WILSON BULLETIN Monday Oct 10 2005 10:14 AM Allen Press • DTPro System GALLEY wils 117_409 Mp_390 File # 09TQ 390 Wilson Bulletin 117(4):390–393, 2005 SEXUALLY DIMORPHIC BODY PLUMAGE IN JUVENILE CROSSBILLS PIM EDELAAR, 1,2,3,6,7 RON E. PHILLIPS, 4 AND PETER KNOPS 5 ABSTRACT.—Sexual dimorphism in color and pattern of contour feathers is rare in juvenile songbirds. We describe how captive-bred juvenile males of Scottish Crossbill (Loxia scotica) and nominate Red Crossbill (L. curvirostra curvirostra) can be differentiated from females prior to prebasic molt by an unstreaked patch on the males’ upper breast. There may be a functional relationship between sexual dimorphism and the formation of pair bonds or breeding while the birds are still in juvenile plumage. Sexually dimorphic Red Crossbills and Bearded Tits (Panurus biarmicus) are known to form pair bonds, and even breed successfully, while still in juvenile plumage. Received 6 August 2004, accepted 10 July 2005. Among songbirds, sexual dimorphism in ju- venile flight feathers (which are often retained until after the first breeding season) is not un- usual, but sexual dimorphism in juvenile con- tour feathers is rare (Pyle et al. 1987, Svens- son 1992). Sexual dimorphism in juvenile crossbills (Loxia spp.) has not been reported in the scientific literature (e.g., Svensson 1992, Cramp and Perrins 1994, Adkisson 1996), but, in an unreviewed bulletin for breeders of captive birds (United Kingdom), Castell (1983) reports that juvenile crossbills are sexually dimorphic. Females are described as completely streaked on the underparts, from the base of the lower mandible to the belly. Males differ in that they have a yellow- ish, unstreaked band or patch at the upper breast, just below the throat (Fig. 1) and an unstreaked chin (but see Fig. 1). In addition, the streaks on the breasts of males are less bold, narrower, and rounder-edged (not square-edged), and the ground color of the breast is a richer color (more yellowish, not whitish). Here, we address the reliability of using the unstreaked breast patch to sex ju- 1 Dept. of Biology, New Mexico State Univ., Las Cruces, NM 88003, USA. 2 Dept. of Zoology, Univ. of British Columbia, Van- couver, BC V6T 1Z4, Canada. 3 Dept. of Theoretical Biology, Univ. of Groningen, 9750 AA, Haren, The Netherlands. 4 Yetholm, St. Catherine’s Place, Elgin, Moray, Scot- land, United Kingdom. 5 Molsteeg 37, 6369 GL Simpelveld, The Nether- lands. 6 Current address: Dept. of Ecology and Evolution- ary Biology, Univ. of Arizona, Tucson, AZ 85721, USA. 7 Corresponding author; e-mail: w.m.c.edelaar@umail.leidenuniv.nl venile crossbills. We only assessed and report on results pertaining to the breast patch; no quantitative information was available to us for evaluating the reliability of other reported sexually dimorphic traits, although we concur that juvenile males are generally more yellow- ish in color than juvenile females. From 1993 to 2003, we tested the validity of using the unstreaked patch to sex 228 ju- venile crossbills bred in captivity. All birds were kept in chicken wire and metal-frame aviaries. Adults and chicks were fed with commercial birdseed, supplemented with grit, eggshell or fish bone, and high protein egg feed, and were provisioned regularly with co- nifer cones. Birds were banded as nestlings with uniquely numbered bands. Pedigrees were known, and most birds were related due to regular inbreeding. The putative and actual sex of each bird was determined by each of three breeders. Our study entailed sexing juveniles from two different crossbill taxa. The identification of some crossbill taxa can be challenging, and birds in the wild should be identified primarily on the basis of vocalizations, measurements, and geographic location (Groth 1993, Sum- mers et al. 2002, Edelaar et al. 2003). Despite the fact that calls of captive birds are unlike those of wild birds, the (nominate) Red Cross- bill (Loxia curvirostra curvirostra) is readily distinguished in captivity from other crossbill taxa by bill and body size, as long as the part- ly overlapping—but typically larger—Scottish Crossbill (L. scotica) can be excluded. Our Red Crossbill stock originated from continen- tal Europe (Germany, Austria, and Russia) and had not been crossed with other crossbill