All birds (for our intents and purposes) undergo one main molt, or growing-in of new feathers, each year. This molt is called the prebasic molt in AHY (after hatch year) birds, and the preformative molt in HY (hatch year) birds. Molts can be complete, incomplete, or partial. During a complete molt, birds grow in all new feathers--this includes all body feathers, and all flight (wing and tail) feathers. Birds that undergo an incomplete molt will grow in new body feathers and some, but not all, flight feathers. Finally, during a partial molt, birds grow in new body feathers and a varying number of wing coverts, but generally no flight feathers.
Now, why is this important? It is important because almost every adult (AHY) bird will undergo a complete prebasic molt, but the HY birds that have a prefomative molt will usually have a partial or incomplete molt. And it is specifically this partial or incomplete molt that will allow us to see a molt limit!
Molt limits are the "boundaries between replaced and retained feathers, resulting from partial or incomplete molts" (Pyle 1997). These boundaries are visible because the newer, replaced, feathers are generally less worn and less faded than older, retained, feathers. So, if all (minus an exception or two) adult birds have complete prebasic molts, then a bird showing a molt limit must be a........................hatch year bird!
We had a typical but interesting day at Palo today, with two Varied Thrush and a brand new hatching year female Spotted Towhee. Check out some photos and descriptions of how we age and sex these species.
|HY male Varied Thrush. Females would generally not have such dark auriculars (cheek band) and breast band. [Photo by Cory Ritter]|
|Male Varied Thrush. Deeper orange and blacker black than the previous HY above. [Photo by Luke Musher]|
|Male Varied Thrush. Note neater, better quality flight feathers, broad and rather truncate primary coverts, indicative of an after hatching-year bird. [Photo by Luke Musher]|
|HY female Spotted Towhee. Its brownish rather than blackish head, wings and back indicate that it's a female.|
|HY female Spotted Towhee. This Is textbook HY rectrix shape, narrow and tapered. [Photo by Dan Lipp]|
|Can you find the molt limit in this photo? [Photo by Dan Lipp]|