June 23, 2012

Birding: Songbird repertoires can be quite complex, and full of variety


In the last column, I wrote about the morning chorus, the marvelous dawn symphony performed by singing birds during the breeding season. The chorus is still going strong (much to the dismay of some would-be sleepers). I'll expand on the topic in today's column, exploring the complexity of bird songs.

The males of some species give a very simple song like the harsh "fee bee" of the Eastern phoebe or the "hey sweetie" or "fee bee-ee" song of the black-capped chickadee. Although some individual variation occurs among males of these species, the repertoire of songs is low.

On the other hand, some of our breeding birds have long, complex songs with great variation. Some of this variation occurs between males, and some of the variation occurs in a single male. 

Red-eyed vireos give their familiar sing-song vocalization, "here I am -- where are you -- over here." By recording red-eyed vireos and analyzing the songs with sound analysis software, my students and I have found that one male may have more than 40 songs. Males are tireless; a single bird may sing over 20,000 songs in a day.

You may be surprised to know that the brown thrasher has the most varied repertoire of any bird studied. A single male can sing more than 2,000 songs.

Species with complex songs include virtuosos like bobolinks, ruby-crowned kinglets and song sparrows. Each male gives a long song with many phrases. A bird may omit, replace or add particular phrases to produce different renditions of its song. The speed of note production can be dazzling.  The ethereal song of a Winter Wren includes over 100 notes in a span of in less than 10 seconds.

Ornithologists have been intrigued by the variety of repertoire sizes of male songbirds and have noted some interesting patterns. Species with large repertoires tend to have high parental care by the male of a pair. Migratory songbirds tend to have greater repertoire sizes than related non-migratory species. Finally, species that are polygynous (males have multiple female mates) have larger repertoire sizes.

In a fascinating paper, Susan Peters of Duke University and three colleagues described the results of a study of song complexity in the song sparrow.

Song sparrows are widely distributed in North America. They have a complex song that usually begins with three clear notes and then a complicated and variable series of single notes, phrases of several notes, and trills. Song Sparrows have a large repertoire of songs. The phrase "maids maids maids put on your tea kettle-ettle-ettle-ettle" captures some of the cadence and complexity of the songs.

Some populations of song sparrows are migratory, while others, typically in more moderate climates, are year-round residents. All song sparrows are monogamous, and males of sedentary and migratory populations contribute equally to brood rearing.

Peters and her colleagues compared the repertoire sizes of two sedentary populations of song sparrows (in North Carolina and Washington state, both of which have relatively mild winters) to migratory populations of Song Sparrows from Maine and Pennsylvania. Most of the song sparrows in the latter two states migrate south in the winter. 

The researchers recorded Song Sparrows in all of these states and then analyzed them to compare the number of different songs from each population. Their analysis revealed that the Washington and North Carolina sedentary populations were more similar to each other than to either the Pennsylvania or Maine populations.

Surprisingly, these ornithologists found that the sedentary populations had more complex repertoires than the migratory populations. These results conflict with the conventional wisdom that migratory species generally have larger and more complex repertoires than non-migratory species.

It is clear from the work of Peters and her colleagues that we have much to learn about reasons for differences in the complexity of bird songs.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at: whwilson@colby.edu


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