Local weather and endogenous factors affect the initiation of migration in short- and medium-distance songbird migrants

Migratory birds employ a variety of mechanisms to ensure appropriate
timing of migration based on the integration of endogenous and exogenous
information. The cues to fatten and depart from the non-breeding area are
often linked to exogenous cues such as temperature or precipitation and
the endogenous program. Shorter-distance migrants should rely heavily on
environmental information when initiating migration given the relatively
close proximity to the breeding area. However, the ability to fatten and
subsequently depart may be linked to individual circumstances, including
current fuel load and body size. For early and late departing migrants, we
investigate the effects of temperature, precipitation, lean body mass,
fuel load, and day of year on the initiation of migration (i.e., fuel load
and departure timing) from the non-breeding region by analyzing 21 years
of banding data for four species of short- and medium-distance migrants.
Temperatures at the non-breeding area were related to temperatures at
potential stopover areas. Despite local cues being predictive of
conditions further north, the amount of variation explained by local
weather conditions in our models differed by species and temporal period
but was low overall (< 33% variation explained). For each species,
we also compared lean body mass and fuel load between early and late
departing migrants, which showed mixed results. Our combined results
suggest that most individuals migrating short or medium distances in our
study did not time the initiation of migration with local predictive cues
alone, but rather other factors such as lean body mass, fuel load, and day
of year, which may be a proxy for the endogenous program, and those beyond
the scope of our study also influenced the initiation of migration. Our
study contributes to understanding which factors influence departure
decisions of short- and medium-distance migrants as they transition from
the non-breeding to the migratory phase of the annual cycle.