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Testis Mass and Subadult Plumage in Black-Headed Grosbeaks

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Testis Mass and Subadult Plumage in Black-Headed Grosbeaks The Condor96:6X-630 0 The CooperOrnithological society 1994 TESTIS MASS AND SUBADULT PLUMAGE IN BLACK-HEADED GROSBEAKS GEOFFREY E. HILL Department of Zoology and Wildlife Scienceand Alabama Agricultural Experiment Station, FunchessHall 331, Auburn University,Auburn, AL 36849-5414 Abstract. Like males of approximately 30 other speciesof North American passerines, male Black-headedGrosbeaks (Pheucticus melanocephalus) do not attain complete definitive alternate plumage until their second breeding season.I collected yearling and adult male Black-headed Grosbeaks in the Rio Grande valley in central New Mexico to test the hy- pothesisthat delayed plumage maturation in Black-headedGrosbeaks is one manifestation of a general, hormonally-mediated developmental pattern. I compared the mass of testes to the degreeof plumagematuration in eleven yearling males and found a significantpositive correlation. Moreover, the mean testis mass of yearling males was significantly lower than the mean testis mass of older males. In contrast, I found no significantrelationship between plumagebrightness and testis massamong sevenadult males in definitive alternate plumage. These observations support the idea that delayed plumage maturation in Black-headed Grosbeaksis part of a general delay in investment in first-year reproduction. Key Words: Delayedplumage maturation; hormones;life history: plumage color;Pheuc- ticus melanocephalus. INTRODUCTION season, and, in recent years, a growing number Males of most speciesof temperatepasserine birds of studies have also begun to focus both on non- attain a definitive alternate plumage in the first breeding functions and constraints to producing spring after their birth and attempt to breed. In ornamental plumage (Rohwer and Butcher 1988, approximately 31 species of sexually dichro- Butcher and Rohwer 1989, Zack and Stutchbury matic North American passerines, however, 1992). Despite the effort that has been devoted males fail to attain a complete definitive alternate to understanding delayed plumage maturation, plumage in their first breeding season and may however, there is still no consensuson why this or may not attempt to breed (Rohwer et al. 1980, developmental pattern evolved and is main- Rohwer and Butcher 1988). In specieswith such tained (Thompson 1991, Zack and Stutchbury delayed plumage maturation, the extent to which 1992) or much data on the proximate factorsthat yearling males develop definitive plumage is determine the extent of plumage development in variable, with some individuals achieving a more yearling males. adult-male-like appearance than others (e.g., Black-headed Grosbeaks (Pheucticusmelano- Flood 1984, Hill 1988a, Enstrom 1992). More- cephalus)are typical of temperate passerinesthat over, whether or not subadult males attempt to show delayed plumage maturation. They are sex- breed is often related to the degree of plumage ually dichromatic with ASY (after second year: maturation that they display-yearlings with a in a second or later breeding season)males dis- more adult-male-like appearance are generally playing a complex, brightly colored nuptial more likely to breed than those with a more fe- plumage and with females having a much drab- male/juvenile appearance (Flood 1984; Hill ber streaked brown plumage (see Hill 1987 for 1988a, 198813;ButcherandRohwer 1989). Thus, a detailed description ofplumage). Yearling male the patterns of reproductive behavior and plum- Black-headed Grosbeaks are intermediate in ap- age development are linked in many species. pearance between females and ASY males but Numerous studies have addressed the adap- highly variable: a few nearly resemble females, tive function of subadult plumage in the breeding showing only a few features of male definitive- alternate plumage; a few look much like ASY males, showing only a few features of female/ juvenile plumage; most show a more balanced ’ ’ Received 10 December 1993. Accepted 7 March mix of ASY male and female/juvenile charac- 1994. teristics(Hill 1988a). Previous researchhas shown f62f.4 GROSBEAK TESTIS MASS 621 0.016’ ’ ’ ’ ’ ’ ’ ’ L 0.044 Yearling Males 0 0 Adult Males 0.012. 0 0.03. 0 0 0 0 0 0 0.02. 0 0.008. 0 0 0 0 0.004- 0 0.01. o 0 0 4 . c 25 30 35 40 45 50 52 54 56 58 60 62 64 female-like _ adult-male-like drab W bright Plumage Brightness Score FIGURE 1. The relationshipbetween plumage brightness and testismass corrected for body mass.Plumage brightnessof yearlingmales reflects extent of first prealtematemolt. Plumagebrightness of ASY malesreflects brightnessof orangeand yellowventral plumage. that most yearling male grosbeaksforego breed- yearling Black-headedGrosbeaks, I compared the ing and that only thoseyearlings with adult-male- plumage scoreand testismass of the 11 SY males. like plumage defend territories and attempt to To correct for the potential confounding effects breed (Hill 1988a, 1988b, 1989). The purpose of of body size, I used testis mass divided by body the present study was to test the idea that degree mass in the comparison. I found a significant of plumage development is related to the degree positive correlation (rs= 0.9 1, n = 11, P = 0.004, of testes development and thus to test the hy- Fig. 1)-yearling males with brighter plumage pothesis that there is a proximate (hormonal) (i.e., more extensive definitive alternate plum- mechanism for the associationbetween breeding age) tended to have larger testes. behavior and plumage coloration in subadult All ASY male Black-headed Grosbeaks dis- males. play definitive alternate plumage and thus show the same pattern of nuptial plumage, but they vary substantially in plumage brightness (inten- METHODS AND RESULTS sity of ventral orange and yellow) (Hill 1987, I collected seven adult and 11 subadult male 1988a). Therefore, I also compared the plumage Black-headed Grosbeaks (Pheucticus melano- brightnessand testismass of adult males. I found cephalus)along the Rio Grande in the vicinity no significant relationship between the plumage of Los Lunas, New Mexico in May and June brightnessofASY males and testis size (rr = 0.40, 1985. For each bird that I collected, I scoredthe n = 7, P = 0.33, Fig. 1). The small number of plumage brightness as outlined in Hill (1987, males on which this last comparison was based 198 8a). For 16 plumage regionsI assigneda score made the power of the test weak and the chance of 0 (dullest/most female-like) to 3 (brightest/ of detecting a significant relationship low. most adult-male-like). The 16 scoreswere then Grosbeaks were collected over a 32-day inter- summed to give an estimate of overall plumage val during the peak period of nesting. To test for brightness that ranged from 0 to 48 (see Hill the potential effects of season on testis size, I 1987, 1988a for details). Immediately after being compared testis massand collecting date for both collected the birds were weighed to the nearest ASY and yearling males. There was no significant 0.1 g, and within a few hours after they were relationship between collecting date and testis collected, the birds were dissectedand both their size for either ASY (rs= - 0.33, n = 7, P = 0.42) right and left testes were removed and weighed or yearling (rs = 0.02, It = 11, P = 0.93) males. to the nearest 0.01 g. I also compared the testis mass of subadult and To see if there was a relationship between the ASY males, again correcting for body size. Adult plumage development and testis development in males had significantly heavier testesthan year- 628 GEOFFREY E. HILL .04- % 0 .03- 0 z o- Yearling Males Adult Males FIGURE2. Testismass of yearling(n = 11)and ASY(n = 7) maleBlack-headed Grosbeaks. Box plots illustrate IOth, 25th, 5Oth,75th and 90th percentileswith horizontallines and showall datapoints outside this range. lings (U = 67, it = 11, 7, P = 0.01, two-tailed stimulate the development of additional struc- Mann-Whitney U Test; Fig. 2). tures and behaviors related to reproduction (Eis- ner 1960, Murton and Westwood 1977). The en- docrinological control of molt is less clear. In at DISCUSSION least one tropical species,molt is stimulated by This is the first report linking extent of yearling testosterone: subadult male Satin Bowerbirds male plumage development to testis mass in a (Ptilonorhynchusviolaceus) implanted with tes- specieswith delayed plumage maturation. Mol- tosteroneproduced definitive plumage years be- ler and Erritzoe (1988) found a similar positive fore untreated males in the same agecohort (Col- relationship between testis volume and the size lis and Borgia 1992). Yearling male Black-headed of the black throat patch in House Sparrows Grosbeaksderive their female/juvenile plumage (Passerdomesticus), a speciesin which yearling characteristicsfrom the retention of first basic males are on average less well ornamented than feathers,not by molting drab feathersduring their adults but lack a distinctive subadult plumage. first pre-alternate molt (unpubl. data). Thus, it Observations presented in this paper as well as is the extent of the first pre-alternate molt that previously published studies of Black-headed determines the degree to which yearlings attain Grosbeakssupport the idea that delayed plumage definitive plumage. For temperate bird species, maturation is part of a generalreproductive strat- fall molt seems generally to be stimulated by egy in which many features of breeding-testes thyroxin and inhibited by androgenssuch as tes- size, pre-alternate molt, timing of migration, ag- tosterone(Payne 1972). In the spring, androgens gressiveness,song output,
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