Penguin Day Care – Safety in Numbers

Gentoo penguin chicks

Gentoo penguin chicks keep company at Copacabana (photo: L. Asato)

Kristen Green

Kristen Green

by Kristen Green, Ichthyology Lab

January 2009:  A mosaic of rock nests that defined the penguin colonies at Copacabana has erupted into a chaos of chicks in the past week. When we arrived in mid-October, each colony had distinct borders, and within a colony, each individual nest was spaced exactly one pecking distance away from its neighboring nest. Three months later, the nests have been almost completely disassembled and thousands of birds have merged into one super colony. Soon, a reddish-pink stain of penguin guano will be all that remains of the colonies until the next spring.

The disintegration of the colonies is simultaneous with the crèching stage of the Adelie and Gentoo chicks. ‘Creche’ is French for ‘day care,’ and is the period when both parents leave the chicks unattended in the colonies while they go to sea to forage for food. The Gentoos will return each day with a full load of krill for their chicks for another month, but most of the Adelie chicks will have molted into their waterproof adult plumage and be on their own in another week. Gentoo chicks have a slight advantage here; they are fed by their parents for much longer, and chicks may also accompany the adults for their first few foraging trips at sea. As a result, the Gentoo chicks are likely to be in better condition and have more experience foraging for krill before winter. Meanwhile, hunger forces the Adelie fledglings into the ocean, where they must rely completely on instinct to catch krill for the first time.

Feeding (photo: L. Asato)A A gentoo penguin chick  scores a meal from mom (photo: L. Asato).

For now though, all the chicks are ignorant in their chunky bliss of these future hardships. They spend most of their time eating, sleeping, or in loosely organized, but highly mobile masses, presumably on their way to either eating or sleeping. Depending on the time and place, Copacabana is either a kind of narcoleptic nursery ground, or brimming with aimless mobs of chicks.

The groups of creched chicks form a security-in-numbers daycare. The skuas still shop the colonies for dinner, but they focus their efforts on chicks that are alone and small. The easy pickings of eggs and tiny chicks in early summer will be over soon, but the skuas are no less determined; they have chicks of their own to feed now. Yesterday, I saw a skua pair (the same pair that fought successfully for control of the lower penguin colonies) bullying a late-breeding Gentoo that was still incubating two small chicks. The skuas attacked with impressive synchrony; one flustered the Gentoo from the front and other worked the rear.

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A Gentoo penguin guards its nest with two chicks (photo J. Warren)

The Gentoo, extremely agitated and trying to fend off an attack from multiple directions, was soon outmatched. One skua found a quick opportunity to nab one of the tiny chicks. This chick was swallowed whole in seconds, while the second chick was pulled out with the same effective technique. The skuas enjoyed a brief tug of war with the second penguin chick, and each gulped down its share. One skua stayed just to harass the Gentoo defending its empty nest for a little while longer. The other skua flew back to the nest to regurgitate the penguin chick to her own chicks; irony is an anthropogenic invention.  It  pays in nature to be doing the same thing as everyone else; the few tiny chicks that still have weeks before creching are an easy target for the skuas.  I watched all this with awe and a little bit of horror, but above all with the feeling that biology has a way of letting you know exactly where you stand in the world.

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