Meeting Abstract
For temperate zone birds, the trade-off between reproduction and self-maintenance is exemplified by the transition from breeding to plumage molt. In most temperate zone species, at most the earliest stages of primary (flight) feather molt overlap with reproductive competence. Crossbills (Loxia sp.) are temperate zone birds with flexible breeding schedules, reputed to breed in all seasons if conifer seeds (their primary food) are common. How they fit molt – essential to long-term survival – into such a reproductive schedule is a puzzle. Our nearly 30 years of field data show an “autumn hiatus” in reproductive competence, even in years with good seed crops. Primary feather molt overlaps extensively with persistent reproductive competence, but body molt coincides with reproductive collapse. Thus, unlike zebra finches, which overlap reproductive competence with a slow, protracted molt, crossbills molt seasonally and segregate the most demanding molt stages from breeding. This pattern may relate to sex steroid sensitivity. Although field data suggest that male crossbills with the highest testosterone (T) levels do not molt, captives with experimentally elevated T do eventually molt. T implants delay primary molt compared with controls, and the effect of T on timing and rate of body molt is greater than on primary molt. Thus, crossbill molt onset appears relatively insensitive to high sex steroids, so molt can proceed while circulating sex steroids are still above breeding baseline. Body molt, which is more sensitive to T, is delayed further if T remains high, so molt completion is facilitated by reproductive collapse, which leads to the decline of T below breeding baseline levels.
