Non-photoperiodic regulation of reproductive physiology in the flexibly breeding pine siskin (Spinus pinus).

In order to time reproduction to coincide with favorable conditions, animals use environmental cues to up- and down-regulate the reproductive axis appropriately. Although photoperiodic cues are one of the best studied of such environmental cues, animals also attend to others such as temperature, food availability, rainfall and social cues. Such non-photic cues are expected to be particularly important for tropical species and temperate-zone species that exhibit flexible or opportunistic breeding schedules. In this study, we investigate the use of non-photic cues, specifically food availability and social cues, to time the initiation of reproductive development in the pine siskin (Spinus pinus), a temperate-zone songbird with a flexible breeding schedule. Following winter solstice, males were housed on a 12L:12D photoperiod with either access to a preferred food, a potential mate (social cue), or both. Control birds received only maintenance diet and no mate. Access to a preferred food had a significant positive effect on testis size and circulating luteinizing hormone (LH). However, we found no effect of social treatment on reproductive development. The effect of the food treatment on reproductive development did not appear to result from effects on body mass or fat, as neither measure differed across treatments. The food treatment influenced not only reproductive physiology, but also reproductive behavior in this species, as access to seeds had a positive effect on affiliation of pairs. This study demonstrates that food is a potent stimulus for the initiation of reproductive development in pine siskins.