2015 Field Season is in the Books.


Abandoned Cassin’s Vireo nest, with two Brown-headed Cowbird eggs (dark brown), one Cassin’s Vireo egg (whitish), and a dead chick.

My 2015 field season was a great one, and likely my last full one.

I headed to the field in early April, hoping to witness the arrival of the birds from their wintering grounds. I found five males on my first day, and watched as several more trickled in over the next few weeks and set up territories in the valley. Seven of the thirteen birds I had banded in 2014 came back and a further four showed up to fill in the vacant space. The biggest surprise of those early days was the discovery of a bird from 2013 that was not present in 2014. He showed up, singing his same old tunes, in precisely the same place where he had nested two years prior — it was a bit like being reacquainted with an old friend.

As the season wore on and the data piled up, I noticed a disturbing pattern: the abundance of Brown-headed Cowbirds had reached unprecedented levels. Cowbirds are the most successful brood-parasites in North America, laying their eggs in the nests of other bird species, out of hopes they can skip out on parenting duties while still passing on their genetic material. They have increased dramatically in number throughout the western part of the continent in recent years — largely in response to human alterations to the landscape — and represent a serious threat to many populations of songbirds. Cassin’s Vireo, it seems, are one of their favorite hosts; I suspect their nests, constructed at the tips of low-lying branches, are easily detected by the cowbirds.

I knew it would be a tough year when nine of the first ten nests were abandoned, most due to apparent brood parasitism. The pairs re-nested, but were quickly parasitized again, and again, and again. By the end of the season, the thirteen pairs had built at least 24 nests, not a single of them successfully fledging a Vireo chick. Several raised Cowbird chicks instead, while others gave up on breeding altogether. The photo above shows an abandoned nest, and graphically illustrates the incessant challenges facing these birds year after year, as they struggle to raise their young in these harsh conditions. Though I can’t help but root for the Vireos, I have also developed an immense respect for the Cowbirds and their malevolent ways. It is endlessly fascinating to watch this high-stakes evolutionary arms race unfold.

Despite the nest failures, I managed to collect a vast amount of recordings and behavioral observations, and completed lots of playback experiments to get a better feel for the functions of the songs and syntax of this species. The birds at my main site were all gone by the end of June (likely in response to the aforementioned cowbirds), so I spend the last couple of weeks of the season recording birds in the furthest corners of California. I traveled to Lassen, Shasta-Trinity, Los Padres, and Sequoia National Forests, recording several birds in each location. All told, I have more data than I know what to do with — my winter will surely be a busy one trying to make sense of it all.

Until next time.


2 thoughts on “2015 Field Season is in the Books.

  1. Hi Richard. Re: cowbird parasitism, why do the vireos abandon the nest? Aren’t there still video eggs in there? Are they upset by the presence (in their nest) of cowbirds a-laying? By cowbird eggs?

    Also the video nest material looks interesting. Almost like a lacquered fiber composite?


    • Hi Ross,
      Thank you for your questions!
      Nest abandonment happens for a few reasons. Most importantly, I have never observed a vireo to successfully raise both a vireo and a cowbird chick. It seems that the presence of a cowbird precludes the survival of the vireo chick. Why, exactly, this is so, I am less sure. Typically, brood parasites have evolved very fast development, so they can both hatch earlier and outcompete their nestmates after hatching. Regardless of the reasons, raising cowbirds clearly hampers vireo reproductive success, so abandonment is perhaps the best strategy.

      As for how they recognize the cowbird eggs, many studies have been done on this in other species, and my understanding is that color, egg size, and egg number are all important characteristics that birds can cue in on to detect a parasitic egg. Cowbird eggs are larger and have more brown speckles than the vireo eggs, but are not so different in overall appearance. The superficial resemblance of the two is probably one reason why the deception is sometimes successful.

      The vireo nest material is mostly vegetation, but the white material around the edge is actually spider silk, probably collected from spider egg casings. All nests I have seen of this species contain significant amounts of spider silk – probably because of its stickiness and its strength.


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