Helpers at the Nest in Australian Passerine Birds

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Abstract
Records of birds additional to the pair helping at single nests in Australian passerine species are summarized. This behaviour is known to occur in the Ground Cuckoo-Shrike, the Yellow-tailed Thornbill, the blue wrens, the Sittella, four species of honeyeater, the White-winged Chough and Apostlebird, the woodswallows, and the Pied Butcher-bird; and there is a likelihood of its occurrence in the babblers. The proportion of birds showing the behaviour is high in comparison with other avifaunas and it is probably due to some common factor, the likeliest being climate. In species such as the blue wrens, which are rather sedentary, this behaviour can help to maintain the population at a high level in a poor habitat by ensuring nesting success. In other species small nomadic parties are more likely to discover suitable areas, and to contain a breeding pair, while additional birds help in nesting and the rearing of young and can also act as replacements for lost members of the dominant pair. In the Australian species the helpers are often adults apparently capable of breeding and it seems possible that the behaviour could have been evolved in adults, and not from immature individuals assisting at the nest, as suggested by Skutch for other regions. Selection for such behaviour could occur if it is assumed that the dominant pair pass on to their offspring the dual capacity to act as a member of a dominant pair or as a helper in different situations. The reduction or loss of male breeding plumage in subordinate males in the blue wrens might help to prevent the arousal of aggressive reactions associated with such plumages. There appears to be no information available concerning possible factors which inhibit breeding behaviour in subordinate birds or determine the selection of dominant individuals.