Changes in daylength links latitudinal and seasonal trends in clutch
AbstractNear ubiquitous reproductive trends across taxa follow a pattern in
which output increases with latitudes and decreases with calendar date.
Research grounded in life history theory provides separate ultimate
explanations for latitudinal and seasonal trends. Here we frame these
dual trends as a Simpson's paradox and attempt to gain insights into
proximate cues that might account for both simultaneously. Using citizen
science data on Eastern bluebirds, we found highest support for a model
of clutch size based on change in day length at clutch initiation.
Describing reproductive trends based on non-biologically relevant
constructs of latitude and calendar date obscured links between
proximate and ultimate explanations. For birds, our findings are
consistent with an internal coincidence model of circadian rhythmicity
as a proximate control of clutch size. Other avian studies might benefit
from viewing clutch size as a circadian behavior of clutch initiation
and termination rather than a quantified trait.