Cooperative breeding in birds

Though there are numerous examples of kin selection among birds, helpers are not always related to the offspring they rear. In these situations, as among Azure-winged Magpies, there are usually ecological factors that promote cooperative breeding as well as numerous benefits that offset the reproductive costs incurred by helpers (Canário et al. 2004).

In most cooperatively breeding birds such as Micronesian kingfishers, helpers are commonly sub-adults, or young individuals that are reproductively mature but choose not to breed (Kesler&Haig. 2007). Other species, such as the brown-headed nuthatch, are conditionally cooperative breeders. Individuals of this type all attempt to breed independently each year and only resort to helping when their nests fail (Cox et al. 2007). In some unique instances, such as in populations of the Seychelles warbler, cooperative breeding is facilitated by not only sub-adults but also post-reproductive grandmothers that have lost their position of breeding dominance (Richardson et al. 2007).

Why be a helper?

This section aims to identify the benefits helpers accrue in compensation for their reproductive costs, as well as the external variables that promote cooperative breeding.

The presence of helpers in cooperatively breeding units increases the survival rate of young hatchlings (Canário et al. 2004). This result is obviously beneficial to the new generation but it comes at a price to the helper, who must sacrifice time, energy, and the opportunity of having their own offspring during that breeding season. In light of these costs, there must be an evolutionary reason for birds to still engage in cooperative breeding. An examination of this form of breeding in the azure-winged magpie provides some clues as to why and when it makes evolutionary sense to be a helper.

The azure-winged magpie is a member of the Corvidae, or crow-family, of birds ( [link] ). It breeds in small colonies and feeds mainly on insects. Helpers of cooperatively breeding families in this species do not follow a pattern of kin selection in that they are not necessarily directly related to the families they assist (Canário et al. 2004). They contribute to nest building and upkeep, feeding nestlings and brooding females, and warding off predators. In a study of Iberian Azure-winged Magpies in Portugal during two consecutive breeding seasons, researchers found the number of helpers and cooperatively breeding units to be heavily influenced by changes in the weather. The year that experienced significantly more rainfall (~30%) witnessed an increase in the number of helpers and cooperatively breeding units (Canário et al. 2004). This may be attributed to the fact that rainfall has a marked influence on the presence of invertebrates, which are the main component of the Azure-winged Magpie’s diet. During rainfall, insects tend to seek cover making it more difficult for Magpies to find them. This reduces the available food supply. As a result, increased numbers of helpers appear because the low probability of successful reproduction caused by limited resources discourages a large proportion of mature adults from breeding and instead encourages them to invest in rearing the offspring of others.