Sunday, 21 April 2024
From the archives:
Shorebird expert and co-author of CSIRO Publishing’s Australian Bird Guide, Danny Rogers, outlines the life and precarious times of our largest shorebird, the rapidly disappearing Eastern Curlew.
This article originally appeared in the March 2016 issue of Australian Birdlife magazine and has been adapted and shortened for web for World Curlew Day. For more great reads, become a BirdLife Australia member and receive our quarterly, award-winning magazine in your mailbox.
22nd January 2016. I’m wading across a channel between barrier islands at The Spit Nature Conservation Reserve, adjacent to the Western Treatment Plant (WTP), Victoria. A bird 300 metres ahead gives an alarm call as it takes off: a strident, hoarse ‘corr-ee, corr-ee’ that rings across the lagoon.
The curlew’s largely brown plumage is intricately patterned in close view; its white eye-ring somehow adds a wary and alert impression to a species that is indeed especially wary and alert. And above all there’s that outrageous bill – almost body-length, deeply decurved and vividly pink at the base of the lower mandible. What a bird!
I happen to know this individual Eastern Curlew quite well, having seen it several times each summer since I took on shorebird monitoring at the WTP in 2004. It’s the only Eastern Curlew at the WTP. I used to regularly see about twenty Eastern Curlews there as a teenage birder in the 1980s, but now only one remains.
This is not just a local problem. The entire Australian population of the Eastern Curlew has been declining by about 3 per cent per year since systematic shorebird counting became widespread in Australia in the early 1980s. The declines are occurring throughout Australia but are happening most quickly in the southeast – it is nearing local extinction in Tasmania and also New Zealand, where a small population used to occur.
These alarming declines have led to the Eastern Curlew being classified as Critically Endangered. It’s clear that a major factor has been the deterioration and destruction of tidal flats on the shores of the Yellow Sea where Eastern Curlews (and many other shorebird species) refuel when migrating between Australasia and breeding grounds in Far Eastern Siberia. This may not be the full story though: we do not yet fully understand why the Eastern Curlew is declining more rapidly than other shorebird species that face similar threats, or why it is declining more quickly in south-eastern Australia than elsewhere, or the role that Australian factors have played in this decline.
The Eastern Curlew is a particularly charismatic shorebird species. A quick overview of some of the distinctive attributes of the Eastern Curlew highlights not only its notable qualities, but may have some relevance to explaining its vulnerability.
The Eastern Curlew is the biggest shorebird species, a little taller and heavier than the closely related Eurasian Curlews and it also pips North America’s Long-billed Curlew for the longest bill of all the shorebirds. As we have uncovered time and again in the fossil record, a greater size is not the best strategy for avoiding extinction – the bigger they are, the further they can fall.
Correlates of large size include specialised diet, longevity, delayed maturity, wariness, and desirability to hunters. It may be that these traits could be contributing to the Eastern Curlew population declining more rapidly than most shorebirds.
Without doubt they are the most timid of our shorebirds. Constantly alert and nearly always the first wader species to take flight if a person or predator approaches, Eastern Curlews take their safety very seriously indeed. In principle one would think this a good thing. But with rapidly increasing human populations, there is inevitably less undisturbed coastal habitat for the Eastern Curlew. We need to refine our knowledge of the ‘buffer distance’ this species requires, and our understanding of the extent to which it can habituate to human presence.
There have been some tracking studies on the Eastern Curlew – hard-won data as the species is so clever and wary that catching birds is a huge challenge – that confirm direct flights from Australia to the Yellow Sea, Yellow Sea to their Arctic breeding grounds, with a similar track on return migration. In short, they are classic long-haul migrants – each migration basically consists of two major flights of several thousand kilometres, each requiring a huge amount of refuelling. Conservation of staging areas is therefore vital.
Pilot studies with satellite transmitters in 1999 were less successful, as the smallest transmitters available in those days seemingly impeded the curlews’ ability to fly long distances. (Contemporary geolocators are far lighter and prove no impediment to the birds’ movements across the globe.) The studies did, however, reveal a great deal about the responses of Eastern Curlews when they had problems making their migrations. The birds proved to be adaptable, managing to find stopover sites on coastlines they would not usually consider and managing to build up the fuel to return to their point of destination, admittedly at the cost of a missed breeding season. Those early studies offered some hope, as they indicate Eastern Curlews can adapt to changing circumstances. If we can leave some suitable habitat in strategic parts of the flyway, curlews will probably find it.
Another point illustrated by the tracking studies was the early timing of migration of Eastern Curlews. They are one of the first migratory shorebird species to arrive in Australia, with the majority turning up in August and early September. They are also the first to depart, with adults most leaving our shores from late February to mid-March. This may be in part why we don’t know all the staging sites used by Eastern Curlews in East Asia – they can be missed because they pass through before the typical schedule of shorebird surveys begins.
We know that the population crash is more pronounced at the southern extremity of the Eastern Curlew’s range but whether this is due to local conditions in areas like Tasmania and Victoria or whether it is a result of pressures along the Flyway remain to be seen. Perhaps birds that migrate further (and thus use more fuel) are more likely to have trouble building the energy stores required for successful migration.
At least, we think they are, though the difficulty of catching them makes it hard to obtain confirmation through banding and survival studies. They certainly take a long time to reach maturity. After they first arrive in Australia, young birds remain ‘down south’ for 2-3 years before they first migrate north and attempt to breed. It’s likely that this time is spent developing the foraging skills required to refuel under tight time-budgets when migrating north. Shorebirds are not especially fecund birds – they lay large eggs and no more than four can be incubated concurrently. Eastern Curlews cannot compensate for those 2–3 missed early breeding seasons by laying large clutches; instead they need to compensate by living longer than most shorebirds, and hence having more breeding attempts. The species depends on high annual survival (probability of living from one year to the next) and the size of the population is likely to be very sensitive to changes (even minor changes) in adult survival.
Eastern Curlews are very much coastal birds. At high tide they often roost on beaches, typically on the water-edge side of a flock of other shorebird species. They also often roost in saltmarsh – being quite tall they can see over short vegetation and thus can use sites that other shorebird species avoid. Eastern Curlew roosts are nearly always exposed, allowing the birds to scan for danger at long range. Their roosts are relatively undisturbed as curlews are typically shy. They are always near tidal flats, as that is the only habitat where non-breeding Eastern Curlews forage. There may be the odd historical record of Eastern Curlews on freshwater wetlands, but basically they aren’t interested in them.
Eastern Curlews feed largely on crustaceans, and most of their energy intake comes from quite large prey such as Ghost Shrimps and Sentinel Crabs. That’s what the long bill is for – they capture a lot of their prey with long neck-twisting probes or lunges of the bill down crab and shrimp burrows. They often give the captured prey a good shake to dislodge dangerous claws, but nevertheless when they swallow they ingest a lot of indigestible carapace as well as meat. These indigestible portions are ejected (vomited) in large distinctive pellets, and the study of their contents can reveal a great deal about their diet.
Most Eastern Curlew prey is large and wary, and you usually have to watch a foraging curlew for quite a while before seeing a successful prey capture. But that’s not the case when they are feeding on soldier crabs, small spherical crabs which occur in great wandering herds on some sandy tidal flats. Eastern Curlews have no difficulty catching them at all, delicately picking them up with the bill tip and swallowing them at a great rate. So why do they spend so much time hunting for big crabs and shrimps which are much more dangerous and harder to catch? One theory is that the ratio of meat to shell in a soldier crab is so low that eating one barely covers the energy cost of digestion, so a soldier crab diet is insufficient when Eastern Curlews are fattening up for migration.
The specialised diet of Eastern Curlews probably limits their range of habitat options. Not all tidal flats have the large prey they are interested in, and large mudflat invertebrate species are often the species least tolerant of habitat alteration and most likely to be harvested as human food.
Female Eastern Curlews are larger than males. They stand a bit taller and weigh about 20 per cent more, but the most obvious difference is in their bill length. Females have much longer bills, about the same length as their body, and with practice it’s quite easy to distinguish males and females in the field. A study in Western Port, Victoria, showed that the difference in bill length of the sexes was associated with differences in diet – only long-billed females could capture Ghost Shrimps reliably and they hunted in sandier habitats where the shrimps occurred, while males preferred muddier substrates with pools where they could find more crabs that were not burrow-dependent.
Australian-based birders are in a position to help the Eastern Curlew. Australia is the perfect place to monitor their numbers – not just to document a decline, but to identify ‘hotspots’ and ‘black spots’ so we can figure out the coastal management practices that work best for this species. If you have Eastern Curlews on your patch, simple non-intrusive behavioural observation can make a big difference – finding all of their roost sites (including the ones used on neap tides and at night), finding where they forage and what they eat, working out how many birds there are, when they arrive, when they depart and determining the sex-ratio. If you can work all this out you will have had some brilliant birding in spectacular settings and you’ll be generating the kind of local knowledge that is usually the foundation of effective local conservation efforts.
It must be remembered that Eastern Curlews are closely related to the only two migratory shorebirds species known to have gone extinct in recent times – the Eskimo Curlew and Slender-billed Curlew. It’s a group we can’t be complacent about – we do not want a third curlew joining its fellows in the extinction club.
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