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Flyway travellers' high energy lifestyles


 
Play  High energy cost of shorebirds' migratory lifestyles  chris hassell14min7.mp3  
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Chris Hassell, shorebird researcher with the Global Flyway Network, discusses some of the extraordinary changes that migratory shorebirds regularly undergo.    
Shorebirds are remarkably attuned to their environments and at the Roebuck Bay mudflats near Broome on the Kimberley coast they cope with huge tidal differences which can be up to ten metres. This means that on some days these mudflats are around four kilometres wide but when the neap tide arrives this will reduce to some three hundred metres.

Below: Extensive mud flats exposed at low tide, Crab Creek entry to Roebuck Bay. Image from Chris Hassell.

 

 
The tides dictate the shorebirds movements so it doesn’t matter whether it’s day or night when they feed and the probing nature of their feeding beaks works well in the dark. Shorebirds are great predictors of large tides. They gather on the mud in front of the mangroves and disperse into areas which they know will flood.

Considering that they have flown from The Arctic where the days are very long and then a week later are in 12 hour days and 12 hour nights, shorebirds cope remarkably well with such drastic changes. The birds behave similarly whether they have just arrived from their migration or whether they have been on the Bay for several months.

They do undergo physiological and hormonal changes before they migrate, which allows them to put on the large amounts of weight they need to sustain their migration. Just prior to migration, their stomachs shrink, their breast muscles increase slightly in size and their hearts increase in size. This flight preparation makes sense as they are not going to eat or drink for days and they will be in constant flight so they will need to maximise strength and endurance. When they eventually land at Roebuck Bay the shorebirds undergo another rapid organ reversal where their stomachs quickly expand while heart and muscles reduce.

This is high energy lifestyle and happens at least three times a year. First there is a long feed in Broome, then a flight to somewhere in the Yellow Sea where they undergo another rapid organ reversal in order to feed again. Just before the flight to the Arctic they undergo another rapid organ reversal which is then reversed at the breeding grounds. Given that they are also changing their feathers for the different circumstances of breeding, flying and camouflage, shorebirds’ physiology is forever changing. They are also quite long-lived. One 28 year old tagged Godwit at Roebuck Bay would have flown some 50,000 migratory kilometres so far, a distance further than to the moon, and is still a regular migrator.

 
Roebuck Bay abounds in birds of prey where at high tide, the narrow roosts may comprise many thousands of shorebirds. Shorebirds have strategies for minimising the risk of individual capture (for example, on the sand they are constantly on the move within their flocks, alert and ready for instant any varied flight), while prey birds have ways to maximise a successful strike. Typically, they will hover in the air, using the wind to remain virtually motionless, poised behind a tree or sand dune. From this hidden vantage point they can scan the shoreline, ready to swoop.
Image from Paul McQueen.
    

 
One of the reasons that shorebirds breed in the Arctic is that their chicks are born well developed, ready to run around on the tundra and feed themselves. Having 24 hour daylight also helps them feed. There is super abundant food in the Arctic summer and within three weeks the chick are the size of the adults and their bills have grown enough to cope with feeding on mudflats. Amazingly, their first migratory flight happens at around five weeks old.

Although the chicks are well developed, with long legs, their bills are short and soft and not suitable for probing in mudflats for shellfish or other prey. If they were hatched for example at Roebuck Bay, they would not be able to feed on the abundant biota in the mud and their down would become clogged with mud and water. By contrast, on the tundra, virtually all they have to do is open their mouths in order to catch the swarming insect life.
Interestingly, the chicks of the larger shorebirds stay at Roebuck Bay for their first year or two because they have to learn to cope with probing feeding. An Eastern Curlew has a 30 centimetre bill which it uses for feeding and it takes practice to differentiate between a shell with food and an empty one, an important preparation for a high energy lifestyle.

But why so far? It is likely that during an ice age period, the edge of the ice (which is where the extraordinarily rich insect life explodes in summer and hence provides suitable breeding habitat for the shorebirds) was much closer to the mudflats. As the ice has receded over many thousands of years, the migration journeys have slowly extended to their present extremes

Text: V.B. November 2009

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