Food and social cues modulate reproductive development but not migratory behavior in a nomadic songbird, the Pine Siskin (Pinus spinus)

Many animals rely on photoperiodic and non-photoperiodic environmental
cues to gather information and appropriately time life history stages
across the annual cycle, such as reproduction, molt, and migration. Here,
we experimentally demonstrate that the reproductive physiology, but not
migratory behavior, of captive Pine Siskins responds to both food and
social cues during the spring migratory-breeding period. Pine Siskins are
a nomadic finch with a highly flexible breeding schedule and, in the
spring, free-living Pine Siskins can wander large geographic areas and
opportunistically breed. To understand the importance of non-photoperiodic
cues to the migratory-breeding transition, we maintained individually
housed birds on either a standard or enriched diet in the presence of
group-housed heterospecifics or conspecifics experiencing either the
standard or enriched diet type. We measured body condition and
reproductive development of all Pine Siskins and, among individually
housed Pine Siskins, quantified nocturnal migratory restlessness. In
group-housed birds, the enriched diet caused increases in body condition
and, among females, promoted reproductive development. Among individually
housed birds, female reproductive development differed between treatment
groups whereas male reproductive development did not. Specifically,
individually housed females showed greater reproductive development when
presented with conspecifics compared to heterospecifics. The highest rate
of female reproductive development, however, was observed amongst
individually housed females provided the enriched diet and maintained with
group-housed conspecifics on an enriched diet. Changes in nocturnal
migratory restlessness did not vary by treatment group or sex. By
manipulating both the physical and social environment, this study
demonstrates how multiple environmental cues can affect the timing of
transitions between life history stages with differential responses
between sexes and between migratory and reproductive systems.