Counting Shorebirds on Land, Sea and Air Delaware Bay May 2007
Victor Ayala, Mandy Dey and I bounced about in the small aluminum skiff trying our best to count red knots, ruddy turnstones, sanderlings, semipalmated sandpipers and dunlins and having a difficult time of it. During the previous few hours, the sea had grown nasty around Egg Island point, probably the most remote place on the Delaware Bay. The boat rolled and pitched in the high, steep waves that were driven by a stiff breeze from the southeast blowing against the tide. Most sailors fear wind against tide conditions in Delaware Bay and we were trying to deal with that as well as scanning the shoreline for birds.
Our goal was to do a ground count while Kathy Clark, Ron Porter and Humphrey Sitters counted from an airplane probably also suffering a bad time from this wind. Our entire team was spread out along the bayshore today trying to conduct counts simultaneously with the team in the airplane so we could, with the help of USGS biologist Jon Bart, develop a more robust estimate of the shorebird population on the bay. The count of red knots has dropped significantly over the last 15 years and is now at an all time low. We don’t consider our counts to be estimates of the total population because we don’t know what proportion stopover in the bay (although we are closing in on that number with our resightings project). Moreover, even when peak numbers are present, some may not have arrived, while others may have already departed. The counts do, however, provide a key indicator of the condition of the Delaware Bay stopover as do the other species counts. For example last year a sudden drop in the number of ruddy turnstones created a great deal of concern.
Our aerial counts are carefully designed so that they not only tell us the peak numbers of each species as accurately as possible but also a measure of the degree to which numbers vary across the stopover season. These are not just sample counts from which the total is estimated by extrapolation. These are counts of all shorebirds using the Delaware Bay. Moreover they have been carried out in exactly the same way with the same principal counter (Kathy Clark), at the same time in relation to the tide every week for the whole 6-weeks of the spring shorebird stopover for over 20 years. This consistency of methods gives the counts great credibility. In sharp contrast, the latest figure for the number horseshoe crabs in Delaware Bay is 18 million. But no one has counted 18 million crabs; it is just an estimate with a very large degree of uncertainty. As such it is a poor basis for judging the condition of the crab population or determining harvest regulations.
Though we have every confidence in the aerial counts, no survey in incapable of improvement and all surveys should be verified using different methods. So this year we are carrying out sample ground counts at the same time as the flights. These will be compared with the aerial counts and provide some estimate of the number of birds not seen from the air. In theory counters standing on dry land should be able to count every bird present on a beach accurately. However, for those bobbing about in a small boat, like Victor, Mandy and me off Egg Island, this may not be true. We intend to do a better job next week.
The aerial count of May 22 is intriguing. As suspected, knot numbers were a bit lower than this time last year probably because adverse northeast winds held up their migration. Kathy and her team counted 7,395 down from the May 23 2006 figure of 8,680. Thankfully, turnstones were up to 18,535, 34% higher than last years May 23 peak of 12,178, but much lower than the May 24 count of 2005, when there were 42,995. The next count should confirm the peak figure for most species so we are looking forward to its result with trepidation.
While we were busy counting the bay, Mandy Dey coordinated with other biologists to produce a US east coast estimate for red knots. On the same day that Kathy Clark counted 7,395 knots, Brad Winn counted 2,155 in Georgia, Bill Mace and Katherine Goodenough counted 125 in South Carolina, Barry Truitt and Bryan Watts counted 5,939 in Virginia, and Dave Allen and Sue Cameron reported a preliminary count of 125 in North Carolina. These figures total 15,763 which is comparable to last year’s figure of 15,494. In 2006 there was less coverage in South Carolina and the northern coast of North Carolina.
The final chapter of the count saga will be told later this week.
Victor looking out over Egg Island