Thursday, November 13, 2014

Skulling 101

It was another good day at the banding station with 77 new birds and 17 recaptures made up of 8 different species.  Similar to yesterday, Yellow-rumped Warblers continued flying over in loosely formed flocks.  Occasionally a small flock would drop down to feed on the wax myrtle berries next to net 9 and this is where we got most of our 66 Yellow-rumps.  Since we are in the midst of Yellow-rump season I decided to bring back a post on determining age and sex of Yellow-rumped Warblers, you can check out the post written by Aaron back in 2012 by clicking here.  In that post, Aaron goes over the plumage characteristics that we examine in order to determine age and sex.  Sometimes we come across birds with conflicting plumage characteristics and we use skulling to determine age. 

When birds are first hatched they only have one layer of bone making up their skull.  Over the first couple of months to a year of a bird’s life a second layer of bone will grow below the first layer.  The second layer of bone usually follows one of two patterns which can be seen in the photo below.  Areas that only have the first layer of bone will appear pinkish.  The areas with two layers will appear more whitish and will also have tiny white spots.  The tiny white spots are columns between the two layers of skull.  When we skull we squirt a little water to the back of the birds head and part the feathers to create a window.  Bird’s skin for the most part is transparent which allows us to see through to the skull.  We move the window around to look for contrast between areas with two layer of bone versus the areas with only one layer.  If we find contrast this means the skull is unossified and the bird is a hatch year.  Fully ossified skulls will lack any contrast and will appear completely whitish with tiny whiter spots.  A bird with a fully ossified skull is not necessarily an adult.  Certain species can have their skull become fully ossified by October and November.  It’s always a good idea to combine multiple characteristics when determining the age of birds.  
The patterns of skull ossification

1 Eastern Phoebe
1 Ruby-crowned Kinglet
4 Hermit Thrush
3 Gray catbird
66 Yellow-rumped Warbler
2 Song Sparrow

3 Hermit Thrush
6 Gray Catbird
6 Yellow-rumped Warbler
1 Swamp Sparrow
1 Northern Cardinal

# of New Birds:  77
# of Recaptures:  17
# of Species:  8
Effort:  140.3 net-hours
Capture Rate:  67.0 birds/100 net-hours
# of Nets:  23

Aaron Given
Libby Natola
Matt Zak
Mattie VandenBoom