An Introduction to Molt Limits: Part I

In the birding vernacular, the term "molt limit" is suddenly all the rage. Formerly, only museum staffers and bird banders used the comparative shape and age of feather sets to identify and age birds. Properly assessing the shape and age of particular feather sets was definitely a hands-on activity. More recently, two factors are bringing molt study out of the museum tray and mist net. First, digital images now allow us to blow up photos of birds to the point where we can examine individual feathers and feather sets with little if any loss in resolution. Secondly, recent works by several authors, most notably those of Peter Pyle and Steve N.G. Howell, have helped demystify molt for average birders, many of whom have never critically examined the feathers of a bird live or dead. 

This first-summer (SY) male Western Tanager was photographed at Malheur NWR, Harney County,

Oregon 30 May, 2010. (Dave Irons photo)

For the purposes of getting started, we offer this one-year-old/first summer (SY in banding terminology) male Western Tanager. Note that nearly all flight and tail feathers are faded brown rather than near black as they would be on an adult male. These feathers, which are retained from juvenile plumage grown during the summer of their hatch-year (2009), contrast sharply with the darker blackish back and covert feathers that were replaced during the partial prealternate molt this Spring (2010). Also notice the sharp contrast between the old feathers and the newer (blackish) tail feather (rectrix) and secondary in the left wing. Further, notice that the fresh back feathers are not quite as black as they would be on an adult (see photo in "Part III") and they show narrow paler olive edges. Finally, the flanks of this bird are dingy with a grayish or olive cast, whereas an adult male would have brighter lemon yellow flanks.