Broome Birds on the Move

An article written by Jan Lewis

Although research on the migration of shorebirds from Roebuck Bay and Eighty Mile Beach to their Arctic breeding grounds has been going on since the 1980s, there are still large gaps in knowledge about migration routes and breeding ecology.

You probably remember the wonderful flight data revealed from the satellite transmitters fitted to Bar-tailed Godwits in 2008 (see / if you have forgotten) and will be pleased to know that two of those birds (C6 and H3), now at least 9 years old, are still seen in Roebuck Bay. New developments in battery miniaturisation will probably allow smaller species of shorebirds to be fitted with similar transmitters in the future but, until recently, researchers have had to rely on leg flag re-sighting to find stopover sites and breeding grounds, the lack of access to most breeding areas being an obvious limitation.

Since 2009, geolocators have been added to the shorebird researcher’s tool box. These are small tracking devices which can be attached to a shorebird’s leg. They record the time of dawn and dusk, from which an estimate of location can be deduced. They also record conductivity every 10 minutes, which identifies whether a bird is in contact with sea water, thereby allowing migration departure and arrival times to be estimated. The upside of this technology is that birds weighing as little as 50gm can carry it. The downside – the geolocator must survive immersion in salt water and extreme temperatures AND each bird carrying one has to be recaptured in the following year(s) so that the geolocator can be retrieved.

There was therefore great rejoicing in the last week of October when shorebird researchers from across Australia, together with Yawuru/ DPaW Rangers and local volunteers, retrieved 10 geolocators from a series of catches in Roebuck Bay: seven from Great Knots, fitted a year ago, two from Red Knots fitted in 2011 and one from a Greater Sand Plover attached in 2010.

Fingers crossed that the latter two will yield 2 years’ data – although success is not certain. Geolocators retrieved from Greater Sand Plovers migrating to breeding grounds in the Gobi Desert in China/Mongolia behaved erratically, exhibiting symptoms that suggested extraneous electromagnetic interference! Researchers have been delighted with the additional information that this relatively simple technology can reveal.

When birds are in the continuous daylight of the Arctic summer, geolocators provide no evidence of their location, but their output has often shown more or less regular patterns of light and dark. The realisation that this variation was caused by the bird shading its geolocator by sitting on its nest during incubation offered the opportunity to study breeding behaviour in species rarely observed on nests.

Information that could be gathered includes:

(i) dates of arrival and departure from the breeding grounds;
(ii) the proportion of time that adults incubate eggs;
(iii) the proportion likely to have successfully hatched eggs (because they incubated for a full term);
(iv) evidence of re-nesting following a failed first attempt;
(v) the total time spent on the breeding grounds; and
(vi) differences in breeding behaviour between males and females.

Wow – what a bonus!

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