Tricky, tricky birds

If you wanted to measure the good effects of cooperative behavior in a species, how would you do it? There are many ways, but common ones are to measure the size of the animals in question (to see if they are eating well) or to measure the number of offspring. Positive effects for cooperative behavior would should result in increases in these measures.

Not always so, say Russell et al. in Science, largely because the birds are being tricky enough to exploit their neighbors.

The Superb Fairy-Wren (superb, I tell you...simply superb!) is a species of bird that raises its young other exclusively in mating pairs or in larger groups where non-parents feed the baby birds. These additional birds are called helpers. When researchers want to know whether these helpers improve survival or the size of the baby birds, they can compare groups with just mating pairs. The prediction is that baby birds with helpers will be bigger because they will be better fed.

The troublesome thing that happens when you do this experiment is that the chicks (chicks, man, chicks!) from the pairs and the groups with helpers are the same size. (This depicted in the figure to the left from Figure 1 in the cited paper.)

How do we explain this discrepancy? Why aren't the helpers effective at raising the size of the offspring?

Russell et al. show why. Superb Fairy-Wren mothers lay smaller eggs in the presence of helpers than they do in just mating pairs. (This is depicted in the figure to the right from Figure 2 in the cited paper. The figure shows the chick masses after they have been adjusted for differences in egg volume. Note how the effects of helpers appear after this adjustment.)

The mothers know that they are going to have help, so they invest less in their offspring. They invest less because the act of laying larger eggs lowers their own chances for survival.

Tricky...little...buggers...

Here are the authors' conclusions:

Given that, in other cooperative breeding systems, offspring of low mass are shown to have reduced chances of recruitment and low future reproductive success, why do females breeding in the presence of helpers lay eggs that yield suboptimal offspring? We know that the durations of incubation and chick rearing are inflexible in superb fairy-wrens and so are uninfluenced by helper presence. One possibility is that mothers are constrained from optimal allocation of resources into eggs because of competition with helpers for food in their territory. Alternatively, females breeding in groups might exploit helper contributions to nestling mass by reducing their own investment in reproduction to save resources for future breeding attempts. The latter explanation is more strongly supported by our evidence.

...

We have shown that in superb fairy-wrens, mothers reduce egg investment when breeding in the presence of helpers and that the subsequent undernourishment of the young at hatching wholly conceals the positive effect of helper contributions to nestling mass. The critical factor that will select for maternal reductions in egg investment in a cooperative bird is a predictable workforce to assist in provisioning young, for this will allow females to make informed decisions at the egg-laying stage concerning how much food their chicks will receive after hatching. This is true of most cooperative birds: The number of helpers present from the onset of egg laying accurately reflects the number of helpers available to feed the offspring after hatching. We therefore predict that load lightening of maternal investment at the egg stage will be a general phenomenon in cooperative birds, as is the case with maternal load lightening at the chick-provisioning stage. We conclude that all studies conducted on species in which helper numbers are predictable from the onset of breeding have the potential to underestimate the contributions made by helpers to nestling condition and/or maternal survival. Such studies will overlook the significant benefits that helpers stand to gain from breeding cooperatively through kin selection if helpers are related to breeders or group augmentation if they are not. (Citations removed. Emphasis mine.)

This is an important study because many biologists are out there looking for examples of animal cooperation and reciprocity. If species are hiding the benefits of reciprocity -- basically by exploiting the animal offering the help, examples of cooperation are going to be more difficult to find. It also says something about how tricky evolution can be. These birds have accurately accounted for the strategy that best maximizes their own chance of survival and their offspring's. They do so by taking as much out of helping birds as they can.

What I would like to know now is why the helpers keep participating in this hustle? Is it a phenomenon of generalized reciprocity or do the helpers offspring receive the same treatment later on? Or are the birds closely related suggesting kin selection? I guess I don't know enough about this particular species to say.