Ecological opportunity leads to the emergence of an alternative behavioural phenotype in a tropical bird

SummaryLoss of a dominant competitor can open ecological opportunities. Ecological opportunities are considered prerequisites for adaptive radiations. Nonetheless, initiation of diversification in response to ecological opportunity is seldom observed, so we know little about the stages by which behavioural variation either increases or coalesces into distinct phenotypes. Here, a natural experiment showed that in a tropical island's guild of army ant‐following birds, a new behavioural phenotype emerged in subordinate spotted antbirds ( Hylophylax naevioides ) after the socially dominant ocellated antbird ( Phaenostictus mcleannani ) died out. Individuals with this behavioural phenotype are less territorial; instead, they roam in search of ant swarms where they feed in locations from which dominant competitors formerly excluded them. Roaming individuals fledge more young than territorial individuals. We conclude that ecological opportunity arising from species loss may enhance the success of alternative behavioural phenotypes and can favour further intraspecific diversification in life‐history traits in surviving species.