Vertical-crested penguin lays two eggs

Vertical-crowned penguins have an unusual and seemingly manipulative breeding strategy in that they lay two eggs per year but only hatch one chick. They are indifferent to the first egg they lay, often not hatching it at all or tossing it out of the nest to be smashed or eaten by scavenging seabirds. Biologists have long been puzzled by this "wasteful" breeding strategy and have tried to understand the reasons behind it.

These medium-sized penguins, native to New Zealand, are some of the world's least-known penguins. This is probably because they breed only on two isolated, inhospitable, and uninhabited islands in southeastern New Zealand, and access to these islands is restricted by the New Zealand Department of Conservation. However, because this penguin species are listed as "endangered" by the, it is important to understand its breeding biology and the status of its populations.

Lloyd Davis of the University of Gotta, New Zealand, and colleagues from several different research institutions in New Zealand have now published the results of a 1998 breeding study of straight-crested penguins on the Antipodes Islands. Although the study was completed nearly 25 years ago, no data had previously been published, and the study remains the most complete and in-depth record of straight-crested penguin breeding behavior to date. The results of the study have now been published in the journal Pols ONE.

In that study, 270 penguins in a colony were captured and tagged with a unique code that could be read by a distant observer. Once tagged, the penguins were not disturbed again, but their behavior in the colony was monitored for more than 249 hours. The colony had 113 nests, and occupancy, mating, fighting, and hatching behaviors were observed at all nests over a month. In addition, the dates of egg laying in each nest were recorded.

Like most other penguin species, the crowned penguin lays two eggs during breeding. However, the first egg laid was 70% smaller than the second egg laid about five days later. Davis and his colleagues found that the first egg is usually lost from the nest before or shortly after the second egg is laid, and that larger penguins sometimes deliberately break the egg or eject it from the nest. In addition, about 40% of mated penguins did not even hatch the first egg. Only after laying a second egg did incubation begin steadily. So, the researchers wanted to know what the purpose of the penguins' failure to hatch the first egg was.

In another experiment in a nearby penguin roost, researchers built rings of pebbles around 14 nests to prevent the first egg from rolling out of the nest. The results showed that 86% of the first eggs stayed in the artificial nests, compared to 4% in the control nests. However, this did not increase the chances of the first egg surviving, and they were all eventually lost due to being broken in the nest, taken by other animals, or not hatched at all. Even if the first egg remained in the nest, it was still rejected by the incubating parent and died.

The vertical-crested penguins travel long distances in search of the food they need to sustain themselves, and they are likely unable to feed two chicks. The reduced number of hatched penguin eggs may be an adaptation to this situation. The researchers suspect that harlequin penguins have retained their ancestral reproductive habit of laying and hatching two eggs. However, unable to provide enough food for two hatchlings, they have adapted to reduce the number of hatching by sacrificing the first egg.

From this perspective, it makes sense for the first egg laid to be small to minimize the cost to the female in terms of wasted nutrition and energy. The vertical-crested penguin exhibits the most extreme egg size isomorphism of all penguins (and indeed of all birds), with the first egg always being significantly smaller than the second. Reducing brood size in this way is a potential strategy to reduce investment in offspring.

Researchers warn that unless vertical-crested penguins receive more research attention and conservation efforts, the species will continue to be poorly understood, and eventually, their survival may be threatened. Evidence suggests that climate change is negatively impacting reproduction in the islands where they live, with more storms and mudslides in recent decades destroying parts of the colony and killing nesting penguins. In addition, changes in ocean productivity around the islands have led to a dramatic decline in penguin populations in the eastern part of the region, and may also be hurting the harlequin penguins.

The researchers write, "This study highlights the paradox that such an interesting and endangered penguin species should be so little known in this day and age that the best data we have come from nearly 25 years ago and that more research and better conservation of this remarkable species is urgently needed."