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Archive for October 3rd, 2012

It is hard to believe it’s already October, but we might have guessed by the number of Myrtle Warblers moving through.  Since the calendar flipped we have caught 142 Myrtles, making up over 55% of the total of newly banded birds in the last 3 days.  As we noted in our last post, these birds can winter at fairly high latitudes owing to a specialized digestive tract that allows them to eat berries that are inedible to most other species.  They are also interesting in that they are 1 of only 6 warblers that breed in North America that undergo marked changes in plumage aspect from season-to-season.  Pictured below are a young female (L) and adult male, both in non-breeding or ‘basic’ plumage.

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Fortunately for us, ageing Myrtle Warblers is fairly straightforward, as this species tends to show obvious molt-limits.  Molt-limits are simply differences in the appearance and structure of feathers due to the different times at which they were grown.  In the wing-spread of the Myrtle Warbler below, the difference between the recently molted, fresh alula covert versus the pale middle alula indicates that this is a young bird…

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…and the lack of a difference between the same feathers on a different bird below indicates that this is an adult (this is the wingspread of the adult male pictured alongside the female in the first photo above).

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Speaking of molt-limits, Red-eyed Vireos can show some pretty dramatic differences between feather generations as well.  Below is a young bird that had a rather limited molt compared with most of the other vireos we have processed.  Note the retained outer greater coverts – most of the hatch year individuals we have seen have molted these completely.  The Red-eyed Vireo is noted for producing multiple clutches per season, so it may be that this individual hatched later in the summer and did not have time to molt as extensively as birds that hatched earlier.

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In addition to the increasing numbers of Myrtle Warblers, we have seen more White-throated Sparrows on the move as well.  Like Myrtle Warblers, these sparrows are relatively short-distance migrants.  They abandon boreal breeding grounds in Canada to overwinter in coastal/southern U.S., as well as northeastern Mexico.  Unlike Myrtle Warblers, there is limited dichromatism between males and females, and identifying the sex of an individual is generally impossible outside of the breeding season.  Pictured below is a young bird making its first foray south.

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We typically catch more Golden-crowned Kinglets here than their Ruby-crowned counterparts, so it was a treat to have been visited by this young male Ruby-crowned Kinglet.  This is a species that shows marked dichromatism between the sexes, as males are characterized by the eponymous red crown that is sometimes used to intimidate rival males in territorial disputes.

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And finally, a photo of Bear Cove, about 200 meters due west of our banding station.

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Being stationed on a peninsula can result in some pretty ‘birdy’ days, as migrants funnel down its length in search of food.  We’ve already banded 100 or more birds on a couple of days, and if previous years are any indication, we’ll likely be in for more during this period of ‘peak passage’.

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