Thanks to everyone who shared their insights both on and off list. It seems
like swallows (including Cliff) are decreasing shockingly in some places,
while increasing in at least a few others. Based on feedback that some
colonies have been suddenly abandoned while new ones have sprung up in
other areas, it does seem that the active breeding sites move around over
time. I'm just somewhat surprised that the Cliff Swallows gave up on all
the colonies in my nearby area this year. It may well have something to do
with the blood-sucking parasites described by David L, as I did notice
several nests whose openings were literally crawling with insects earlier
this spring. It'll be interesting to see whether the birds come back in a
few years once the bugs have died off. (It's also kind of ironic that an
insectivorous bird could be driven off by a bug!)
Peter Ruprecht
On Thu, Aug 14, 2025 at 10:16 AM Nathan Pieplow <npieplow...> wrote:
> Peter et al.,
>
> This year, the canal between Twin Lakes and the Boulder Country Club
> neighborhood has multiple Cliff Swallow colonies, which it hasn't had the
> past couple of summers. Cliff Swallows are still present in their usual
> numbers at the other two regular colonies around Twin Lakes, so this
> represents a local increase in the number of Cliff Swallow colonies. It
> will be interesting to see how long these new colonies persist.
>
> Nathan Pieplow
> Boulder
>
> On Thu, Aug 14, 2025 at 10:01 AM DAVID A LEATHERMAN <daleatherman...>
> wrote:
>
>> 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?
>>
>> Thanks,
>> Peter Ruprecht
>> Superior
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