Intra-specific plasticity in parental investment in a long-lived single-prey loader

Seabirds exhibit considerable behavioural flexibility
in foraging investment in order to meet the nutritional
needs of their chicks during variable environmental conditions.
Although regulation of offspring provisioning is generally
thought to be related to species-specific constraints
imposed by central place foraging, some studies suggest
different responses within the same species linked to local
differences in foraging conditions. Under adverse conditions,
seabirds are expected to be less flexible because they
must secure their own survival chances first before investing
in current reproduction. Short-ranging single-prey loaders
are expected to show large intra-specific variation in time
spent on foraging because their mode of foraging is energetically
expensive, and because they face restricted possibilities
to increase the numerical prey input to the colony
compared to multiple prey loaders. In this study, we examined
if and how the single-prey loading Sandwich Tern
Thalasseus sandvicensis varies colony attendance based on
the nutritional status of their chick as well as parental body
condition in two study colonies. The proportion of time that a
chick was left unattended at the colony negatively correlated
with chick body condition, suggesting that the parents tried
to counterbalance poor feeding conditions by investing more
time in foraging. Energy transport rates to the chicks
(corrected for time spent away from the colony) and body
condition of the chicks were similar in both colonies. However,
at Zeebrugge, where adults were in poor body condition,
parental non-attendance was much lower than on
Griend, even when chicks were in poor condition. Still, our
results suggest that parental nest non-attendance in Sandwich
Terns is merely a corrective response to food loss to
kleptoparasitic gulls in order to meet the nutritional status of
the chick, although an effect of adult body condition could
not be excluded.