Egg discrimination by hosts and obligate brood parasites: a historical perspective and new synthesis

With the knowledge that cuckoos and cowbirds lay their eggs parasitically, and that some hosts eject parasitic eggs, ornithologists began to ponder the question of how host females discriminate between a foreign egg and their own eggs, wondering how hosts "know" which egg to remove. Results of one of the first uncontrolled experiments were inappropriately interpreted to imply ejection was based on discordancy, with hosts simply ejecting the egg in the minority, or the "odd-looking" egg. Controlled experiments eventually revealed that hosts first learn the appearance of own their eggs and discriminate between them and any odd egg in their nest, regardless of which egg type is in the minority. Recent work has shown that discordancy may play a role in discrimination by males mated successively with females that lay polymorphic eggs. We examine the details of the early experiments, in light of recent advances in studies of egg recognition. An ability to recognize eggs also has been extended, implicitly, to include obligate brood parasites, as it underlies several hypotheses in explanation of the behavior of parasites toward their hosts. Egg recognition in parasites, however, has not been experimentally confirmed, nor has a mechanism been identified by which parasites could discriminate between their own eggs and the other eggs in a nest. We review hypotheses (parasite competition, egg removal and multiple parasitism, mafia, farming) that require the ability of obligate brood parasites to discriminate eggs at different levels and the potential mechanisms used by parasites to recognize their own eggs and suggest experiments to test for egg discrimination. An assessment of the egg recognition ability of parasites is germane to our understanding of how parasites counteract defenses of hosts.