EFFECT OF HELPERS ON NESTLING GROWTH AND SURVIVAL IN A NEOTROPICAL HOST OF A BROOD PARASITIC COWBIRD

ROJAS RIPARI JUAN MANUEL; REBOREDA JUAN CARLOS; DE MÁRSICO MARÍA CECILIA

Congreso; NORTH EASTERN NATURAL HISTORY CONFERENCE; 2021

Cooperative breeding in birds is a reproductive system in which one or more adults (helpers) assist others in rearing their offspring. It has been found that cooperative breeding is more prevalent among hosts of obligate brood parasites than non-host species in Australasian and sub-Saharan African avifaunas. This pattern suggests possible evolutionary and ecological interactions between cooperative breeding and brood parasitism, but the interplay between these two systems is poorly understood. The presence of helpers at the nest could benefit host parents if it reduces the costs of parental effort and/or enhances nesting success by limiting within-brood competition at parasitized nests. In turn, brood parasites could benefit from using cooperatively breeding hosts if they obtain more resources at helped nests. Here we examined the association between cooperative breeding and nestling growth and survival in nests of Agelaioides badius (Greyish Baywing, hereafter Baywing), primary host of the specialist Molothrus rufoaxillaris (Screaming cowbird). We intensively monitored 84 breeding attempts over four breeding seasons (2015 to 2018) and collected data on brood composition, daily mass growth of host and parasitic nestlings (n= 107 and 47, respectively), group size (parents plus helpers) and nest predation events. Richard?s models of mass growth of Baywing and Screaming Cowbird nestlings did not detect differences in growth patterns between nests with and without helpers. However, Baywing nestlings showed reduced mass growth in nests with multiple parasitism. Generalized linear models of nest predation revealed a negative association between the presence of helpers and nest predation rates during the nestling stage. Our results suggest that helpers? contribution to nest provisioning does not enhance mass growth of host and parasitic nestlings, nor does it fully compensate for the detrimental effects of parasitism on body condition of host broods. However, cooperative breeding could be beneficial to both Baywing and Screaming Cowbird in terms of reducing nest predation rates. Whether this effect is the result of collective nest defense or a side effect of larger groups occupying safer territories needs further investigation.