Arrival-breeding interval is flexible in a songbird and is not constrained by migration carry-over effects

As spring phenology advances with climate change, so too must the timing
of life cycle events. Breeding at the right time is critical in many
species as it maximises fitness. For long-distance migratory birds,
flexibility in the duration of the arrival-breeding interval (pre-breeding
period) may allow populations to adjust their timing of breeding. However,
whether first egg-lay dates are flexible to local environmental conditions
after arrival, and if they are constrained by the time needed to replenish
energy lost during migration, remains unclear. We investigated the
regional flexibility of the arrival-breeding interval in an avian migrant,
the purple martin Progne subis, across their breeding range. We evaluated
whether the duration of the arrival-breeding interval was flexible to
temperature and precipitation at breeding sites, and if timing was limited
by migration rate and stopover duration. We also tested if longer interval
durations were associated with higher fledging success. To address our
hypotheses, we used a combination of migration tracking, weather, and
breeding data collected from four regions across eastern North America
(26.1ºN to 52.4ºN latitude). We found the arrival-breeding interval to be
shortest in the north and longest in the south. Across all regions, warmer
temperatures encountered at breeding grounds were associated with shorter
intervals, and faster migration rates led to longer intervals. The length
of the interval was not influenced by precipitation or stopover duration.
Lastly, longer intervals were not associated with higher fledge success.
Currently, the longer arrival-breeding intervals in this study system, on
average 28.3 days, may provide both early- and late-arriving birds with
ample time for recovery so birds can lay eggs according to temperature.
Any negative effects of faster migration may have been buffered by longer
arrival-breeding intervals, as interval length did not determine fledge
success. With ongoing climate change, further research is needed to
examine if arrival-breeding intervals become constrained by migration
timing, which may limit opportunities for migrants to match the timing of
breeding with key resources.