Data from: Shifts in timing and duration of breeding for 73 boreal bird species over four decades

Breeding timed to match optimal resource abundance is vital for the
successful reproduction of species, and breeding is therefore sensitive to
environmental cues. As the timing of breeding shifts with a changing
climate, this may not only affect the onset of breeding, but also its
termination, and thus the length of the breeding period. We use an
extensive dataset of over 820K nesting records of 73 bird species across
the boreal region in Finland to probe for changes in the beginning, end,
and duration of the breeding period over four decades (1975-2017). We
uncover a general advance of breeding with a strong phylogenetic signal,
but no systematic variation over space. Additionally, 31% of species
contracted their breeding period in at least one bioclimatic zone, as the
end of the breeding period advanced more than the beginning. We did not
detect a statistical difference in phenological responses of species with
combinations of different migratory strategy or number of broods.
Nonetheless, we find systematic differences in species responses, as the
contraction in the breeding period was found almost exclusively in
resident and short-distance migrating species, which generally breed early
in the season. Overall, changes in the timing and duration of reproduction
may potentially lead to more broods co-occurring in the early breeding
season – a critical time for species’ reproductive success. Our findings
highlight the importance of quantifying phenological change across species
and over the entire season, to reveal shifts in the community-level
distribution of bird reproduction.