Research in Action – How to Age a Tree Sparrow

Banding projects of the Kalamazoo Valley Bird Observatory take place year round, even during the winter.  Fewer species are in Michigan during the colder months, but the ones that are here can be abundant.  One such species is the American Tree Sparrow (ATSP).  Each winter since 2007, we’ve banded 300-400 ATSP.  Last winter, an additional 50 ATSP, first banded in previous years, were recaptured.

Our technical banding guide, Identification Guide to North American Birds, Part I by Peter Pyle, indicates that, from January through March, 95% of ATSP can be aged as either second year (SY), meaning the bird hatched in the previous year, or after second year (ASY), meaning the bird hatched two or more years ago.  (All birds are assigned a birthday of January 1.)  Banders use the molt pattern in the wing to determine the bird’s age:  The primary covert feathers (short feathers directly above the outer flight feathers) are retained during the first molt while the adjacent coverts over the secondary flight feathers are replaced, creating a contrasting “wear and sun fading” pattern.  (This is true for most, but not all, songbirds.)  While this ageing system is reliable for most species in the summer, we are not certain that it’s accurate for ATSP during the winter.  From our observations of recaptured birds, this contrasting pattern seems to appear on all ages in January through March.

Since October 2011, we have been taking two pictures of each known-age ATSP.  One picture shows the primary and secondary coverts to document presence or absence of contrast.  The second photo documents any contrast in the tertiary feathers (the three innermost feathers of the wing) and the secondary flight feathers, as these feathers may molt at different times too.  For our known-age birds, we used a different ageing technique:  “Skulling” looks at the ossification of the skull.  Any bird that is still growing in the second layer of bone is a young bird and hatched during the previous breeding season.  These are the known SY birds.  Sparrows banded in previous years are the known ASY birds.

At this writing, we have photographed 66 known SY and 57 known ASY Tree Sparrows.  We’ll continue to take photos every three or so weeks as we recapture these same birds.  By this spring, we will either confirm or refute our ability to age American Tree Sparrow as second year or after second year during the January to March period.