Which Bird Crosses an Ocean Twice a Year? One That Barely Flaps

The characteristic flight is a series of long glides alternating with brief bursts of flapping, banking from side to side so that the wingtip nearly touches the water, which is what gives the group its English name. The technique exploits the fact that wind near the sea surface is slowed by friction while wind higher up is faster, so a bird that climbs into the faster air and then turns and dives back into the slower air gains energy from the difference on each cycle. Rising air on the windward face of a swell provides further lift. The result is travel over enormous distances at a small fraction of the energy that continuous flapping would require.

Tracking devices have produced figures that were not believed beforehand:

• •Sooty shearwaters recorded covering around sixty five thousand kilometres yearly
• •Figure-of-eight routes crossing the whole Pacific in a season
• •Manx shearwaters breeding in Britain and wintering off South America
• •Daily distances of several hundred kilometres sustained for weeks
• •Breeding birds making foraging trips of over a thousand kilometres
• •Individuals returning to the same burrow after months at sea

The contrast between their competence at sea and their helplessness ashore is extreme. Their legs are set far back for swimming, so they can barely walk and shuffle on their bellies, which makes them easy prey for anything on land. They therefore nest in burrows on islands without ground predators and visit only at night, arriving and departing in darkness and calling with the eerie screams that gave several colonies their local reputations for being haunted. A single egg is laid, both parents incubate in long shifts while the other feeds far out at sea, and the chick is fed on regurgitated oil-rich food until it is heavier than its parents and then abandoned to find the sea alone.

Navigating back to a burrow on a specific island after months over featureless ocean is a serious puzzle and the experiments testing it are famous. A Manx shearwater taken from its burrow in Wales and released in Boston returned to that burrow in twelve and a half days, having crossed the Atlantic over a route it had never flown. Displacement experiments since have shown that the birds use a map sense as well as a compass, since they correct towards home from unfamiliar places rather than simply flying a fixed direction. Smell appears to be involved, since birds with the olfactory nerve blocked orient poorly over the sea while navigating normally over land.

The group is among the most threatened families of birds and the pressures are specific to how they live. Rats, cats and pigs introduced to breeding islands eat eggs, chicks and adults that cannot escape on land, and this has eliminated colonies entirely. Long-line fisheries hook birds that dive for the bait, killing large numbers, which mitigation gear has reduced substantially where it is used. Plastic is ingested at sea and fed to chicks, with some populations showing it in nearly every bird examined. Light pollution disorients fledglings making their first night flight, grounding them in towns. Island eradication of introduced predators is the single most effective remedy and has worked repeatedly.

Alternating climbs into faster wind above the sea with dives back into slower air near the surface extracts energy from the wind gradient, which is why these birds glide for hours and flap rarely. Tracking has recorded annual journeys around sixty five thousand kilometres. Legs set far back for swimming leave them helpless ashore, so they nest in burrows on predator-free islands and come in only at night.