Communally Breeding Australian Birds with an Analysis of Distributional and Environmental Factors

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Abstract
SUMMARY Dow, D. D. 1980. Communally breeding Australian birds with an analysis of distributional and environmental factors. Emu 80: 121–140. A terminology is introduced that can be used for describing as well as theorizing and is thus useful for hierarchically classifying observations. All known communally breeding species in Australia are tabulated: sixty-five in twenty families. Evidence is presented for species being described as communal breeders for the first time and for species in which additional data have become available since the last major review (Rowley 1976). Published maps of the ranges of thirty-one species were coded digitally by degree-block for analysis by computer as were maps of vegetation, faunal zones and eleven environmental variables. The distribution of these species is tabulated by the vegetational and faunal zones that they occupy and a detailed statistical tabulation is included of all the environmental variables for each species separately. A composite map shows the density of species throughout Australia. The greatest number (24) of communally breeding species was found in northern New South Wales. The pattern of distribution did not merely reflect that of land birds in general and numbers of species were not strongly associated with particular faunal zones or vegetational types. A regression model was constructed that accounted for seventy-eight per cent of the variation in numbers of species by simple geographical variables, longitude being the most important. Two analytical strategies were used. The first suggested that the most important environmental variables were moisture in the driest sixteen-week period, temperature in the coldest week, and moisture in winter; the second, plant growth in summer, temperature in the coldest week, variation in rainfall and seasonal variation in plant growth. The possible relations between these variables and communally breeding species are discussed and it is concluded that the enviromental data neither strongly favour nor exclude three hypotheses regarding the origin or adaptive maintenance of communal breeding generally. The analysis does not support the view that because taxonomic diversity of communally breeding species is high, there must be common factors in their ecology. Species are considered to be opportunistic communal breeders or obligate communal breeders and a simple model incorporating physiological condition, social attraction and group facilitation illustrates the concept of ‘levels’ of communal breeding that have been observed in nature. The distinction between opportunistic and obligate species is considered important because these may represent different evolutionary strategies for behaviour hitherto lumped as communal breeding.