Sex-specific provisioning of nutritious food items in relation to brood sex ratios in a non-dimorphic bird

Abstract: In birds, the frequency with which the parents feed the young can vary considerably. Because of sexual differences in the begging behaviour and/or differences in the food requirements of the nestlings, brood sex ratio (BSR) is an important factor that may influence parental provisioning behaviour. Disparities in the quantity and quality of prey received by the sexes have been reported in a range of sexually size-dimorphic birds. However, to our knowledge, no study has evaluated prey composition delivery to nestlings in relation to BSR in a non-dimorphic size bird species. Because BSR influences provisioning rate in dimorphic and non-size dimorphic species and because in dimorphic species, BSR influences prey composition delivered to the nest, we hypothesised that similar to dimorphic species, BSR may influence prey composition delivered to nestlings in non-size dimorphic species. We quantify parental provisioning rate and prey composition of prey delivered to nestlings in relation to BSR in the Thorn-tailed Rayadito (Aphrastura spinicauda) a non-dimorphic and altricial passerine bird. At the population level, we found that Thorn-tailed Rayadito mothers delivered more insect larvae to the nest when compared to the father, who provided the brood with a diet more diverse in composition. However, when we considered BSR, mothers delivered a greater quantity of arachnida and lepidoptera items (high-quality foods) in male-biased BSR. In addition, nestling weight gain increased in line with the proportion of high-quality food in the diet. Our results suggest that when considering non-dimorphic species, there may be more subtle, but nevertheless important, differences, in explaining parental care behaviour in species with bi-parental care. Significance statement: In birds, the frequency with which the parents feed the young can vary considerably. Because of sexual differences in the begging behaviour and/or differences in the food requirements of the nestlings, brood sex ratio is an important factor that may influence parental provisioning behaviour in sexual size species. For the first time, we evaluated prey composition delivery to nestlings in relation to BSR in a non-size dimorphic bird species. We found that the mother of the Thorn-tailed Rayadito delivered a greater quantity of lepidoptera and arachnida (high-quality food) items in a male-biased brood. In addition, nestling weight gain increased with the proportion of high-quality food in the diet. Our results suggest that in non-dimorphic species, there may be more subtle, but nevertheless important, differences in explaining parental care behaviour in species with bi-parental care.