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Left behind? Shorebirds at Roebuck Bay in winter

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Danny Rogers provides an insight into why some young shorebirds stay in their Australian feeding grounds rather than migrating to their northern breeding grounds.

Roebuck Bay in Western Australia is an important feeding ground for northern shorebirds that have migrated from their distant breeding grounds in the steppes of Mongolia and the high Arctic. In April these shorebirds make the long return flight back to their northern breeding grounds. Setting off on a direct flight across the Timor Sea, they first make landfall on the Chinese coast before continuing their migration to their northern summer breeding grounds.

Left: A juvenile marsh sandpiper awkwardly and probably unsuccessfully attempting to probe for prey - a hard skill to master.

Photo: Jan van de Kam from Life along land's edge p 110

The preparation for this migration takes months and the feeding grounds of Roebuck Bay are the source of the stored energy that is required for this long journey.

The birds must virtually double their body weight to reach the Chinese Coast, their main stopover on the way north. Young birds, still learning the art of successful foraging, cannot feed fast enough to build up their bodies sufficiently by the departure deadline. They must reach the breeding grounds by the beginning of June to have any chance of breeding successfully in the short nesting season of the Arctic summer. So, not surprisingly, the many birds left at Roebuck Bay over the Australian winter are too young to make the return flight north.

Above right: A young striated heron practises manipulating an inedible leaf - with more success than with attempts to capture life prey,  Photo: Jan van de Kam from Life along land's edge p 111      
What is surprising is that the saltwater habitats and the freshwater habitats of some species are a determinant of why some shorebirds are forced to stay around Roebuck Bay. In particular, the intertidal habitat proves much more difficult as a feeding source for young shorebirds, given that they are probing for bivalves that burrow into the mudflats.
Left: Two juvenile Great Knots show their inexperience by feeding far from the main flock - a dangerous practice. Photo: Jan van de Kam from Life along land's edge p 110

A comparison of stints gives a fuller picture. The coastal Red-necked Stint never migrate back north in their first year, whilst the freshwater Little Stint migrates when it is one year old. It is all to do with the ease of feeding.
However, salt water migratory shorebirds seem to make up for their slow start to breeding by being exceptionately long lived with very high survival rates despite their annual gruelling migrations.



Above: A flock of immature shorebirds at Roebuck Bay in June, a time of year when the adults are thousands of kilometres away at their northern breeding grounds
Photo: Jan van de Kam from Life along land's edge p 111

Text: V.B.

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Life along land's edge Feeding styles at Roebuck Bay

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