A little update on what I actually
think about/do on a day-to-day basis:
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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).
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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.
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My guess is that these feathers are only 8-10 months old! Tachycineta stolzmanni, first-year male |
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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.
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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. |
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And just to reinforce the notion that big birds are always in molt. A Southern Screamer, Chascomús, Argentina. |
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