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Anacapa Island Nesting Season

June 17, 2016

Birds have it tough. About 90% die within their first year, with mortality heaviest on nestlings and unhatched birds. Many birds hide their nests in foliage or holes, but ground-nesting birds have a special challenge: their nest is right on the ground, where any bird soaring overhead or reptile or mammal wandering by can find it. Dense vegetation or grass may conceal small nests, enabling nesting success. Seabirds created other solutions.

To minimize nest predation, seabirds usually nest on small islands, where mammal and reptile predators are few or absent. Many take the addition step of nesting on small ledges of steep cliffs. Such birds may build no nest at all, laying their eggs directly on the bare rock. In such cases, the egg is sharply tapered at one end: if it moves, it rolls in a tight circle rather than off the ledge.

Pigeon Guillemot (G. Murayama 6-14-16)

Pigeon Guillemot (G. Murayama 6-14-16)

Other seabirds, such as this Pigeon Guillemot, nest in rocky crevices, cavities or self-excavated burrows in dirt. Such sites can be defended by the adult. If the island is free of mammals and reptiles, some birds may nest openly on the ground. When they are large, as are Western Gulls, and can protect the nest from raptors, they have little to worry about. Still, when chicks are hatched, it helps if they are cryptically plumaged, as they are less likely to be spotted by predators if they wander from the adult’s protection.

Western Gull cryptic chick (G. Murayama 6-14-16)

Western Gull cryptic chick (G. Murayama 6-14-16)

Western Gulls typically lay three eggs (one to five eggs is possible) in a nest built of grass or brush pieces. They nest in colonies; nests may be closely grouped, but not so close that adults in adjoining nests can peck one another. Unlike non-colonial birds which may maintain large territories, the gull’s territory consists only of how far they can peck without leaving their nest.

Chick pair (G. Murayama 6-14-16)

Chick pair (G. Murayama 6-14-16)

In about a month the eggs hatch, after which the chicks are fed a diet of invertebrates, small vertebrates, carrion, and eggs and chicks of other birds. While embryos can survive brief temperatures up to 114°F, adult birds may soak their belly feathers in water to cool heated eggs. Western Gulls are one of the many species that lay more eggs than they can be expected to successfully fledge; later-born chicks are “insurance” against loss of early chicks, and do not often survive.

Up comes lunch (G. Murayama 6-14-16)

Up comes lunch (G. Murayama 6-14-16)

The hungry chicks peck at red spot on the adult’s lower mandible in order to trigger food regurgitation by the parent. Food is brought up from the internal carrying pouch, called the crop. This is not the stomach, and digestion within it is minimal. Chicks often grab more than them can swallow; the meal moves in and down as it is digested.

Western Gull feeding chicks (G. Murayama 6-14-16)

Western Gull feeding chicks (G. Murayama 6-14-16)

Chicks fledge in six to seven weeks, and may leave their home island in another three to four weeks, at which point they may begin appearing at mainland beaches like Surfrider Beach. Western Gulls reach adult plumage in their fourth spring, and begin breeding between ages four to eight. They can live to age sixteen. Along our coast they are frequently seen prying mussels off rocks, dropping them onto rocks from thirty to fifty feet up, then dropping to eat the exposed flesh from the broken shell. Other local gull species seem unable to learn this trick.

Island Tree Mallow (G. Murayama 6-14-16)

Island Tree Mallow gives the gulls something to admire (G. Murayama 6-14-16)

Western Gulls breed from NW Washington to central Baja California. Their winter range extends slightly, reaching the top of Vancouver Island and nearby Canadian mainland, and to the southern tip of Baja. They are extremely reliable at Malibu Lagoon, present on 99% of our visits.

Many thanks to Grace Murayama for her photos from Anacapa which inspired this message.
[Chuck Almdale]

 

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