One Egg Good, Two Eggs Bad?

Forest and Bird, Issue 303, 1 February 2002, Page 22

One Egg Good, Two Eggs Bad?

In 1991, following the disastrous crash in yellow-eyed penguin populations on the mainland, a radical decision was made to remove one of the two eggs from each of the nests at designated study areas. In the event of repeat problems in the food chain, breeding pairs were thought to be much more likely to rear and fledge one chick than two — and still end up in good condition themselves. It was a controversial experiment, but in this particular season it appeared to work. Every adult survived to breed the following year. In the 20 years John Darby has studied the species, it is the only time he has recorded a 100 percent survival rate among breeding adults. And for the next five seasons, the population climbed towards a record high. Now, 10 years on, two PhD students at the University of Otago, Melanie Massaro and Alvin Setiawan, are looking at an allied issue — comparing the breeding strategy of yellow-eyed penguin to that of the Fiordland crested penguin, which invariably raises only one chick, although there are two eggs in a clutch. Whether they sacrifice the smaller first egg or starve the ensuing chick, Fiordland crested penguins deliberately set out to raise only one chick. Yelloweyed penguins do it differently, with two same-size eggs producing two chicks more or less on the same day — a strategy that aims to despatch two healthy chicks to sea come February. In a poor food year, though, this is problematic. Two chicks may be one too many for the parents, and both chicks are put at risk. The PhD studies, due for completion at the end of next year, will compare the different reproductive pathways and why these species are stuck with them. Perhaps, too, the studies will shed light on the advantages and disadvantages of such a breeding regime.