Iberian magpie

Abstract: In many species of cooperative breeding birds, breeders and helpers participate in the parental care with different food provision rules. Normally, helpers feed nestlings less frequently and with smaller quantities of food than breeders. But studies analysing the reaction of feeders to nestling demand are scarce and rarely measure the quantity of food that is actually delivered. In this study, we analysed the provisioning effort of breeders and helpers in the Iberian Magpie, Cyanopica cooki, and how this effort varies with brood demand. We did so by measuring the nestling feeding rate and the biomass supply of each individual. In this way, we obtained a more accurate measurement of the investment assumed by each individual belonging to each status. We found that breeding males visited the nest more often than both breeding females and helpers (mean = 2.24; 0.85 and 1.58, respectively). Furthermore, breeding males delivered more biomass in each feeding visit to the nest than those from other statuses. Breeders, both male and female, increased their parental effort (i.e., provisioning rate and biomass) when brood demand was higher (i.e., more siblings and older nestlings), whereas helpers contributed differently to the nest, but depending on the two types of helpers occurring in this species. Differences in the possible benefits obtained by breeders and helpers may explain these different strategies. In addition, male and female breeders (but not helpers) reduce the feeding rate throughout the breeding season. Thus, in the Iberian magpie, breeders and helpers reveal different patterns of investment depending on nestling food demand.

Abstract: Cooperative breeders receive help with offspring care from non-breeding individuals, named helpers. In the presence of helpers, parents can reduce their work-load without consequences for their offspring, a phenomenon called load-lightening. Load-lightening at the feeding stage is a common strategy among cooperative breeders. Russell et al. 2017 have shown that for the superb fairy-wren, load-lightening can already occur at the egg stage, in which females that are assisted by helpers lay smaller eggs. They predicted that load-lightening at the egg stage also is a widespread phenomenon among cooperatively breeding species. In this study, this prediction is tested by examining the effect of helper presence on egg size in ten cooperatively breeding bird species: the superb fairy-wren, carrion crow, sociable weaver, brown-headed nuthatch, acorn woodpecker, southern lapwing, Iberian magpie, laughing kookaburra, dunnock and the red-winged fairy-wren. For four of these species, it was demonstrated that females breeding in the presence of helpers laid smaller eggs than in the absence of helpers. In the other six species the effect was the other way around (helper presence increased egg size) or there was no effect at all. Therefore, load-lightening at the egg stage does not appear to be a widespread phenomenon in cooperatively breeding birds. However, there is a chance that the correlation between helper presence and egg size is non-causal, since both egg size and helper presence might be correlated to the quality of the territory and/or breeders. Future studies investigating the role of helper presence in cooperative breeders should take this into account.