WILSON BULLETIN Monday Oct 10 2005 10:14 AM wils 117_409 Mp_390 Allen Press • DTPro System GALLEY File # 09TQ

Wilson Bulletin 117(4):390±393, 2005

SEXUALLY DIMORPHIC BODY IN JUVENILE

PIM EDELAAR,1,2,3,6,7 RON E. PHILLIPS,4 AND PETER KNOPS5

ABSTRACT.ÐSexual dimorphism in color and pattern of contour feathers is rare in juvenile songbirds. We describe how captive-bred juvenile males of Scottish (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 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 crossbills. We only assessed and report venile ¯ight feathers (which are often retained on results pertaining to the breast patch; no until after the ®rst breeding season) is not un- quantitative information was available to us usual, but sexual dimorphism in juvenile con- for evaluating the reliability of other reported tour feathers is rare (Pyle et al. 1987, Svens- sexually dimorphic traits, although we concur son 1992). Sexual dimorphism in juvenile that juvenile males are generally more yellow- crossbills (Loxia spp.) has not been reported ish in color than juvenile females. in the scienti®c literature (e.g., Svensson From 1993 to 2003, we tested the validity 1992, Cramp and Perrins 1994, Adkisson of using the unstreaked patch to sex 228 ju- 1996), but, in an unreviewed bulletin for venile crossbills bred in captivity. All birds breeders of captive birds (United Kingdom), were kept in chicken wire and metal-frame Castell (1983) reports that juvenile crossbills aviaries. Adults and chicks were fed with are sexually dimorphic. Females are described commercial birdseed, supplemented with grit, as completely streaked on the underparts, eggshell or ®sh bone, and high protein egg from the base of the lower mandible to the feed, and were provisioned regularly with co- belly. Males differ in that they have a yellow- nifer cones. Birds were banded as nestlings ish, unstreaked band or patch at the upper with uniquely numbered bands. Pedigrees breast, just below the throat (Fig. 1) and an were known, and most birds were related due unstreaked chin (but see Fig. 1). In addition, to regular inbreeding. The putative and actual the streaks on the breasts of males are less sex of each was determined by each of bold, narrower, and rounder-edged (not three breeders. square-edged), and the ground color of the Our study entailed sexing juveniles from breast is a richer color (more yellowish, not two different crossbill taxa. The identi®cation whitish). Here, we address the reliability of using the unstreaked breast patch to sex ju- of some crossbill taxa can be challenging, and birds in the wild should be identi®ed primarily 1 Dept. of Biology, New Mexico State Univ., Las on the basis of vocalizations, measurements, Cruces, NM 88003, USA. and geographic location (Groth 1993, Sum- 2 Dept. of Zoology, Univ. of British Columbia, Van- mers et al. 2002, Edelaar et al. 2003). Despite couver, BC V6T 1Z4, Canada. the fact that calls of captive birds are unlike 3 Dept. of Theoretical Biology, Univ. of Groningen, those of wild birds, the (nominate) Red Cross- 9750 AA, Haren, The Netherlands. 4 Yetholm, St. Catherine's Place, Elgin, Moray, Scot- bill (Loxia curvirostra curvirostra) is readily land, United Kingdom. distinguished in captivity from other crossbill 5 Molsteeg 37, 6369 GL Simpelveld, The Nether- taxa by bill and body size, as long as the part- lands. ly overlappingÐbut typically largerÐScottish 6 Current address: Dept. of Ecology and Evolution- Crossbill (L. scotica) can be excluded. Our ary Biology, Univ. of Arizona, Tucson, AZ 85721, USA. Red Crossbill stock originated from continen- 7 Corresponding author; e-mail: tal (Germany, Austria, and Russia) [email protected] and had not been crossed with other crossbill 390 WILSON BULLETIN Monday Oct 10 2005 10:14 AM wils 117_409 Mp_391 Allen Press • DTPro System GALLEY File # 09TQ

Edelaar et al. • DIMORPHIC PLUMAGE IN JUVENILE CROSSBILLS 391

FIG 1. A juvenile female (left) and male (right) (Loxia scotica). Note the continuous streaking across the underparts of the female and the unstreaked patch on the upper breast of the male (upper arrows), the denser streaking on the underparts (lower arrows) of the female (dark stripes wider than pale stripes) than those of the male (pale stripes wider than dark stripes), and the more yellowish ground color of the male than that of the female. Contrary to initial reports, however, both sexes seem to have a streaked chin.

taxa. The Scottish Crossbills used in this study Benkman 1995). If carotenoids are not pro- were all progeny of wild birds caught in the vided, the normally red males develop a yel- Scottish highlands and subsequently isolated lowish, female-like plumage. On the other from other crossbill taxa during breeding. Our hand, unlike many other species, female Scottish Crossbill stock has an average bill crossbills in captivity may develop a red size thought to correspond closely with that of plumage when provided with food that is ar- wild Scottish Crossbills (REP and R. W. Sum- ti®cially enriched with carotenoids (e.g., Hill mers unpubl. data), con®rming that it did not and Benkman 1995). Hence, overall color derive from sympatrically breeding smaller- (yellowish or reddish) is of little use when at- billed Red and larger-billed Parrot crossbills tempting to sex adult birds in captivity. Ca- (L. pytyopsittacus). rotenoid-enriched food was given only to The putative sex of individuals in juvenile some juveniles (putative males) during the plumage was determined only by the presence study, but this did not preclude us from cor- (males) or absence (females) of an unstreaked rectly sexing all of them after they had un- patch on the upper breast, before any juvenile dergone prebasic molt. Whether in yellow or contour feathers had been molted. Actual sex red plumage, males have brighter, unmarked was determined by plumage after prebasic feathers on the crown and have a colored (yel- molt. It is known that adult coloration of cap- low or reddish) chin; females (at least in the tive crossbills is dependent on whether many taxa we investigated here) have small dark carotenoids are provided during molt (Hill and spots on the crown and have a grayish chin WILSON BULLETIN Monday Oct 10 2005 10:14 AM wils 117_409 Mp_392 • Allen Press DTPro System GALLEY File # 09TQ

392 THE WILSON BULLETIN • Vol. 117, No. 4, December 2005

TABLE 1. Reliability of body plumage for sexing captive-bred juvenile crossbills. Putative sex of juveniles was based on the presence (males) or absence (females) of an unstreaked patch on the upper breast and compared to actual sex following prebasic molt. P is from a Fisher exact test.

Males Females

Taxon n Putative Actual Putative Actual P Scottish Crossbill 205 101 99 104 104 Ͻ0.001 Red Crossbill 23 8 8 15 15 Ͻ0.001

(Phillips 1977, Castell 1983; REP pers. obs.). Red Crossbills, may even reproduce success- In offspring kept for subsequent breeding, sex fully while still in juvenile plumage (Glutz determined on the basis of these adult plum- von Blotzheim and Bauer 1993, Adkisson age characteristics was always con®rmed by 1996; K. van Eerde pers. comm., PE pers. reproductive behavior. obs.). Because reproductive behavior is at We used Fisher exact tests to determine the least as rare as sexual dimorphism among correspondence between the putative sex de- birds in juvenile plumage, the coincidence of termined by juvenile plumage and the actual these two traits suggests a possible functional sex determined after prebasic molt. A signif- relationship. However, a few other icant P-value (P Ͻ 0.05) indicates that the pu- species that exhibit sexual dimorphism in ju- tative and actual sex correspond better than venile body plumage (e.g., Serinus citrinella predicted by chance. We assumed that there [Borras et al. 1993], S. serinus [Senar et al. were no effects of relatedness, parental care, 1998], Parus major [DomeÁnech et al. 2000], or rearing environment; we also assumed no P. caeruleus [Johnsen et al. 2003]) are not differences between observers. known to form pair bonds or breed while in Scottish and Red crossbills were sexed in juvenile plumage. Thus, we hypothesize that juvenile plumage with a high degree of reli- species that form pair bonds or reproduce ability (Table 1). Only 2/205 juvenile female while still in juvenile plumage will show sex- Scottish Crossbills were sexed incorrectly (P ual dimorphism in juvenile body plumage; the Ͻ 0.001), and all 23 Red Crossbills were reverse is not necessarily true. For instance, sexed correctly (P Ͻ 0.001) in juvenile plum- the Red Crossbill subspecies L. c. tianschan- age. We also obtained small sample sizes for ica is often reported to breed in juvenile plum- Parrot, Himalayan (L. c. himalayensis), and age (Edelaar et al. 2003); therefore, we predict Two-barred crossbills (L. leucoptera bifascia- that juvenile males of this taxon can be dis- ta). Preliminary information suggests that sex- tinguished easily from juvenile females on the ual dimorphism in juvenile plumage of these basis of traits we describe in this paper. taxa is not as evident as in Scottish and Red crossbills, as several individuals of both sexes ACKNOWLEDGMENTS were identi®ed incorrectly as males or fe- We thank M. Kleijnen for kindly providing data on males: 6/20 (30%), 2/6 (33%), and 3/7 (43%), his Parrot Crossbills. We thank K. Roselaar for corre- respectively. In order to determine the useful- sponding with us about sexual dimorphism in the ness of plumage dimorphism to sex juvenile plumage of Western Palearctic birds and life history of crossbills of different taxa, more data on sex- Bearded Tits, and C. W. Benkman for discussing sex- ual dimorphism in juvenile body plumage ual dimorphism in White-winged Crossbills and for hosting PE. We thank M. Marquiss, R. Rae, J. C. Sen- should be collected, especially in wild cross- ar, and an anonymous reviewer for comments on a bills and for the many Eurasian and North previous version of the manuscript. Our study was sup- American subspecies of Red Crossbill. ported by a TALENT stipend from the Netherlands There appears to be a correlation between Organization for Scienti®c Research and an Honorary life history and the occurrence of sexual di- Scholarship from the University of Groningen to PE. morphism in juvenile body plumage. Juvenile Bearded Tits (Panurus biarmicus), which LITERATURE CITED have sexually dimorphic contour feathers, ADKISSON, C. S. 1996. Red Crossbill (Loxia curviros- normally form pair bonds and, like juvenile tra). The Birds of , no. 256. WILSON BULLETIN Monday Oct 10 2005 10:14 AM wils 117_409 Mp_393 Allen Press • DTPro System GALLEY File # 09TQ

Edelaar et al. • DIMORPHIC PLUMAGE IN JUVENILE CROSSBILLS 393

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