Saturday, March 7, 2015

Molt and Identification: Long-Distance Migrants

Many birders don't pay much attention to the molt patterns of birds.  But, it can be an important aspect in the identification process.  One way we can use molt to help with identification is to realize that short-distance migrants and long-distance migrants differ in their molt strategies.  

American Golden-Plovers (juveniles) - What do American Golden-Plovers
and Buff-breasted Sandpipers have in common?  They are both
long-distance migrants and share similar molt patterns.
Buff-breasted Sandpiper (juvenile) - Another long-distance migrant that
shares the same molt strategy as the American Golden-Plover
Many long-distance migrants (excluding songbirds which mostly molt on their breeding grounds) don't undergo any wing molt until they reach their wintering grounds, outside of the United States.  However, this only applies to adults.  The flight feathers of juveniles grow in simultaneously, from the time they hatch through the fledging period.  Thus, these young birds have fresh juvenile feathers when they migrate south.

How does this affect identification?  To be able to use this information to the fullest extent, you need to be able to identify the birds as adults or juveniles.  There are many cases where this won't be possible.  In these situations, if the birds are molting flight feathers, it can help in the identification process.  If the flight feathers are not molting, it won't be of any help due to the lack of molt (during this period) of juvenile birds.

Using Molt to Identify Cliff and Cave Swallows

Cliff Swallows are long-distance migrants and thus molt after they migrate south to their non-breeding range.  A species that is very similar to Cliff Swallow, the Cave Swallow, is a short-distance migrant.  So, if you see a Cliff/Cave Swallow that is molting flight feathers in the late summer-fall, it's highly likely that what you are looking at is a Cave Swallow.  

Referenced Literature:
Pyle, P. 1997. Identification Guide to North American Birds, Part I. Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.

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