In cooperatively-breeding species, the presence of male helpers in a group often reduces the breeding female’s fidelity to her social partner, possibly because there is more than one potential sire in the group. Using a long-term study of cooperatively-breeding superb fairy-wrens (Malurus cyaneus) and records of paternity in 1936 broods, we show that the effect of helpers on rates of extra-pair paternity varied according to the helpers’ relatedness to the breeding female. The presence of unrelated male helpers in a group increased average rates of extra-pair paternity, from 57% for groups with no unrelated helpers, to 74% with one unrelated helper, to 86% with 2+ unrelated helpers. However, this increase was due in equal part to helpers within the group and males in other groups achieving increased paternity. In contrast, helpers who were sons of the breeding female did not gain paternity, nor did they affect the level of extra-group paternity (which occurred at rates of 60%, 58%, 61% in the presence of 0, 1, 2+ helper-sons respectively). There was no evidence of effects of helpers’ relatedness to the female on nest productivity or nestling performance. Because the presence of helpers per se did not elevate extra-pair reproduction rates, our results undermine the ‘constrained female hypothesis’ explanation for an increase in extra-pair paternity with helper number in cooperative breeders. However, they indicate that dominant males are disadvantaged by breeding in ‘cooperative’ groups. The reasons why the presence of unrelated helpers, but not of helper-sons, results in higher rates of extra-group reproduction are not clear.

Please see associated publication and publications cited therein for details of data collection for this long-term study.

For convenience, here is an excerpt of the Methods from the associated publication:

The study population consisted of a color-banded population of superb fairy-wrens living in a ~60 ha area located in and around the Australian National Botanic Gardens, Canberra, Australia (35 ◦ 16 S, 149 ◦ 06 E). The study population was censused all 52 weeks of the year (Cockburn et al. 2003), and census records included details of the presence and identity of helpers on each territory. During the breeding season the progress of all nests was monitored, with nestlings banded 5-8 days post-hatching, at which point blood samples were taken for parentage analysis using microsatellite genotypes (see methods in Hajduk et al. (2018)). Fairy-wrens are multi-brooded: due to heavy nest predation, a female may initiate up to eight clutches in a given year, but will only ever raise a maximum of three broods to fledging. Clutches may contain 1-5 eggs, with a strong mode at 3 eggs (Cockburn et al. 2008b).[...] We categorized all offspring in a brood as to whether they were the result of within-pair, within-group extra-pair, or extra-group paternity: see Box 1 for definitions. [...] data spanned 26 years (1988-2013) and contained a total of 1936 broods and 5485 nestlings. [...]

Please see the Hajduk-etal-2020_hlpr-rltdnss-fmle-EP-rprdctn_README.md file.