FACULTATIVE HELPING DOES NOT INFLUENCE REPRODUCTIVE SUCCESS OR SURVIVAL IN COOPERATIVELY BREEDING WHITE-BROWED SCRUBWRENS

1, The most common form of cooperative breeding in birds involves a pa ir and their adult offspring ('helpers') provisioning young, Many stud ies show that pairs with helpers have higher reproductive success than pairs alone, but the differences could be due to confounding variable s, like parental or territory quality, rather than to helping behaviou r, 2, One method of testing whether helping increases reproductive suc cess is to compare the success of intact groups with those from which helpers have been removed. The disadvantage is that variables other th an provisioning by, helpers (e,g. group size) are affected, which them selves could affect reproductive success. Alternatively, one can attem pt to control statistically for confounding variables, but this is dif ficult in territorial species because it may be impossible to ensure t hat all confounding variables are measured. 3. We took advantage of na tural variability in provisioning behaviour among subordinate white-br owed scrubwrens. Sericornis frontalis, to assess the effect of helping . Scrubwrens are a small passerine in the family Pardalotidae in which male offspring may remain on the natal territory; as adults, We compa red the reproductive performance of pairs, groups with nonhelping subo rdinates and groups with helping subordinates, using data from 4-years ' study of an individually colour-handed population resident in the Au stralian National Botanic Gardens, Canberra, 4, The total rate of feed s to nestlings vc as greater when helpers were present, but we could d etect no increase in nestling weight. Furthermore. we found no effect of helping behaviour on reproductive performance or reproductive succe ss. Our measures included: (i) duration of the nesting cycle and inter val between nesting attempts: (ii) success of individual nesting attem pts, measured in over 20 ways: and (iii) reproductive success over the whole breeding season, measured as the total number of fledglings and independent young. 5, There was also no effect of helping on survival of the female or dominant mole, 6, We conclude that helping behaviour does not have a substantial, consistent effect on reproductive succes s. but cannot eliminate the possibility that it might increase reprodu ctive success by a small amount or in some territories or years. Alter natively, helping behaviour might be the nonadaptive consequence of ge ne flow from populations in which it is adaptive, Nonetheless, we reje ct Jamieson's hypothesis that helping behaviour is a nonselected conse quence of strong selection on parental behaviour, because, unlike many other species, helping is not an invariant consequence of natal philo patry.