Abstract
ABSTRACTTo time reproduction optimally, birds have evolved diverse mechanisms by which they respond to environmental changes that help them anticipate and prepare for the breeding season. While residents initiate reproductive preparation and breed in the same geographic location, migrant birds simultaneously prepare for breeding and migration far from their breeding grounds. As a result, it is hypothesized that migrant and resident birds use environmental cues differently to prepare to breed and that there is adaptive specialization in mechanisms regulating reproductive preparation. Specifically, residents are expected to rely more on non-photic cues (e.g. food, temperature, social cues) than migrants. We tested this general prediction using a social cue manipulation. First, we compared the effects of subspecies-appropriate recorded male song on reproductive development in migrants and residents on a naturally increasing photoperiod. Second, we tested the sensitivity of migrant-specific life history events (fattening and pre-alternate molt) to song treatment. After 82 days, residents had higher luteinizing hormone and greater ovarian development than migrants, but song treatment had no effect on these metrics in either subspecies. Song advanced pre-alternate molt but had no effect on fattening in migrants. While our study does not support specialization in social cue use in migrants and residents, it is consistent with findings in the literature of specialization in photoperiodic response. It also demonstrates for the first time that social cues can influence molt in a migrant species. Additional findings from a pilot study looking at responses to a live male suggest it is important to test other kinds of social cues.
Highlights
IntroductionThe decision of when to breed can have dramatic fitness consequences
For seasonally breeding vertebrates, the decision of when to breed can have dramatic fitness consequences
Effects of song treatment on reproductive readiness in migrants and residents luteinizing hormone (LH) secretion was higher in residents than in migrants (β=0.315, t=3.378, P=0.002; Fig. 1; Table S2), and the change in LH increased over time in both subspecies (β=0.003, t=3.794, P
Summary
The decision of when to breed can have dramatic fitness consequences Selective environmental factors such as weather, food availability and predators can affect the optimal time frame for reproduction. Reproduction can occur year-round (e.g. sooty terns; Chapin and Wing, 1959), seasonally (e.g. most temperate zone birds), or opportunistically in response to resource peaks (e.g. wild zebra finch response to rain; Zann, 1996). These variable reproductive schedules are coupled with complementary variation in timing of maintenance activities such as molt, as well as migration and dispersal, which require careful timing decisions
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