Apologies for the wordplay in three languages but I could not resist! Sooty Terns lay one egg. But we sometimes find nests with two eggs!
Sooty Tern eggs are highly variable in background colour, the pattern and colour of spots and smudges, and also in shape, but the eggs that each female lays are consistent in these characteristics each time she lays an egg, be it to replace a lost egg within a breeding season, or between years. When we find two eggs in a nest they are almost invariably different in one or more of the characteristics, clearly indicating that the eggs have been laid by two females. For a long time I had wondered how these two-egg clutches could be produced. Did a second female lay an egg in the nest of another bird as some form of parasitism, hoping that the first bird would do all the work of rearing the second bird’s chick? Or did the two-egg clutches result from adoption of a second egg by an incubating bird? I had observed that when incubating birds are disturbed, their take-off sometimes rolls the egg out of the nest, which is usually little more than a shallow scrape in the sand, and when the bird returns to its nest site it reaches out with its bill and rolls the egg back into the scrape. The second explanation thus seemed more likely.
In 2014 Christine and I designed a small experiment to investigate. We took eggs from a part of the Bird Island colony where eggs were harvested for human consumption. At our experimental nests we placed a second egg at a measured distance from the nest owner’s egg. Eggs that were placed close (8 cm) to the nest owner’s egg were commonly adopted by the incubating bird, while eggs that were placed further away (up to 20 cm) were not adopted and often disappeared, having been taken by predators (most likely land crabs).
While we now know how 2-egg clutches form and that it derives from a behaviour that has evolved to safeguard the nest owner’s egg, adoption of a second egg has its dangers. Sooty Terns seem unable to incubate two eggs satisfactorily and sometimes neither egg hatches. Even where one hatches, the owner cannot be certain that this is its own egg. And there remains another mystery – why are some eggs laid close to existing nests, rather than in nests of the bird laying the egg that becomes adopted?
Full details of our study have been published: FEARE, C.J. & LAROSE, C.S. 2014. Egg adoption by incubating Sooty Terns Onychoprion fuscatus. Marine Ornithology 42: 27–30.