Data from: The stress hormone corticosterone in a marine top-predator reflects short-term changes in food availability

In many seabird studies, single annual proxies of prey abundance have been
used to explain variability in breeding performance, but much more
important is probably the timing of prey availability relative to the
breeding season when energy demand is at a maximum. Until now,
intraseasonal variation in prey availability has been difficult to
quantify in seabirds. Using a state-of-the-art ocean drift model of larval
cod Gadus morhua, an important constituent of the diet of common
guillemots Uria aalge in the southwestern Barents Sea, we were able to
show clear, short-term correlations between food availability and
measurements of the stress hormone corticosterone (CORT) in parental
guillemots over a 3-year period (2009–2011). The model allowed the
extraction of abundance and size of cod larvae with very high spatial (4
km) and temporal resolutions (1 day) and showed that cod larvae from
adjacent northern spawning grounds in Norway were always available near
the guillemot breeding colony while those from more distant southerly
spawning grounds were less frequent, but larger. The latter arrived in
waves whose magnitude and timing, and thus overlap with the guillemot
breeding season, varied between years. CORT levels in adult guillemots
were lower in birds caught after a week with high frequencies of southern
cod larvae. This pattern was restricted to the two years (2009 and 2010)
in which southern larvae arrived before the end of the guillemot breeding
season. Any such pattern was masked in 2011 by already exceptionally high
numbers of cod larvae in the region throughout chick-rearing period. The
findings suggest that CORT levels in breeding birds increase when the
arrival of southern sizable larvae does not match the period of peak
energy requirements during breeding.