The Warbler With a Word for Cowbird
If you live near willows by a creek in Canada or the United States, the bright yellow bird singing there in late spring may well be a Yellow Warbler. Its song is the easy part. Field guides write it as sweet sweet sweet, I’m so sweet, and once you hear the words, they are hard to unhear. The more interesting sound is a thin, high alarm note, written as seet, that the warbler saves for one particular visitor.
A Call That Means Cowbird
Brown-headed Cowbirds lay their eggs in other birds’ nests and leave the hosts to raise the chicks. Yellow Warblers are regular targets, and they can recognise one. In a 1997 study, ornithologists Sharon Gill, Diane Neudorf and Spencer Sealy showed female warblers model cowbirds. The females reacted more strongly to a normal cowbird than to one with a longer, thinner bill, and more strongly to a female cowbird’s chatter than to a Song Sparrow’s song. The females answered the cowbirds with seet alarm calls, among other things.
Later work found that the warblers give seet specifically to warn other warblers about cowbirds. Red-winged Blackbirds nesting nearby listen in. When researchers played seet calls to them in 2020, the blackbirds responded as strongly as they did to cowbird chatter or predator calls. The team thinks the blackbirds hear it as a general warning about danger to the nest rather than a cowbird alert in particular. A 2023 follow-up found that blackbirds nesting close to warblers were less likely to have cowbird eggs laid in their nests, though that is only a correlation.
The Nest With Extra Floors
When a cowbird does lay an egg, the warbler has an unusual answer. The female builds the nest on her own, over about four days. If she finds a cowbird egg in it, she often builds a brand new nest directly on top, burying the cowbird’s egg along with any of her own already laid. According to the Cornell Lab of Ornithology, this can leave nests with up to six tiers.
Most people notice this bird because it is yellow, but a surprising amount of its life is about cowbirds. It keeps a call just for them. When the call is not enough, it often buries the whole clutch and builds again.
Sources:
- Cornell Lab of Ornithology: Northern Yellow Warbler overview, All About Birds
- Cornell Lab of Ornithology: Northern Yellow Warbler life history, All About Birds
- Animal Behaviour (via PubMed): Gill, Neudorf and Sealy (1997), Host responses to cowbirds near the nest: cues for recognition
- Communications Biology: Lawson et al. (2020), Heterospecific eavesdropping on an anti-parasitic referential alarm call
- Ecology and Evolution: Lawson et al. (2023), Red-winged blackbirds nesting nearer to yellow warbler and conspecific nests experience less brood parasitism
- Wikipedia: American yellow warbler