The Shorebird Project: Delaware Bay, New Jersey, USA

The entire team waited patiently for a few of the 300 red knots roosting and feeding on Stone Harbor point to be twinkled by Peter Fullagar into the catch area of the three cannon net. We had being trying for over an hour to move the birds into position but without luck. Peter was working hard twinkling the birds so slowly that you couldn’t really tell he was moving. The team including Sue, Mandy, Dick, Pablo, Victor, Alice, Barry, Angela, Jeannine and Philippa sat in rapt attention by the firing box while an unseasonably cold wind pushed sand across the long sandy flats adjacent to Hereford Inlet. But the wind-blown sand forced the birds into a new place just outside of our catch area. We decided to relocate the net. Eventually, Peter twinkled the birds onto position and we caught 84 red knots. This was an important catch as it represents the birds that feed on the Atlantic Coast which are most often birds from the southeast US wintering population. It was also the second catch in a very long day that started at dawn.

But the most spectacular event of the evening was the gradual disappearance of three small flocks of knots and sanderlings into the north. It always seems such an impossible coincidence to actually be there when a group of birds, that have been around the bay and Atlantic coast for two or three weeks, suddenly make the decision to lift up and fly on a 2,500 mile journey. And yet there was no mistaking it. They took off as usual, flying together but in a loose formation. Usually they are responding to some unknown cue, the need to move for a better place to feed, a real or imagined threat from a predator. But on this evening of strong winds from the south they took off and gradually gained altitude. Flying higher and higher, in widening circles the flock started to reform into a ‘V’, the most efficient formation for flying long distances. Turning into the north they slowly disappeared from sight. Next stop Southampton Island or some other Arctic tundra breeding site. It was thrilling!

Our catch that evening told the whole story. Although the mean weight was 171 grams or 9 grams below what is considered to be average lift-off weight of 180 grams, the numbers disguised the existence of two different weight-groups. The majority were in the higher weight-group that averaged 182 grams while the lower weight-group averaged only 128 grams. This illustrates an important characteristic of the migration: most of the birds that feed on the Atlantic coast arrive on time and gain sufficient resources of fat and protein to fuel their onward migration while others are less fortunate; perhaps they arrived late or in poor condition, or perhaps couldn’t find sufficient food. Either way, they are behind in the race to get to the Arctic in time to breed successfully.

Distribution of red knot weights from May 27 catch at Stone Harbor

A similar weight pattern was evident among the knots we caught earlier the same day at Fortescue on the Delaware Bayshore. Probably the fat birds came to the bay 10-14 days ago arriving around the time of the last spring tide when there was a good horseshoe crab spawn. This led to an abundance of eggs and they fattened quickly. The second group probably arrived later and found fewer eggs because it was a period of neap tides. Before the collapse of the horseshoe crab population, eggs were available throughout the month regardless of the tide. Now with far fewer crabs they can all spawn on the best tide, the high spring tides, and avoid the low high tides or neap tides.

When the fatter birds reach sufficient size they leave for the Arctic. This pushes the average weight of the birds in the bay downwards, because those left behind have lower weights. Each year we see groups of birds coming to the bay late, but it is only in about the last five years that they seem to be having a more difficult time. We will have to wait and see what will happen to them this year. But it’s always good to watch fat birds spiral up into the sky chattering excitedly and disappear northwards!

Red knot weight gain (grams) tracked throughout the stopover period (early May to early June) on Delaware Bay; 1997 to 2007