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.