Our work on cuckoos led us to experiment with a cuckoo host in Australia, the superb-fairy wren.
These birds have an interesting social life in their own right, even before becoming exploited by cuckoos. Generations of research by Australian evolutionary ecologists has found that they are facultative cooperative breeders. In plain English, this means that sometimes a pair of birds will raise offspring together, but more commonly the pair will be assisted by helpers at the nest. Helpers are always adult males in this species, and often sons from previous breeding attempts (daughters disperse away to a territory far from their relatives).
Although pairs can be assisted by as many as 5 helpers when feeding offspring, strangely the chicks themselves seem to gain no benefit from all this extra provisioning and fledge the nest weighing no more than when raised by a pair.
We solved this puzzle by looking at the fairy-wren’s eggs (Russell et al 2007, 2008). When breeding females are assisted by lots of helpers, they systematically undernourish their eggs, so that they are less rich in fat, protein and carbohydrate and yield smaller chicks at hatching. After hatching, the helpers compensate for this under-investment with their extra provisioning at the nest so that by the time they fledge, the chicks have caught up in mass.
By skimping on egg investment, females are able to put more resources into survival and live longer as a result. So the extra resources provided by helpers are effectively taken by breeding females to promote their own fitness, which explains why we see no benefit of helpers for chicks.
Russell A. F. et al 2007 Reduced egg investment can conceal helper effects in cooperatively breeding birds. Science 317:941-944
Russell, A. F. et al 2008 Maternal investment tactics in superb fairy-wrens. Proc R Soc B 275:29-36