops, we have brought about a widespread
decline in such insects. Violet-green Swallows are decreasing generally as
breeding birds in the Pacific states, although Rocky Mountain populations are
doing well.
Even with local declines, swallows are thankfully still among
our most common and visible birds.
Dennis Paulson
1 comment:
The WDFW has put up 30+ nest boxes along the spur dike at Wiley Slough, the Skagit Headquarters unit on Fir Island. Tree Swallows occupy these nests. It's an amazing experience to walk along the dike in the midst of soaring swallows.
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