While the process of skulling or looking at the pneumatization of the skull makes for a generally definitive aging tool, some circumstances--weather conditions, number of birds waiting to be banded, and lighting conditions to "see" the windows in the skull of Hatch Year (HY) birds--can make this method sub-optimal. Use of plumage characteristics has become a more reliable alternative with many advantages.
Returning birds of known age and sex can make for a valuable comparison tool, as well as providing useful characteristics of aging and or sexing individual species. Here are a few examples of known-aged Common Yellowthroats (COYE).
|COYE banded August 21, 2013 as a HY of unknown sex.|
Recaptured on September 26, 2015 as an AHY female.
|COYE Banded September 13, 2011 as HY Unknown|
Recaptured on September 27, 2015 as an AHY male
Note the bill color is dark signifying an AHY bird
|Common Yellowthroat underside bill colors|
HY bill on left and AHY on right
Note the darkness to the AHY bill
|Tail shape can assist in aging some species, including COYE.|
Left is HY bird with more pointed tips to the retrices.
Right is more truncate or rounded retrices of an AHY bird.
As a note. there are always exceptions to every rule. For warblers, the Prothonotary Warbler bill coloration differences appear to be reversed for the age classes, with AHY bills presenting much lighter and pinkish in color. Banding is a constant learning process, but there are so many rewards--for us and for the birds. The better we understand species demographics the better we can ensure their future.