Data from: Provisioning tactics of great tits (Parus major) in response to long-term brood size manipulations differ across years

Parents provisioning their offspring can adopt different tactics to meet
increases in offspring demand. In this study, we experimentally
manipulated brood demand in free living great tits (Parus major) via brood
size manipulations and compared the tactics adopted by parents in 2
successive years (2010 and 2011) with very different ecological
conditions. In 2011, temperatures were warmer, there were fewer days with
precipitation, and caterpillars (the preferred prey of great tits) made up
a significantly larger proportion of the diet. In this “good” year,
parents responded to experimental increases in brood demand by decreasing
mean inter-visit intervals (IVIs) and reducing prey selectivity, which
produced equal average long-term delivery of food to nestlings across the
brood size treatments. In 2010, there was no evidence for effects of brood
size manipulations on mean IVIs or prey selectivity. Consequently,
nestlings from enlarged broods experienced significantly lower long-term
average delivery rates compared with nestlings from reduced broods. In
this “bad” year, parents also exhibited changes in the variance in
inter-visit intervals (IVIs) as a function of treatment that were
consistent with variance-sensitive foraging theory: variance in IVIs
tended to be lowest for reduced broods and highest for enlarged broods.
Importantly, this pattern differed significantly from that observed in the
“good” year. We therefore found some support for variance-sensitive
provisioning in the year with more challenging ecological conditions.
Taken together, our results show that variation in brood demand can result
in markedly different parental foraging tactics depending on ecological
conditions.