Data from: Unpredictable perturbation reduces breeding propensity regardless of pre-laying reproductive readiness in a partial capital breeder

Theoretically, individuals of migratory species should optimize
reproductive investment based on a combination of timing of and body
condition at arrival on the breeding grounds. A minimum threshold body
mass is required to initiate reproduction, and the timing of reaching this
threshold is critical because of the trade-off between delaying breeding
to gain in condition against the declining value of offspring with later
reproductive timing. Long-lived species have the flexibility within their
life history to skip reproduction in a given year if they are unable to
achieve this theoretical mass threshold. Although the decision to breed or
not is an important parameter influencing population dynamics, the
mechanisms underlying this decision are poorly understood. Here, we
mimicked an unpredictable environmental perturbation that induced a
reduction in body mass of Arctic pre-breeding (before the laying period)
female common eiders (Somateria mollissima; a long-lived migratory
seaduck) while controlling for individual variation in the pre-laying
physiological reproductive readiness via vitellogenin (VTG) - a
yolk-targeted lipoprotein. Our aim was to causally determine the
interaction between body condition and pre-laying reproductive readiness
(VTG) on breeding propensity by experimentally reducing body mass in
treatment females. We first demonstrated that arrival body condition was a
key driver of breeding propensity. Secondly, we found treatment and VTG
levels interacted to influence breeding propensity, indicating that our
experimental manipulation, mimicking an unpredictable food shortage,
reduced breeding propensity, regardless of the degree of pre-laying
physiological reproductive readiness (i.e., timing of ovarian follicles
recruitment). Our experiment demonstrates that momentary environmental
perturbations during the pre-breeding period can strongly affect the
decision to breed, a key parameter driving population dynamics.