Friday, April 13, 2012

Swallow molt


A little update on what I actually think about/do on a day-to-day basis:

Blue-and-white Swallow (adult), in molt, with helpful labels! near Illimo, Peru

Generally, the life of tropical birds has two large energy expenditures per year: reproduction and molt. In temperate zones, there is also migration for some birds. A bird is therefore faced with striking a balance between these activities in such a way as to maximize its fitness. With hundreds of exceptions, the general rule is that the three events are separated. Take, for example, the Purple Martin. It breeds in MA May-July, then migrates to South America, where it molts all of its feathers (not at the same time!), then migrates back north and repeats the process (if it is lucky). 

Southern Rough-winged Swallow with 6 new, 1 missing and 2 old primaries per wing, Pacora

But why molt feathers in general? The simple answer is that feathers do not last long. And why is this is a big deal? Birds generally have between several hundred (a hummingbird) and over ten-thousand (a swan) feathers on their body… replacing all these is not a trivial investment of protein and energy for the bird – especially for a large one, such as a swan, vulture or albatross. Interestingly, feathers do not grow much quicker in these large birds than in small birds – leading to large birds being constantly in molt in order to replace all their feathers (though there are times where they are in more heavy molt). In a great study, albatrosses in heavier molt were shown to have lower breeding success… one of the only times any trade-off between reproduction and molt has been investigated. 

My guess is that these feathers are only 8-10 months old! Tachycineta stolzmanni, first-year male
Southern Giant-petrel, Ushuaia, Argentina. Big birds are always in molt.

The first day I was here last year, I caught an adult male Tumbes Swallow on a nest who was molting his primaries… After gathering a lot more data both last year and this year, it seems this is the norm. Males begin their molt first, generally the last two weeks of February, followed by females a few weeks later. So my lofty goal this year, which I am working hard on now, is to see if the swallows invest less heavily in reproduction while molting – which I hypothesize to occur because of reduced foraging efficiency in molting birds. Of course, this is taking a long-leap, as this has never been shown, either, but it seems reasonable that a bird like a swallow, which forages entirely on the wing, might have less success or have to expend more energy foraging with missing feathers in the wings.

Strange molt - notice that there is one old feather amidst two new - usually they molt in order. In another study in over 100 specimens of Tachycineta bicolor (Tree Swallow) only 2 examples of this were found.

And just to reinforce the notion that big birds are always in molt. A Southern Screamer, Chascomús, Argentina.

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