2365 Ecology, 79(7), 1998, pp. 2365–2375  1998 by the Ecological Society of America REPRODUCTIVE SEASONALITY IN AN OPPORTUNISTIC BREEDER, THE RED CROSSBILL, LOXIA CURVIROSTRA THOMAS P. HAHN 1 University of Washington, Department of Zoology, Box 351800, Seattle, Washington 98195-1800 USA Abstract. Crossbills (genus Loxia) are thought to breed opportunistically whenever food is abundant. If strictly true, this would distinguish them from most temperate zone birds that use photoperiod and supplementary cues (e.g., temperature, food supply) to time changes in reproductive physiology and behavior. I describe patterns of changing repro- ductive physiology in free-living Red Crossbills (Loxia curvirostra) as functions of season and conifer phenology. Nonbreeding spring migrants had elevated plasma luteinizing hor- mone (LH) and began gonadal development prior to finding a summer breeding site. The birds began breeding in July while feeding on maturing seeds of western hemlock (Tsuga heterophylla). Circulating LH declined in August, and gonads regressed completely as molt accelerated in September, although seed availability was at an annual high. Slight gonadal growth occurred after molt in late autumn and early winter, but reproductive maturation and nesting did not occur during winter in this study, possibly because western hemlock cones had shed most of their seeds. Winter/spring breeding (reproductive opportunism) at other locations was inferred from the presence of independent juveniles in early summer. Thus, activation of the reproductive system in summer and termination of breeding during molt in autumn appear to be regular seasonal phenomena in Red Crossbills, as in other temperate zone birds. Their opportunistic capabilities are superimposed on this underlying pattern. Key words: conifers; crossbills; environmental cues; Loxia spp.; luteinizing hormone; opportu- nistic breeding; photoperiodism; photorefractoriness; reproduction; reproductive cycles; seasonal breeding; western hemlock. INTRODUCTION Fluctuating environmental conditions require most animals to adjust their physiology, morphology, and behavior at various points in their lives. Appropriate timing of these adjustments depends on sensitivity to proximate cues from the environment (see Baker 1938). For example, most north-temperate zone birds use pho- toperiod as an ‘‘initial predictive’’ cue to synchronize reproductive cycles with the seasons (e.g., Farner 1970, 1975, Farner and Lewis 1971, Farner and Gwinner 1980, Wingfield and Farner 1980, Follett 1984), and use ‘‘supplementary cues’’ (cf. Wingfield 1980, 1983) such as temperature and food availability to fine-tune their reproductive schedules to local conditions. When changes in environmental conditions occur regularly (seasonally), then predominant reliance on a single, initial predictive cue such as photoperiod can evolve (see Cohen 1967, Wingfield et al. 1992). Environments that change less predictably require greater sensitivity to supplementary information as well (see Wingfield et al. 1992, 1993). Crossbills (genus Loxia) are north-temperate zone Manuscript received 18 December 1996; revised 17 July 1997; accepted 5 September 1997; final version received 19 December 1997. 1 Present address: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544 USA. finches that specialize on an unpredictable food source, conifer seeds (see McCabe and McCabe 1933, Lack 1944, Southern 1945, Newton 1973, Benkman 1987b). The erratic spatial and temporal distribution of this food supply (e.g., Fowells 1965) evidently has led crossbills to evolve an opportunistic reproductive schedule. Crossbills are renowned for breeding in any month (e.g., Griscom 1937, Bailey et al. 1953, Tordoff and Dawson 1965, Burleigh 1972), and are believed to use food supply as the primary proximate cue in timing reproduction (e.g., Newton 1973, Benkman 1990). This has led to the perception that crossbills are exceptional among temperate zone birds, most of which breed strictly seasonally and are prevented from breeding for part of the year by a period of reproductive refracto- riness to environmental stimuli (see Lofts and Murton 1968, Farner et al. 1983, Nicholls et al. 1988, Wilson and Donham 1988, Ball and Hahn 1997, Hahn et al. 1997). It remains unclear whether or not the physio- logical mechanisms that underlie reproductive sched- ules of seasonal breeders also permit opportunistic breeding (see Ball and Hahn 1997, Hahn et al. 1997). Data from captive Red Crossbills Loxia curvirostra (Tordoff and Dawson 1965, Hahn 1995) and free-living White-winged Crossbills L. leucoptera (Hahn et al. 1995) do not support Farner and Serventy’s (1960) hy- pothesis that tonic secretion of gonadotropic hormones from the pituitary might maintain the gonads of op-