Wednesday, September 23, 2015

Diversity of Warblers at Its Fall Peak

Migration continues to progress slowly along the shores of Lake Erie. Considerable SW winds have resulted in a slow but continuous change over in birds. Twenty warbler species were captured or seen over the first half of September including: Ovenbird, Northern Waterthrush, Golden-winged, Brewster's, Black-and-white, Tennessee, Nashville, Mourning, Common Yellowthroat, American Redstart, Cape May, Magnolia, Bay-breasted, Blackburnian, Chestnut-sided, Blackpoll, Black-throated Blue, Black-throated Green, Canada, and Wilson's.

The presence of Blackpoll Warbler, Gray Catbird, Swainson's Thrush, and Gray-cheeked Thrush has been constant. Their numbers will continue into early October.

Brewster's Warbler (BRWA)
A pleasant surprise for the season
Important knowledge gained through banding includes demographics of populations, timing of migration, productive success, and age ratios of various species. 

One such species beginning to arrive that can be easily aged and sexed (often through binoculars) is the Rose-breasted Grosbeak. They often reveal their presence with their distinctive and loud "chink" call. In fall migration the males, both Hatching Year (HY) and After Hatching Year (AHY) are donning their basic plumage where the body feathers match closely to those of the female grosbeak.
HY Male Rose-breasted Grosbeak (RBGR)
Note the begining of a pre-supplemental molt
in the greater and median coverts of this individual.
Note the pink or "rose" under the wings and a tiny spot of rose on the breast.
Dorsal View of the HY Male RBGR
The basic plumage looks similar to female. The few black feathers seen are a result of a pre-supplemental molt. It will molt into its alternate plumage on the wintering grounds.
Now, about those "Baypolls"...
As a group, the Bay-breasted and Blackpoll warblers have to be two of the most confusing fall warblers. There are distinctive leg coloration differences with the BLPW having lighter colored legs and yellow pads to its feet and the BBWA having darker legs with gray pads to its feet. These are helpful even in the field as the lighter leg color of the BLPW will reflect light differently than the dark gray leg of the BBWA. Additional field marks that help both in the field and the hand are the many streaks found on the flanks of BLPWs and the somewhat clear flanks of the BBWA. Both species show considerable streaking in their backs that separates them from the similar Pine Warbler that has a clear back.

Even in the hand, banders must work hard to determine the age and sex of BLPWs. Knowing the sex and age demographics of a population gives information supporting the condition of the population. For example, having no juveniles or HY during fall migration is a signal that there may have been poor production on the breeding grounds. Understanding the age and sex structure of the banded sample can give clues to where in migration a location is at in migrational timing for the given season, age ratios for life cycle modeling, and even some indication of how important a site may be to the species as adults tend to show more site fidelity.

Here are examples of After Hatch Year (AHY) male BLPW and BBWA. 
Blackpoll Warbler (BLPW) AHY male
Note: the light colored legs and streaking on the flank (just below the wing).
Dorsal view of the AHY male BLPW
Note: the many black streaks on the back. This often adds to the picture in determining the sex of a BLPW in the hand.
AHY Male Blackpoll Warbler
The dark bill is another useful tool in determining the age of this AHY bird.
AHY Male Bay-breasted Warbler 
Note the rounded alula edged in white and the "bay' wash on the flanks.
Dorsal view of the AHY male Bay-breasted Warbler
Note the "bay" feather on its head.
Diversity of species will continue to increase through the end of the month. Volume is just beginning to pick up for this fall, and diversity will continue to increase for a couple of weeks. Many species have not arrived at our latitude as of yet including: Hermit Thrush, White-throated Sparrow, kinglets, Brown Creeper, Fox Sparrow, and juncos. ---There's still a lot of migration to come. 

BSBO will host a free Public Banding Demonstration this Saturday, September 26th at 10:00 AM at the Observatory.

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