The Push-Me-Pull-Me Allure Of Bird Eggs

Playing the Easter Bunny has a long and illustrious history in the field of animal behavior. Two of three ethologists to ever win the Nobel prize, Konrad Lorenz and Niko Tinbergen, made fundamental discoveries by placing eggs near bird nests and observing how the nest owners responded.

For many ground-nesting birds, putting an egg near the nest causes a very predictable, stereotypical response — the incubating bird quickly rolls the egg into its nest. Lorenz and Tinbergen studied egg retrieval behavior to understand the mechanistic basis of behavior, and the behavior became the poster child for what Lorenz termed “fixed action patterns” — instinctive behaviors, triggered by a simple stimulus, that are relatively invariant within a species.

Egg retrieval makes adaptive sense because birds sometimes do accidentally kick eggs from their nest, and retrieval rescues these otherwise doomed eggs. However, egg retrieval can have a downside in species with reproductive cheats. In a diversity of birds, females lay eggs in each other’s nests — this brood parasitism within species is analogous to the more widely-known interspecific brood parasitism seen in birds like cuckoos and cowbirds. Researchers have shown that parasitic snow geese (Chen caerulescens) take advantage of egg retrieval behavior to parasitize their own kind — parasites simply lay their egg next to an active nest, which stimulates the sitting bird to retrieve the egg into its nest. And since snow geese do not distinguish between foreign eggs and their own, the host ends up raising the parasitic offspring.

Unlike snow geese, some birds do recognize brood parasitic eggs, raising the question whether such a species would show egg retrieval behavior. For example, conspecific brood parasitism is very common in American Coots (Fulica americana), a marsh-nesting waterbird, and the hosts reject about a third of the parasitic eggs that other females lay in their nest. Hosts use egg features like egg color, and probably the color and pattern spots as well, to distinguish the parasitic eggs, which are then buried deep in the nesting material where they are no longer incubated.

My collaborator Daizaburo Shizuka and I were interested in understanding whether coots, given their egg recognition and rejection, would retrieve eggs placed outside their nests. Coots are not ground nesters — they build mound-like platforms of vegetation over the water in wetlands — so egg retrieval was not a given. And, if coots do retrieve eggs, another question was whether the evolution of parasitic egg recognition has influenced the cues used to retrieve eggs — would coots be more picky retrievers than other birds?

To answer these questions, we put a variety of eggs and egg-like objects just outside the nest cup and slightly down the slope of the nest mound so if these eggs appeared back in the nest, we could be sure that they had been retrieved and not accidentally knocked into nest bowl. In the first experiment, we removed one egg from each of the 17 nests and placed it outside below the rim of the nest.  Evidence for retrieval came quickly — all of the eggs were safely back in the nest when we checked the nest the next day. Coots are egg retrievers!

Given that coots often reject parasitic eggs that other coots lay in their nests, an obvious next experiment was to see if coots would retrieve parasitic coot eggs (Figure 1). One possibility is that the same recognition system coots use to recognize parasitic eggs might have been co-opted for the retrieval response so that eggs that would normally be rejected would not be retrieved. The idea is that failure to retrieve is analogous to rejection — in both cases, the birds do not want an egg in their nest. The experiment did not support this idea, as coots retrieved 100% of the parasitic eggs we put outside their nest rims, an identical response to their own eggs.  Importantly, the fraction of eggs they failed to retrieve — zero — differs significantly from the 40% rejection rate observed for parasitism with a single parasitic egg. Clearly, brood parasitism has not influenced egg retrieval behavior in coots.

We then conducted a variety of experiments to assess whether there is a limit to what a coot might retrieve and, if there is, what cues are important in stimulating egg retrieval. We gave the birds an amusing assortment of eggs: coot eggs painted white, natural white chicken eggs, chicken eggs painted white, and white chicken eggs treated with an ultraviolet (UV) light-blocking shellac (birds see light in UV wavelengths, and it has been shown to be important in egg rejection in some species). Coots retrieved almost all of these eggs at a very high rate, a pattern consistent with the early ethological studies that showed that anything with an egg shape is sufficient to stimulate retrieval.

We were finally able to push the birds to the point where they showed diminished enthusiasm for retrieval when we gave them objects that were not egg-shaped. Wooden cubes and cylinders that were either painted white or adorned with photographs of coot egg markings were all retrieved at rates between 30% and 50%, depending on treatment. On the one hand, that these objects were retrieved at rates below the rates shown for real eggs confirms that egg shape does matter. On the other hand, many individuals did retrieve these objects — perhaps what constitutes an egg shape varies among individuals. This could be tested by showing that the same individuals are consistent in their response to these objects.

One of the more astonishing discoveries of our study was what a difference just a few centimeters makes. Many of the objects the birds retrieved into their nests were later removed from the nest by burial. Paradoxically, when the same object was outside the nest, the birds wanted it inside the nest, but once it was in the nest, the birds wanted it back out. This observation supports an important conclusion: egg rejection and egg retrieval are based on unrelated behavioral and cognitive mechanisms. For the objects that were rejected once in the nest, the efficient thing would have been to have never retrieved them in the first place, but this is not an option because two independent behaviors are involved. As in real estate, the response depends on location, location, location.

These findings are described in the article entitled Context-dependent response to eggs: egg retrieval versus egg rejection in conspecific brood parasite, recently published in the journal Animal Behaviour. The work was conducted by Bruce Lyon from the University of California and Daizaburo Shizuka from the University of Nebraska, Lincoln.