Timing of breeding

The timing of seabird breeding activity on the GBR is complex and can vary with location, year and/or species. In general, both northern and southern colonies

BOX 31.1 CRITICAL FORAGING LOCATIONS FOR WEDGE-TAILED SHEARWATERS

Recently it has been shown that adult shearwaters in the southern GBR cannot simultaneously replenish their own body reserves and maintain chick development using only prey caught near breeding colonies. Because of this adults use a specialised dual-foraging strategy where they alternate multiple short foraging trips in resource-poor, near-colony waters with long trips to highly productive areas at large distances from colonies. During near-colony trips adults sacrifice their own body condition to satisfy chick energy requirements. Then at some critical body mass they perform an extended trip to regain body condition. This foraging strategy is well known in southern ocean tube-nosed sea-birds but has not previously been seen in the tropics.

Using satellite transmitters, shearwaters from Heron Island have been observed travelling over 2000 km on a single 'at-distance' foraging trip to visit multiple seamounts in the Coral Sea. Still others have been observed foraging off the northern coast of New South Wales. At these locations extensive upwelling of nutrients supports very high levels of prey abundance. In both areas shearwaters were observed foraging along the edges of these upwelling zones. The conservation significance of such key foraging refuges cannot be overstated. The breeding success of shearwaters and probably other sea-bird species on a regional scale is likely to be totally dependent on the continued stability of upwelling at these sites. How climate change will affect productivity at these locations is currently unknown.

tend to have specific seasonal peaks in activity. Most breeding peaks occur over summer from October through to April each year, but for some species and especially in northern colonies breeding activity can occur at all times of the year. Different species may also use the same areas at different times of year. For example, masked booby breeding in the Swains Reefs peaks in winter, while brown booby breeding at the same locations peaks during the austral summer.

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