Diverse evolution patterns of cooperative breeding in Australian birds

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1.During the 3-year study period, we found 14-22 groups of the Grey-crowned Babbler and conducted observations : Mean group sizes were 4.21,3.85 and 4.59 respectively : most groups comprising single monogamous pairs with 1-7 auxiliary birds. Sex ratio was almost unity for groups in which all members were successfully sexed for each year : 11:11(six groups), 25:20(13 groups), and 14:12 (nine groups), respectively. Clutch size was 1-4 eggs (two was commonest) and joint-nesting was observed. Major cause of failure was predation but brood reduction due to starvation was rare. Pairs without helpers could not fledge offspring at all. However, for groups with helpers, there was no significant correlation between the number of fledglings and that of helpers. Almost all members of groups provisioned chicks, while there were a few birds giving no help at all. Birds of one-year old provisioned less than those of two-year old or older. Total provision rate was higher in the larger group than the s … More maller groups, but the difference was not significant. This species could be an obligate cooperative breeder, but the number of helpers did not influence the reproductive success remarkably.2.During the 3-year study period, 29 broods (45 nestlings) in total were sexed with the DNA sexing method. Sex ratio of nestlings was male biased : 30 (66.7%) were males and 15 (33.3%) were females. Almost all broods with more than one chick were occupied by either sex. For groups with female-biased helper sex ratio, the sex ratio of broods was male-biased, and vice versa. It is likely that female parents of the Grey-crowned Babbler adjust the sex ratio of a brood according to the sex ratio of helpers and the number of helpers weighted by the age of helpers.3.We obtained the evidence about the cooperative breeding in the population of the Red-backed Fairy-wren in the tropic monsoon area. In 2004, five of 17 groups observed accompanied with one or more auxiliary birds (helpers). Total provisioning rate for chicks was higher in the groups with helpers than in the groups without helpers. Although both sexes became helpers, males outnumbered females. All dominant males had black and red plumage, while helper males (=subordinates) had female-like dull plumage. Less