Delaware Bay, Delaware Bay 2011, shorebird ecology

The early news is knot good -Delaware Bay Shorebirds 2011

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The most important variables describing the status of red knots on Delaware Bay are the size of the migrant population and the percentage of the flock that reaches 180 g, the minimum weight necessary to make it to the Arctic to breed successfully.  We knew before the May 2011 season that the number of red knots in their main wintering areas were down by as much as 30% or more.   We knew that the percentage of knots making weight is far below what it once was in the late 90’s.  This was a direct result of the lack of any improvement in the number of horseshoe crabs and the density of crab eggs available to shorebirds for at least the last 10 years mainly because the harvests have stayed the same in this period.   Most worrying was the growing evidence that the drug companies, bleeding predominantly female crabs for the chemical lysate, were killing more than they were admitting.  Given all of this bad news and uncertainty, our team worked hard to get accurate data to help us understand.

We still can’t be certain because all the data must be analyzed, but the early results suggests the number of knots on Delaware bay are down by 2000-3000 birds probably around 12,000-13,000.  The percentage of birds making it to 180 g on the NJ side of Delaware Bay is similar to last year – about 40% — which is still half what it should be.  These numbers are not good, but what do they mean?

Survey plane flying low over Kimbles Beach, NJ

First the baywide survey numbers are difficult to tease apart.  There were three baywide counts done this year, including ground and aerial counts.  The counts took place every 4 to 5 days starting around the 21st. of May.  In this way the counts would cover the peak of the migration, usually about May 24th.   Counting too soon might mean missing late arriving flocks, too late and we might miss the first of the heavier birds flying out of the bay to the Arctic.  There is considerable room for error.  Over- and under- counting are possible, and the less experienced the observer, the greater the chance of error.

Even a good count is difficult to understand.   We caught 5 knots with geolocators this May (which help us track their movements) including one recovered from a bird wintering in Florida that lasted long enough to record data for two northward spring migrations.  In the first trip north, the knot skipped the Delaware Bay, but in the second he came to the bay where he was caught again.  The big questions are, how many short-distance migrant knots do the same thing, and how does this affect the estimate of the peak stopover population especially as long-distance migrant knots decline?  There were once 67,000 red knots in Tierra del Fuego wintering areas, now there are about 10,000.

The average weight of red knots caught in NJ.(1997 to 2011). The dotted curve describes the average of all catches the dashed line is an estimate of weight increase necessary for birds to reach 180g by late May. Average weights stayed well below the 180 g threshold in 2011

The percent of knots making 180g must also be considered seriously.  We know this variable, based on knots caught just prior to departure (May 26 to May 28th), is strongly correlated with the density of horseshoe crab eggs on Delaware Bay.   It is also influenced by the number of birds that leave the bay as they achieve sufficient weight.  This might change year to year.  This year we had suitable winds for migration almost the entire ten-day period from May 20th to May 30th.  Did small groups of birds leave as soon as they reached sufficient weight?  In years when conditions for migration are mostly unfavorable — as are most years on Delaware Bay — do birds tend to stay longer?  All of this would affect the percentage of birds reaching 180g.

Storms developing in the Atlantic in 2010

Despite all of these considerations, the decease in the number of knots and the lack of any improvement in all the other variables spell rough times ahead for the knot because other factors influencing mortality are also afoot.  Crossing the Atlantic-Caribbean in August and early September is becoming more hazardous, and this year is forecasted to be an active hurricane season.   Moreover, there is growing evidence of mortality from toxic events like red tide — an algal bloom that creates a buildup of toxins in seawater so dense that it can kill beach invertebrates and cause human respiratory problems.

Every year the case for action to recover the knot and other shorebirds gets stronger but a declining number of birds, and a growing list of problems makes the feasibility of restoration grow dimmer.