Wednesday, May 9, 2012

Seasonal variation in Central American Sedge Wren songs?

Up here in Cerro de Hula, we have a sizable population of Sedge Wrens. This morning alone, while doing point counts, we heard 13 different individuals vocalize in an area no bigger than 25 hectares (500 x 500 m). The other side of the hill has more Sedge Wrens, and they are also found in nearby areas. We have never systematically searched for all territories in the area, but a conservative estimate would be somewhere between 25 and 50 pairs, possibly more.

We occasionally saw these birds back in December of last year and the following January, but we didn't realize how common they are in these parts until they started singing, in early February. 

At first, the males sang elaborate series of songs, repeating a song only once or twice or not at all before switching to another song. They sang all morning, with multiple afternoon singing bouts as well. Each one of them seemed intent on demonstrating his vocal prowess with a seemingly endless variation of songs.

But now, three months later, almost all of them are singing buzzy, repetitive songs with little variation. Gone are the musical flourishes from earlier. This, this and this to my ears sounds similar. (The last two fragments are from the same individual.) The pauzes between phrases are noticeably longer now than they were back in February.

Curious if this apparent temporal shift in vocal array signifies a different stage in the breeding process, I decided to consult other recordings in Xeno Canto. I speculated that perhaps the elaborate, musical songs were sung by unmated males attempting to attract a female, and the buzzier, more repetitive songs were sung by mated males simply marking the boundaries of their territories. If true, this pattern would be reflected in recordings from the beginning of the breeding season versus later recordings. However, the Sedge Wren has an extremely wide range, from Canada well into South America, with marked life history differences between populations.

Northern populations of Sedge Wren are migratory, and the Sedge Wren is unique among North American passerines in that it first breeds in northern parts of its breeding range (upper midwestern US and southern Canada), then undertakes a short-distance migration to more southern (Nebraska, Kansas, Missouri) and northeastern (Vermont, Massachusetts) parts of its breeding range, to breed again late summer, before it migrates south to its wintering grounds in the southeastern US and northeastern Mexico.

Central and South American populations, however, are sedentary.

This ecological difference has vocal repercussions, as Kroodsma and others demonstrated in a 1999 paper in Animal Behaviour: the semi-nomadic habits of northern Sedge Wrens favor song improvisation, while birds from more sedentary populations learn each other's songs and develop regional dialects. In Costa Rica, for example, Kroodsma's team found that color-banded birds remained at banding sites year-round and that many songs they heard there were shared by neighboring males. Costa Rican birds from a population only 27 km apart, however, sang different songs. In contrast, northern Sedge Wrens do not have these regional dialects; they have large song repertoires that enable them to communicate with other Sedge Wrens from throughout the northern range.

Back to my hypothesis, then: would Sedge Wrens from comparable (i.e. Central American) populations show this pattern, i.e. greater song variation early on in the breeding cycle versus simpler, buzzier, more repetitive songs later on? Xeno Canto has an impressive number of Sedge Wren recordings — no fewer than 145 — but, as is so often the case, the vast majority from the U.S. and Canada, and South America. In fact, besides my own Honduran recordings, there is one brief recording from Guatemala in the XC database, made by Jesse Fagan in August 2008, for all of Central America. Only 20 seconds in duration, it is really too short to assess the vocal variation of that individual. I hear the repetitive chatters, without any of the melodic flourishes that birds here early in the breeding season were producing, but who really knows what else it was singing that day.

If we consider Mexico as biogeographically comparable to Central America, then the two Mexican recordings, from June 2006 and June 2010 respectively, sound remarkably similar to my simple, buzzy 'late season' songs from Honduras. The recordist, Manuel Grosselet, notes that to his ears, the June 2010 recording from near Mexico City sounds "atypical". However, to me it sounds typical of a late season song, possibly — speculation — from a bird that has already attracted a mate and only needs to mark his territory. As the phrase goes, "more study is needed…"

Cited literature:
Kroodsma, D. E., J. Sánchez, D. W. Stemple, E. Goodwin, M. L. da Silva, and J. M. E. Vielliard. 1999b. Sedentary life style of Neotropical Sedge Wrens promotes songs imitation. Anim. Behav. 57:855-863.

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