Function, Variation, and Implications for Food Allocation of the Nestling Gape in the Greater Ani

Abstract

The greater ani, a cooperatively breeding neotropical cuckoo, forms groups of 2 – 4 unrelated, socially monogamous pairs that lay their eggs in a single communal nest with all group members contributing to caring for the clutch and raising the chicks. Data was collected from nest groups that are part of an ongoing study in the Barro Colorado Nature Monument. In this study, I analyze the ontogeny of the conspicuous black and white flanges and yellow gape pattern of the nestlings, how the signal changes with competition, and how this influences adult feeding decisions. For the first 4 – 5 days after hatching for 19 nests, we took daily morphometric measurements of the chicks and daily photos of individual nestling’s gapes. We marked chicks with a unique paint combination and set up a camera at the nest to monitor feeding events. I used MATLAB to select 7 regions of the gape signal and determine the average colors within those selections. We coded nest videos – identifying who was fed, who ultimately received the food, prey size and type, the begging intensities of all nestlings, proximity to the adult, and relative heights – and I used this data in conjunction with the color values to analyze whether the gape signal plays a significant role in parental feeding decisions. Nestling gapes showed a significant increase in size for groups that were subject to higher levels of nestmate competition. Increased size in nestlings was predictive of higher yellow coloration in the rictal flanges and inner mouth. Other measured regions of the gape showed no relationship with nestling fitness. Mouth coloration was not a significant predictor of whether a nestling was fed, however, it did near significant when looking at who actually received the prey.