Social organisation and breeding biology of the White-shouldered Fairywren (<i>Malurus alboscapulatus</i>)

The White-shouldered Fairywren (Malurus alboscapulatus) is a tropical passerine bird distributed across much of New Guinea. White-shouldered Fairywrens are among few species of fairywren with exclusively tropical distributions and differ from better studied congeners in Australia because subspecies vary by female, but not male, coloration and morphology. As with many bird species in New Guinea, basic demographic, social, morphological, and breeding data are limited. From 2011 to 2018 we documented the basic biology of two subspecies representing extremes of the female ornamentation spectrum. Both subspecies form groups having an even operational sex ratio and appear to breed year-round. Extra-pair paternity occurs in the subspecies with female ornamentation; comparable data are lacking for the subspecies having unornamented females, but the greater scaled cloacal protuberance volume of males suggests similar or higher extra-pair paternity rates. Females of the ornamented subspecies are generally larger than those lacking ornamentation, but exhibit reduced tail lengths, which is thought to serve as a signal of social dominance in other fairywrens. After first achieving adult-like plumage, males and ornamented females retain ornamented plumage year-round; however, only males in the subspecies with unornamented females appear to exhibit delayed plumage maturation. Our discussion highlights similarities and differences between White-shouldered Fairywren life histories and those of better studied Australian Malurus species; we focus on tropical vs. temperate environments and variable female ornamentation, and we identify priorities for future research.