Manipulations of brood age reveal limited parental flexibility in an insectivorous passerine

Abstract

For birds with altricial nestlings, behavioural flexibility in provisioning may be essential to success, as it enables parents to respond to changes in offspring demand. While parents often increase the quantity of food they provide as nestlings grow, they may also adjust different aspects of this suite of behaviours (i.e. feeding rate, prey type and size and distribution within broods) flexibly in response to nestling age. We tested whether generalist insectivores, mountain bluebirds, Sialia currucoides , respond to abrupt shifts in the body size of nestlings the same way they do when nestlings age normally. During a short-term (within-day) manipulation of nestling age, we recorded provisioning behaviour of parents feeding their own (‘natal’) and an age-manipulated (increased or decreased by 4 days) brood on the same day, in addition to recording provisioning ∼4 days before or after, when natal broods were the same age as manipulated broods. Parents responded to the age manipulation by altering feeding rates, but when broods aged naturally, parents altered the types and sizes of prey they delivered. Parents fed less often and provided larger prey when feeding younger-than-expected broods than when feeding young natal broods. Parents given older-than-expected broods fed more larvae, and fed more often, but provided smaller prey than supplied to older natal broods. Parents feeding older natal broods distributed prey more equitably among nestlings than those feeding either experimental older broods or younger broods (both natural and experimental broods), but this was not due to biased feeding of nestlings according to their body weight. Females fed more often than males and increased the quantity of larvae they provided to younger-than-expected broods. We suggest that parent mountain bluebirds can perceive and respond to both rapid and gradual changes in offspring size, but they may use different cues to adjust feeding rate and prey use. • Provisioning flexibility may allow parents to cope with changes in offspring demand. • Mountain bluebirds typically alter prey type but not feeding rate as nestlings grow. • We briefly manipulated nestling age (± 4 days) to examine parental responses. • During manipulation of brood age, parents altered feeding rate but not prey use. • Parents may adjust feeding rate and prey use in response to different cues.