As the avian embryo grows and develops within the egg, its metabolic rate gradually increases. Obligate avian brood-parasitic birds lay their eggs in the nests of other species to avoid the costs of parental care, and all but one of these brood-parasitic species are altricial at hatching. Yet the chicks of some altricial brood-parasitic species perform the physically demanding task of evicting, stabbing or otherwise killing host progeny within days of hatching. This implies a need for high metabolic rates in the embryo, just as precocial species require. Using flow-through respirometry in situ, we investigated embryonic metabolic rates in diverse avian brood parasite lineages which either kill host offspring (high virulence) or share the nest with host young (low virulence). High-virulence brood parasite embryos exhibited higher overall metabolic rates than both non-parasitic (parental) species and low-virulence parasites. This was driven by significantly elevated metabolic rates around the halfway point of incubation. Additionally, a fine-scale analysis of the embryos of a host-parasitic pair showed faster increases in metabolic rates in the parasite. Together these results suggest that the metabolic patterns of the embryos of high-virulence parasites facilitate their early-life demands.

Advanced Science Research Center and Programs in Biology and Psychology Graduate Center of the City University of New York New York NY 10031 USA

American Museum of Natural History New York NY 10024 USA

Auckland University of Technology Auckland 1010 New Zealand

c o Musumanene Farm Choma Zambia

Department of Biological Sciences School of Life and Environmental Sciences Royal Holloway University of London Egham Surrey TW20 0EX UK

Department of Biology The University of Oxford Oxford OX1 3SZ UK

Department of Zoology University of Cambridge Downing Street Cambridge CB2 3EJ UK

FitzPatrick Institute of African Ornithology DST NRF Centre of Excellence University of Cape Town Rondebosch Cape Town 7701 South Africa

School of Biological Sciences Monash University Clayton Victoria 3800 Australia

The Czech Academy of Sciences Institute of Vertebrate Biology Květná 8 Brno 603 65 Czech Republic

The Natural History Museum Tring Herts HP23 6AP UK

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Highly virulent avian brood-parasitic species show elevated embryonic metabolic rates at specific incubation stages compared to less virulent and non-parasitic species