Song functions for joint territory defence and within-pair communication in female and male lovely fairy-wrens

Song has been extensively studied in birds and is considered a classic example of a sexually selected trait. However, this interpretation is based predominantly on studies of males. There is growing evidence that female song is common and phylogenetically widespread, but there are still relatively few species for which song similarities and differences between the sexes have been described. We investigated the function of female and male song in the lovely fairy-wren, Malurus amabilis, a tropical species that maintains territories year-round. We tested whether song structure and sex-specific seasonal variation in song rates across breeding and nonbreeding stages were consistent with expectations of key hypotheses for song function. We did this with temporarily caged and free-living birds. We also experimentally examined female and male responses to simulated territorial intrusion, using playbacks simulating solo intruders of either sex singing songs that varied in complexity (i.e. the variety of elements in song). Females and males had similar song structure, complexity and natural song rates and both sang more during nonbreeding than breeding periods. Within each breeding stage, males sang more than females during incubation, a period of female-only investment in parental care. Both sexes sang more when they were more than 5 m apart than when close together in their dense tropical habitat. Females and males responded similarly to simulated intrusion and coordinated their responses. They responded more strongly to male than female song playbacks and to simple playback songs after they were exposed to complex songs, during both breeding and nonbreeding periods. Our observational and experimental data show that female and male songs are similar and suggest that convergent selective pressures may explain similar functions of song, primarily in joint territorial defence of resources and within-pair communication for coordination.(c) 2022 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.