Saturday, September 19, 2015

Aging Red-eyed and White-eyed Vireos

Captain Sam's 
It was an average day at Captain Sam’s today with 46 new birds comprised of 10 species and 1 recapture.   As you may have guessed, Common Yellowthroats were the most common species banded today with 34.  It seems we will continue with a similar bird activity pattern for the next few days with things possibly picking up midweek. 

Mixed among the Common Yellowthroats today were our two most common species of vireos, Red-eyed Vireo and White-eyed Vireo.  Most of the birds we band at KIBS are aged based on a molt limit found in the wing but when it comes to these two vireos species, the first thing we look at is eye color.  Interestingly enough, when these species are first hatched out they do not have the eye color that they are named for.  Red-eyed Vireos start off with a dark brown iris and over a period of a couple months will attain the bright red iris that the species is known for.  White-eyed Vireos begin life with a gray brown iris that progressively gets whiter over a period of several months.  The full adult eye color is usually reached during their first winter or late fall, making aging based on eye color only useful during fall migration.  When using eye color to age, as with any other aging technique, it is usually a good idea to look at other characteristics such as skull ossification and molt limits to age with certainty.  This was certainly the case today when we captured a White-eyed Vireo with a white eye.  After studying the bird's plumage, we noticed a couple of things that pointed away from an adult bird.  The outer tails feathers were narrow and tapered, and the primary coverts were faded.  So we checked the bird's skull to see if it was completely ossified, and sure enough, it was not, meaning that this individual was a hatch-year bird (see last photo).            

Red-eyed Vireo (top: hatch-year, bottom: after hatch-year)

White-eyed Vireo (top: hatch-year, bottom: after hatch-year)
Hatch-year with white eye similar to an adult.
This bird was probably hatched early in the summer which gave it plenty of time to acquire a white eye by mid-September.  

Little Bear
We had another slow day at Little Bear today, although better than yesterday. We captured 22 new bands and 6 recaptures, of 11 species. About half of our new birds were Common Yellowthroats, but we also had a few other warbler species around today and Little Bear got its first-of-season Gray Catbird! There will be many more to come.

Gray Catbird (Hatch-year, sex unknown)
Photo by Casey Weissburg

  Species Captain Sam's Little Bear
New Recaps New Recaps
White-eyed Vireo 1- - 1
Red-eyed Vireo1 1 - 1
Gray Catbird - - 1 -
Ovenbird 1- 1 -
Northern Waterthrush1 - - -
Common Yellowthroat 34 - 12 -
American Redstart 1 - - 1
Yellow Warbler 3 - 1 -
Palm Warbler (western) 1 - 1 -
Prairie Warbler 1 - 1 -
Northern Cardinal - - - 2
Painted Bunting 2 - 4 1

 Banding Stats Captain Sam's Little Bear TOTAL
# Birds Banded 46 21 67
# of Recaptures 1 6 7
# of Species 10 11 12
Effort (net-hours) 127.5 50 177.5
Capture Rate (birds/100 net-hours) 36.9 42.0 41.7
Nets 25 10 35