Hornbills are omnivorous and the breeding male delivers all food required by the nest-confined female and chicks. The contributions of different food types, in terms of breeding nutrition, have not previously been documented. In Khao Yai National Park, Thailand, we sampled the identity and number of food items delivered daily to the nest, during each week of the nesting cycle, by two small and two large sympatric species of hornbills. We then recorded the mass and estimated the nutrient content of each food type from analyses of protein, fat, carbohydrate, calcium, and energy. The overall pattern of nutrient delivery during the nesting cycle was the same for each of the four hornbill species, and was related to sequential demands for egg, feather, and chick development. The two larger species delivered mainly carbohydrates (Great Buceros bicornis 50%, Wreathed Aceros undulatus 57%) and less fat and protein. The smallest, Oriental Pied Hornbill Anthracoceros albirostris, also delivered mostly carbohydrate (45%), but the small White-throated Brown Hornbill Anorrhinus austeni delivered equivalent proportions of protein (32%), fat (30%), and carbohydrate (37%). Comparison of the incubation and nestling phases showed that more protein was delivered during the nestling phase for all species, except for Great Hornbill where the compression of egg production, incubation, and molt had to be completed by midway through the nestling phase and so high levels of fat and protein were delivered during incubation. We confirmed that fruits are an important source of all nutrients, especially fat, for all four hornbill species, but suggest that delivery of animal protein may be linked, in some way, to breeding success. Oriental Pied Hornbill broods, that received protein at about 1.05% of brood mass per day, had the highest breeding success (96%) whereas Wreathed Hornbills received only 0.57% protein and had only 67% success, while the other two species delivered intermediate amounts of protein and had intermediate breeding success.

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