Serenade of a whimbrel: understanding the function of display behaviour in a sub-Arctic territorial wader

Animal sounds contain important information used in intra- and inter-species communication. For species exhibiting elaborate and energetically expensive signals such as aerial displays accompanied by a call, the nature of the message being transmitted is honest and usually reflects individual fitness. Display events that combine flight and calls and are executed during the breeding season have traditionally been associated with two main functions: repelling rivals and/or attracting mates. In waders, the distinction between the two may rely on the timing of the breeding season at which displays occur, but also on the frequency at which displays occur in relation to local breeding density. Here, we investigated the function of display behaviour in the Icelandic whimbrel throughout the breeding season and along a breeding density gradient. We used a site-based approach in the southern lowland plain in Iceland where whimbrel breeding density and display frequency were recorded for two years. Although whimbrels showed display behaviour throughout the entire season, display frequency was higher at low densities during pre-incubation but similar at different densities during incubation and post-incubation. Our results indicate that this behaviour is context-dependent and multi-functional, supporting the importance of mate attraction at low conspecific density during pre-incubation, while other functions such as resource defence may be important throughout the breeding season.