Aspects of the brood reduction process of the Fiordland crested penguin, Eudyptes pachyrhynchus

Abstract

The brood reduction process of tawaki, or Fiordland crested penguin, Eudyptes pachyrhynchus was studied on Taumaka Island (Open Bay Islands) from July to November 1988. During the study I made observations on adults, eggs and chicks from both manipulated nests, where I swapped eggs to produce clutches of equivalent sized and aged eggs, and natural nests, where parents' retained their own eggs. I observed adult behaviour, egg laying, egg incubation and egg hatching to determine how the initial asymmetries between chicks in a brood were produced, and observed chick and adult behaviours to determine the process of brood reduction for tawaki. I also assessed the implications that brood reduction would have on a management strategy for the species.

The initial size difference between offspring was created by the female who laid two, different sized, eggs. The parent undertaking the first incubation of the eggs did not produce a fully vascularised brood patch until after the second, and larger, (B) egg was laid. This meant that the smaller - first laid - (A) egg was unlikely to begin developing before the 8 egg was laid and consequently unlikely to hatch significantly earlier.

In both natural and manipulated nests, B eggs typically hatched first. First hatched chicks were usually fed before the second egg hatched. This increased further the difference in mass between the two chicks. I determined that, starvation of chicks, resulting from the brood reduction process, was the largest cause of chick mortality and that significantly more larger chicks of a brood survived to the fledging stage.

For the first five days after hatching, when chicks were not physically coherent, the female would preferentially feed one of the chicks. Which of the two chicks she fed determined the pattern of brood reduction. If the larger chick was favoured the smaller chick starved to death soon after hatching. These nests were typified by high begging rates for the small chick and low levels of interference interactions between chicks during feeding. If the smaller chick was favoured it would begin to grow and the brood would coexist for a longer period, although the smaller chick would still starve to death. These nests were typified by even begging levels and much higher levels of interference interactions between chicks during feeding. Overt aggression between chicks, or between chicks and adults was not witnessed.

Because brood reduction is an integral part of the tawaki breeding system, any species management strategy should concentrate on increasing adult survival rather than increasing reproductive output.