Migration distance, migratory and breeding phenology in Icelandic waders

Recent changes in phenology are widely reported across taxa, particularly
in relation to migration and breeding, and rates of change are often
greatest in species migrating over shorter distances. Species migrating
over longer distances might be less able to advance breeding phenology, if
they arrive on the breeding grounds later and breed very soon after
arrival, but opportunities to quantify trends in timings of arrival and
breeding across species that vary in migration distances but experience
similar environmental conditions in their breeding area are
rare. Several internationally important populations of waders,
spanning a wide range of migration distances, breed in Iceland. Decades of
monitoring of the phenology of these populations provide a unique
opportunity to explore the links between migration distance and phenology.
Timings of first spring arrival and egg laying were collected for common
waders breeding across lowland Iceland from 2007 to 2022. Information on
migration distances were collated from the literature or from individual
tracking of different populations. Waders wintering closer to
Iceland arrive significantly and substantially earlier in spring than
those wintering further away. However, the difference in lay dates among
these species is, on average, only ~1-2 weeks, because the timing of
breeding of early-arriving species varies with spring temperatures
substantially more than later-arriving species. Timing of laying is
advancing more in short- than long-distance species while, over this
survey period, time of arrival advanced more in long-distance than
short-distance species. Consequently, short-distance migrants are breeding
earlier despite little advance in arrival, while long-distance species are
arriving earlier but showing little advance in lay dates. The longer
arrival-laying gap in species travelling shorter distances therefore
appears to provide opportunities to breed early in warmer springs that are
not available to species travelling further and arriving later. Among
migratory species, population declines are currently more evident in
long-distance than short-distance species. Given the benefits of breeding
early in migratory systems, including opportunities to re-nest following
failure and greater recruitment rates of early-fledged offspring, the
opportunity of arriving sufficiently early to take advantage of warming
springs could be contributing to these divergent population trajectories.
Quantifying the phenology of nest success and fledging of species
migrating over different distances will help to identify the costs of
travelling further and arriving later during this period of rapid
environmental change at high latitudes.