23 June 2015

A Knot eggstravaganza

Delaware Bay, on the east coast of America, is positioned on the West Atlantic Flyway and is the final staging area for many thousands of waders on their migration from their South American wintering grounds to their Arctic breeding grounds. In spring, the Bay is also home to Horseshoe Crabs. These prehistoric-looking creatures spawn on the beaches in their thousands and their tiny eggs are full of protein – just what hungry waders need to replenish their body fat and to provide them with enough energy to complete their journey north.

The Horseshoe Crab eggs that the birds rely on. Photo by Ruth Walker

Unfortunately, for a number of complex reasons, the shorebird populations have diminished in recent years. As a result, in the mid-1990’s, Wash Wader Ringing Group members helped American scientists to establish the Delaware Shorebird Project. The project aims to research and monitor the health of the wader populations on the Delaware side of the Bay in order to better understand the connection between the birds, Delaware Bay and the Horseshoe Crabs. A sister project runs concurrently on the New Jersey side of the Bay. The birds use both sides of the Bay to feed, often swapping shores to avoid inclement weather. The data gathered are helping to identify and protect the resources that are so critical to the success of the wader migration. In part due to the work of the project, the key target species for the project, Red Knot, has recently been officially recognised as ‘threatened’ by the US Fish & Wildlife Service.

Red Knot. Photo by Ruth Walker

Location of Delaware Bay on the Red Knot's spring migration route

A number of BTO ringers, including BTO staff, still travel out to Delaware each May using their holiday time to volunteer alongside other researchers from around the world on this important project. The study aims to catch and fit individually coded colour flags to 350 individuals of each target species each year: Red Knot, Ruddy Turnstone, Sanderling (all of which are colour flagged) and Semi-palmated Sandpiper (though these are not flagged on the Delaware side of the Bay). Samples of Dunlin and Short-billed Dowitcher are also ringed and measured if caught. During their time in the Bay, birds can double their body weight and therefore catching birds throughout the season is vital to ascertain their fitness levels. In order to survive their migration, it is thought that Red Knot need to weigh at least 180g when they depart (having arrived a couple of weeks earlier weighing 110-120g).

The very similar looking Semi-palmated Sandpiper (left) and Sanderling (right). Photo by Ruth Walker

We aim to ensure that approximately 10% of the Red Knot population passing through the Bay are flagged, along with smaller percentages of Ruddy Turnstone, Sanderling and Semi-palmated Sandpipers. Because waders are global travellers, flagging projects are undertaken in a number of different countries. The flag colour indicates the country the bird was flagged in, so birds caught in Delaware have a green flag, but we also see birds with red (Chile), orange (Argentina), blue (Brazil) and white (Arctic) flags . We attempt to catch approximately every three days so that we have weight samples throughout the stopover period, with the rest of the time spent counting the numbers of birds using each of the beaches each day, re-sighting the colour-flagged birds as well as the seemingly endless task of entering and checking all of the data. Re-sighting colour-flagged birds not only gives information on arrival and leaving dates and movements of individual birds within the Bay, but also information on survival and the total number of birds using the Bay.

An American-ringed Ruddy Turnstone foraging. Photo by Ruth Walker 

This year, 362 Red Knot were caught, along with 284 Ruddy Turnstone, 75 Sanderling, 407 Semi-palmated Sandpipers, 472 Dunlin and 78 Short-billed Dowitchers. It was very pleasing to see that of the Red Knot ringed during the final catch of the season, 74% weighed more than 180g, 23% were over 200g with the heaviest bird weighing in at a whopping 228.6g! Thankfully, only 4% weighed less than 150g and would need to remain for longer in the Bay.

As the birds departed the Bay, so did the BTO staff and other volunteers. We look forward to catching up with the birds again next year! 

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