An Introduction to Molt Limits: Part III


This adult male was Malheur NWR headquarters on 30 May 2010 (Dave Irons photo)

We figured it best to save the easiest of the three birds for last, so here's a little eye candy. Few North American birds rival the stunning appearance of an adult male Western Tanager in alternate plumage. On a bright sunny day, they can be absolutely aglow. 

In fresh alternate plumage, this species is entirely lemon yellow below, on the lower back and rump, and on the lower neck. The forehead is fire engine red fading to bright reddish-orange behind and above the eye and on the throat. The back of an adult is solid black with none of the pale margins shown by first summer males. As seen on the bird to the right, the wing and tail feathers of alternate-plumaged adult males are very dark dusky chocolate brown (not quite black). These feathers are the only ones that are not replaced during the prealternate molt (Spring 2010), so they are retained from the last prebasic molt, which occurred in the late Summer/early Fall of 2009. Thus the tail and flight feathers are approximately nine months old, while all the other feathers (the really bright ones) are only 2-3 months old at most.

If this three-part series has piqued your interest in molt, suggested reading includes the Peterson Reference Guide to Molt in North American Birds by Steve N.G. Howell and the Identification Guide to North American Birds Part I, by Peter Pyle.

An Introduction to Molt Limits: Part II


This SY male Western Tanager was at Diamond Craters, Harney County, Oregon 31 May 2010 (Dave Irons photo)

In Part I we examined a particularly obvious example of  a bird showing feathers of different ages within the same feather set. That bird had a few stray new feathers growing into mostly older feather sets, so the contrast was easily seen. In this piece we will look at a SY (first summer) male Western Tanager that is more uniform in appearance, thus the age-related molt limits are not as apparent.

In overall plumage this bird is duller and less colorful than the SY individual in Part I. Aside from three new tertials on each wing (the long black, white-edged inner-most wing feathers that bracket the rump and lower back) the flight feathers of this bird are uniform in age. Similarly, all of the tail feathers (rectrices) are faded brown or olive-brown. This bird also shows less red feathering in the face and seems even paler on the flanks. Like the other SY bird in Part I, the back feathers of this bird are neatly edged in buffy olive, creating a scalloped look.

You may be wondering why two birds of the same relative age don't look exactly the same at the same time of year? While there is a basic timing to when different molt sequences occur, the manifestation of prealternate molts (which are partial) vary from individual to individual. Conversely, the prebasic molt, which occurs from late summer to late fall depending on the species, is complete (after hatch-year) with all feathers being replaced in advance of either fall migration or the winter season. Upon completing a prebasic molt, after hatch-year (AHY) birds within a species will show little if any individual variation.

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.