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.