North American Birds

VOLUME 69 No1 2016

A Quarterly Journal of Ornithological Record Published by the American Birding Association. The mission of the journal is to provide a complete overview of the changing panorama of our continent’s birdlife.

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Page 14 of 179

13 V O L U M E 6 9 ( 2 0 1 5 ) • N U M B E R 1 T H E M E N D O C I N O S H R I K E ment proceeds bidirectionally (both proxi- mally and distally) from a molt node (or center) at p3, p4, or p5 in most individuals (Figure 5), although in some individuals it may proceed distally from p1, while other individuals may show both of these sequenc - es, e.g., p1 molting before the proximal wave from a central node reaches p2 (Stresemann and Stresemann 1971, Svensson 1992, Cramp and Perrins 1993). Such variation between primary replacement sequences has been recorded for other passerine species (Jenni and Winkler 1994, Neto and Gossler 2006, Junda et al. 2012) and allows birds showing bidirectional replacement to com - plete molt of primaries more quickly than during unidirectional molt (Pyle 2013a). It is possible that those individuals with nodes at both p4-p5 and p1 are undergoing the second prebasic molt after retaining the ju - venile p1 from the year before (see below), as this sort of sequential "memory" between molts (beginning where the previous molt had arrested, as in Staffelmauser, or step - wise molt, in larger birds) has been recently grants within each taxon tend to molt more extensively on winter grounds than shorter- distance migrants. This is likely due to ad - aptations related to elevated solar exposure, which degrades feathers, necessitating more molt, and to increased nutrient availability, which may allow for more feather produc - tion, both in the context of two summer sea- sons per year. Overlaid upon this variation, birds that did not breed or that failed breed - ing early (including many one-year-olds) tend to commence fight-feather molt earlier and on the breeding grounds, whereas suc - cessful breeders tend to migrate southward before commencing this molt at a later date (Pyle 2008: 500-505; Pyle and Reid, in press). Juveniles of these shrikes proceed to un - dergo a preformative (post-juvenile) molt that can vary between partial and incomplete or (probably) complete. All body feathers are replaced primarily in August–December, usually commencing on breeding grounds and completing on winter grounds in longer- distant migrants. A few to most or all fight feathers are then replaced primarily in De - cember through April or May. As in shore- birds such as Calidris sandpipers, the farther south an individual winters, the more fight feathers are often replaced, from a few ter - tials and/or central rectrices in northern-win- tering individuals, to most or all fight feath- ers in trans-equatorial and longer-distance migrants. This variation appears to occur at the individual level, as confounded by each taxon's average wintering latitude. A partial frst prealternate (frst pre-breeding) molt of some to most body feathers occurs in March– May, which can overlap the completion of preformative fight-feather molt in southern- wintering individuals. Adults undergo a defnitive prebasic (adult post-breeding) molt that parallels the preformative molt but is complete, with body feathers replaced primarily in August– December and fight-feather molt variably commencing sometime in August–Janu - ary and completing sometime in October– March. In northern-wintering individuals and failed breeders, the molt is often initi - ated or completed in fall, whereas in trans- equatorial migrants and successful breeders, the fight-feather molt can be suspended for southbound migration and be protracted through spring due to lack of winter-related constraints (Pyle 2008: 500-505); or it can occur entirely on the winter grounds as late as January–March. A defnitive prealternate (adult pre-breeding) molt of body feathers then occurs in February–April, overlapping some or most of the fight-feather molt in southern-wintering individuals. During both preformative and defnitive prebasic molts, sequence of primary replace - Figure 8. Specimens at AMNH illustrating frst-cycle upperpart plumages in male Red-backed Shrikes. From left to right, juvenile (661117 collected 15 October 1902), individuals undergoing the preformative molt (661058 collected 12 October 1886, 661043 collected 8 February 1905, and 661046 collected 7 February 1905), and an individual in frst alternate plumage (463918 collected 22 April 1941). Note the dark subterminal chevrons throughout the juvenile feathering, and the variation in the three birds undergoing the preformative molt, including the expression of alternate-male-like plumage as early as October and the lack of chevrons in these birds by commencement of primary molt in February. Note also the variation in the head, back, and rump plumage in the two birds collected a day apart in February. The bird in alternate plumage (463918) was collect - ed in Iran from duller eastern populations of Red-backed Shrike (see also Figure 9). Photograph by Peter Pyle.

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