Cliff Swallows disappearing

[Peter Ruprecht]

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

[DAVID A LEATHERMAN]

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

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From: cob...@googlegroups.com <cob...@googlegroups.com> on behalf of Peter Ruprecht <prup...@gmail.com>
Sent: Thursday, August 14, 2025 9:09 AM
To: Colorado Birds <cob...@googlegroups.com>
Subject: [cobirds] Cliff Swallows disappearing

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[Jim Nelson]

Peter,

We visit in Estes Park spring, summer, and fall every year.

For many years, Cliff Swallows have nested under the roof overhang of the power plant at the west end of Lake Estes.  This year, there have been no active nests there when we have visited.  Also, I always have seen Cliff Swallows flying in the lake area, along with Barn, Tree, and Violet-green Swallows, during the nesting season.  This year I have not seen any Cliff Swallows around the lake on our visits in May and July.  My only observation of Cliff Swallow in the Estes Park area this season was on May 26 at Lily Lake, south of Estes on Highway 7.

Looking at eBird bar charts for 2025 for the Lake Estes and Matthews-Reeser Bird Sanctuary eBird hotspots at Lake Estes, it appears that sightings of Cliff Swallow at Lake Estes are down considerably compared to the previous 4 years.

Maybe someone who is resident in Estes Park can chime in to confirm or correct my observation.

Jim Nelson

Bethesda, MD

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[Nathan Pieplow]

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

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[Peter Ruprecht]

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