Thursday, June 26, 2008

How Singing Helps Warblers Find Breeding Habitat

Spring is accompanied by an explosion of birdsong that lasts well into the summer. Every morning for the past few months I have woken to the sound of a catbird singing outside my window. We are all familiar with the role birdsong plays in the early breeding season. Males sing to win and defend their territories and, in turn, attract females to mate with them. As the breeding season progresses, continued singing by adults helps juvenile birds learn to sing. It turns out that late season singing has one more function: it helps fledglings find appropriate breeding habitat for the following year.

That was the conclusion of a recent study involving one of my favorite birds, the black-throated blue warbler (Dendroica caerulescens). The researchers wanted to test whether social cues, such as late season birdsong or the presence of fledglings, helped other birds determine which sites had the best potential for breeding.

They set up 54 test sites in White Mountain National Forest in places with inappropriate habitat (to eliminate the possibility that birds followed vegetation cues rather than social cues). At the end of the breeding season in 2006, each site was given one of three treatments: left alone (a control group); song playbacks with male decoys (location cues); song playbacks with male decoys, plus female decoys attending fledgling decoys with with playbacks of begging calls (public information).

That summer, the researchers checked all three types of sites for the presence of black-throated blue warblers that might be looking for future breeding habitat. Sites with an artificial social cue were more likely to receive visits from fledgling warblers than the control sites. Both males and females were observed at the test sites.

The following year (2007), researchers conducted point counts at the test sites to check for warbler activity. Male warblers were far more likely to set up territories at sites where they had heard playback of territorial songs or fledgling calls the year before than at control sites. Females seemed to follow the presence of males; females were observed only at test sites where a male warbler was present.

Since warblers were equally likely to return to sites with location cues (song playback) and public information (song playback and dummy nests), the researchers surmised that late-season birdsong alone was a reliable indicator of good nesting habitat. To test this, researchers checked 60 known warbler territories for singing males.

Song frequency within territories was positively correlated with reproductive success, but only towards the end of the period observed.... By late in the breeding season (31 July), singing was 5.1 times (95% CI: 1.89-22.28) more likely on territories that successfully fledged young than those that did not. Conspecific song in the post-breeding season was therefore a reliable indicator of breeding success.
This result may be of interest to birders who volunteer for breeding atlases or other types of nest surveys.

Birdsong turns out to be a complex and powerful communication tool. For a short-lived species such as the black-throated blue warbler, which has to migrate thousands of miles between its wintering and breeding grounds, individual birds have relatively few chances to produce offspring. Reliance on a social cue like birdsong helps young birds avoid making some nesting mistakes in their first breeding season. They thus can produce more offspring over the course of their lives. It also holds advantages for the species as a whole, since local populations will be able to adapt more quickly to changes in their environment.

Matthew G. Betts, Adam S. Hadley, Nicholas Rodenhouse, and Joseph J. Nocera, "Social information trumps vegetation structure in breeding-site selection by a migrant songbird." Proceedings of The Royal Society B (online edition, published June 17, 2008). doi:10.1098/rspb.2008.0217