Data from: The effects of food supply on reproductive hormones and timing of reproduction in an income-breeding seabird

Current food supply is a major driver of timing of breeding in
income-breeding animals, likely because increased net energy balance
directly increases reproductive hormones and advances breeding. In capital
breeders, increased net energy balance increases energy reserves, which
eventually leads to improved reproductive readiness and earlier breeding.
To test the hypothesis that phenology of income-breeding birds is
independent of energy reserves, we conducted an experiment on
food-supplemented (“fed”) and control female black-legged kittiwakes
(Rissa tridactyla). We temporarily increased energy costs (via weight
handicap) in a 2 × 2 design (fed/unfed;
handicapped/unhandicapped) during the pre-laying period and observed
movement via GPS-accelerometry. We measured body mass, baseline hormones
(corticosterone; luteinising hormone) before and after handicap
manipulation, and conducted a gonadotropin-releasing hormone challenge.
Females from all treatment groups foraged in similar areas, implying that
individuals could adjust time spent foraging, but had low flexibility to
adjust foraging distance. Consistent with the idea that income breeders do
not accumulate reserves in response to increased food supply, fed birds
remained within an energy ceiling by reducing time foraging instead of
increasing energy reserves. Moreover, body mass remained constant until
the onset of follicle development 20 days prior to laying
regardless of feeding or handicap, implying that females were using a
‘lean and fit’ approach to body mass rather than accumulating lipid
reserves for breeding. Increased food supply advanced endocrine and laying
phenology and altered interactions between the
hypothalamic-pituitary-adrenal axis and the hypothalamic-pituitary-gonadal
axis, but higher energy costs (handicap) had little effect. Consistent
with our hypothesis, increased food supply (but not net energy balance)
advanced endocrine and laying phenology in income-breeding birds without
any impact on energy reserves.