Bird Study, Conserving Wildlife, Delaware Bay, Science, shorebird conservation, shorebird ecology, Uncategorized

Shorebirds out in the cold

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It’s well known that the Delaware Bay shorebird stopover depends on the horseshoe crabs.  Few know that the Delaware Bay is a near perfect horseshoe crab habitat.

There are many places on the eastern seaboard where horseshoe crabs breed.  Most are too small to provide sustenance for energy starved shorebirds.  Places like Cape Romain Refuge in South Carolina, have enough horseshoe crabs so that one breeding female unearths eggs of another and thus lays out a tidy meal for shorebirds.  But the areas are small and at this time unimportant to the population of shorebirds. Most of the others are too small to have eggs reach the surface. Its only in Delaware Bay where crab numbers reach into the millions and spawn in such great numbers that they spread like a carpet over nearly all beaches from Gandy’s Beach to Villas, approximately 20 miles of spawning habitat.  The number of eggs and ultimately hatched young reach staggering numbers.

 

 

The huge population of horseshoe crabs on the bay is no accident.  The bay almost seems built to suit the crabs.  Crabs need beaches with large and deep sand flats, allowing just enough water to sufficiently oxygenate the eggs without drowning them.  They need the sea floor to gently rise into breeding beaches, allowing easy access.  While breeding, crabs have to eat small bivalves, which they find in abundance in the bays extensive inter tidal and subtidal flats.

horseshoe crabs spawning at dusk, on Reeds Beach on Delaware Bay

horseshoe crabs spawning at dusk, on Reeds Beach on Delaware Bay

The most important aspect of the bay is its quickly warming waters in the spring.   In Delaware Bay horseshoe crabs don’t breed until waters reach 59 degrees. You won’t see this temperature on the Atlantic Coast until June. Not so on Delaware Bay.  Although the bay has deeper water, mostly in sloughs that snake under the surface out to its mouth at Cape May, most water is relatively shallow – usually less than 18 feet.   That may sound deep but keep in mind the Chesapeake has 100 feet water for most of it length and deeper water throughout.  The Delaware Bay shallow water allows it to heat up soon after the air temperature rises.

 

A graph of water temperature on Delaware Bay in April and May 2016. The critical temp is 59 degrees f. determines whether crabs spawn.

A graph of water temperature on Delaware Bay in April and May 2016. The critical temp is 59 degrees f. determines whether crabs spawn.

 

Unfortunately, it also cools down quickly too and because of this the crab spawn has stopped. The bays water temperature went up dramatically in late March and April, so much so that we worried it might reach the critical 59 degree threshold in April long before the birds arrived in May.  But then the warm weather stopped and the bay temperature dipped than rose, several times in fact.  By early May it had gotten just above the threshold, heating up to about 61 degrees at the Cape Henlopen marine buoy.   We hoped for the best.  Crabs started to breed in good numbers on a few beaches, like Reeds Beach, but were thin elsewhere.   We had about 20,000 shorebird relying on the spawn and the eggs that were brought to the surface.

Shorebirds and gulls on Reeds Beach at high tide, but with no horseshoe crab spawn.

Shorebirds and gulls on Reeds Beach at high tide, but with no horseshoe crab spawn.

Than the bay cooled down again. A nasty western wind and cold front enveloped our area on the weekend and the cool weather followed.  By Monday the temperature went down again and the crab spawn stopped.  This is bad.

 

When the spawn stopped the birds hovered up the remaining eggs in a few days.  Than they started wandering to find eggs in odd places, under houses, along bulkheads.  Some even went to areas like the oyster aquaculture racks to find eggs to the delight of the people trying to expand aquaculture.  But what they saw was desperation.

Worse the Black Back and Herring Gull that feed on eggs and overturned crabs couldn’t find eggs and started eating shorebirds.   By the end of Thursday (May 19) we found 8 dead red knots.  These gulls can swallow small sanderling and semipalmated sandpipers like gum drops so we really don’t know how many shorebirds died.

Herring gull killing red knot, Cooks Beach, NJ Photo by Jack Mace

Herring gull killing red knot, Cooks Beach, NJ Photo by Jack Mace

 

Relief might be in sight though.  The last two days and tomorrow will be warmer.  The bay’s temperature is back up over 60 degrees.  Last night we had a fairly good spawn.  Hopefully this morning we may be back in business.