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July 2010

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From:
Kenn Kaufman <[log in to unmask]>
Reply To:
Kenn Kaufman <[log in to unmask]>
Date:
Fri, 16 Jul 2010 11:35:44 -0400
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Bob Barrett raises some interesting questions about molt in Purple
Gallinules and its relation to timing of migration.

It has been well established that synchronous or near-synchronous molt of
the flight feathers is the norm for members of the Rallidae (rails,
gallinules, moorhens, and coots).  Most birds molt their flight feathers a
few at a time, maintaining their ability to fly, but most rallids drop all
their primaries, secondaries, and tail feathers at once, more or less,
becoming flightless for a period of a few weeks until those feathers grow
back.

Among Ohio species, the replacement of the flight feathers (and resulting
flightless period) happens mainly during August in Sora, Virginia Rail, King
Rail, and Black Rail.  In American Coot and Common Moorhen, it happens
during late summer and early fall, again centered on August.  Information
from farther south indicates that the flightless period in Purple Gallinules
is generally sometime between June and August, although it's plausible to
suppose that it might be later this far north.  The general pattern in this
family, at least in North America, is for the flightless period of the molt
to occur after breeding and before fall migration.

It's interesting to look at other groups of birds that also become
flightless because of a synchronous molt of flight feathers.  The best-known
example involves ducks, geese, and swans, which likewise have their
flightless period in late summer, after breeding and before fall migration.
Loons and most alcids also go through a flightless stage, but the timing of
this varies by species, and even among different ages of the same species.
The common denominator is that these birds undergo the synchronous molt at a
time and place where they don't really need to fly.  Think about it:  a rail
in a large marsh, a gallinule or duck on a large marshy pond, or a loon or
an alcid out on the ocean can get by for long periods without flying
anywhere; they get their food by swimming or wading, and as long as they're
not pressed by predators, they'll be fine.

For anyone interested in details of molt, the most thorough source is the
Identification Guide to North American Birds, by Peter Pyle, part I (1997)
and part II (2008), both published by Slate Creek Press.  These exhaustive
compilations are not for the faint of heart, though.  For a more popular and
readable source, see the brand-new Molt in North American Birds, by Steve
N.G. Howell (Houghton Mifflin Harcourt, 2010).

Incidentally, Purple Gallinules may not appear to be strong flyers; but
records of their vagrancy indicate that once they get going in the wrong
direction, they can fly amazing distances.  There are records for many
islands in the Atlantic, north to Greenland and Iceland, south to the
Falklands and even South Georgia I.; there are many records for Europe, as
far north as Norway, and a surprising number of records for South Africa.
(Indeed, Purple Gallinule and Buff-breasted Sandpiper are among the most
frequent New World birds found in South Africa, despite the evident
difference in flying ability of those two species!)

Within South America, Purple Gallinules are apparently present only during
the austral summer in Uruguay and perhaps parts of Argentina.  They also
move around considerably in seasonally flooded regions such as the llanos
and the Pantanal; in those regions, the timing of the flightless period of
their molt may be dictated mainly by the timing of wet seasons and dry
seasons, but I haven't seen any data on that.

Kenn Kaufman
Oak Harbor, Ohio

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