Date: 8/14/25 9:01 am From: DAVID A LEATHERMAN <daleatherman...> Subject: Re: [cobirds] Cliff Swallows disappearing
Peter,
One reason for some of these historical colony sites being abandoned is a build-up of nest parasites called Swallow Bugs (Oeciacus vicarius). Like their relatives the Bed Bug, they suck blood. The bugs overwinter in the nests and jump on returning birds the next year. They account for significant nestling mortality and I think if adult swallows have a particularly bad year in terms of minimal brood fledging, they start over somewhere else the next year. I have not read anything confirming this but maybe warmer summers lead to more robust Swallow Bug activity, more CLSW mortality, more moving new colony site start-ups. Would make for a good grad student project to determine how climate change is altering this relationship both in terms of whether mortality is increasing and/or if, to compensate, the birds are just forced to find more and more sites which they use for shorter durations before moving again.
When I was in my chair as forest entomologist for the Colorado State Forest Service, I got a few calls from humans living in the mountains in buildings that had both Barn and Cliff Swallow nests on them. When the swallows migrate, sometimes a portion of the Swallow Bug populations comes inside and bites humans. I always told folks to tolerate swallow droppings on the sidewalk, let the birds eat a bunch of mosquitoes, but then wash off the nests after the birds have migrated in late summer-early fall.
Dave Leatherman
Fort Collins
________________________________
From: <cobirds...> <cobirds...> on behalf of Peter Ruprecht <pruprecht...>
Sent: Thursday, August 14, 2025 9:09 AM
To: Colorado Birds <cobirds...>
Subject: [cobirds] Cliff Swallows disappearing
Hi birders,
I live in Superior and have enjoyed watching several largish colonies of Cliff Swallows (dozens of nests) around town for many years. However, starting this year, several of these nesting areas are completely unoccupied even though the mud nests themselves are still largely intact. Has anyone else seen something like this? Any idea what might cause it?
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