Clive Minton report from Delaware Bay May 31, 2009

This is a 'good news' report. After twelve years of depressing reports of reduced horseshoe crab spawning, decreasing shorebird numbers and poor rates of weight gain we at last have what appears to be a turn round in the situation. It's too early to be absolutely certain that things will be better in the future because we cannot separate this years results from the obvious major contribution deriving from the best May weather conditions we have experienced in the thirteen years of the study.

Let me give you some more detail - on the weather, on the horseshoe crabs, on the shorebird numbers, on our banding, on the weights of birds and on our data logger application.

Firstly the weather. We have only had two wet days and one really windy day in the three and a half weeks since we arrived here on May 7th. The relatively calm sea conditions have been particularly beneficial to crab spawning and contrast markedly with the ten continuous days of cold wet and windy weather we experienced in May last year. We have only had the one cool (beany) day as north winds have been almost completely absent. Most days it has been in the 20-27C range - perfect.

Good numbers of crabs came ashore at high tide to spawn during the full moon period in the second week of May. Because of the calm weather they continued right throughout the neap tide period. Then there was a huge bonanza of spawning over the spring tide period of the New Moon in the fourth week of May. Crabs were ten deep on some of the beaches, absolutely covered some of the sheltered creek mouths and eventually spawned along every beach of the Delaware Bay coast. Sufficient food was therefore available for even the earliest arriving shorebirds and by the last week there have been winnows of crab eggs along every shoreline and heaps accumulating in all the pools on the foreshore. This is the first time we have seen this in New Jersey for five years or more and it is much more like the situation that existed when we first started the studies here in 1997. Whilst a part of the increase in spawning horseshoe crabs is associated with the relatively calm seas it does appear that a genuine increase may have come from the first beneficial effects of the restrictions on horseshoe crab harvesting which began to be introduced in 1998-9. Horseshoe crabs take ten years to reach breeding maturity.

Waders started to arrive in good numbers at the end of the first week in May, rather earlier than usual. With the settled weather conditions arrivals were fairly continuous until the last birds got here around May 23rd. With feeding conditions being good it seems that they all remained, though as usual Red Knot gyrated around the Bay with their distribution patterns changing quite markedly over each two or three day period. As usual Misspillion Harbor in Delaware, became the focal point when Red Knot numbers were estimated to be 17 000 on May 24th and an amazing 25 000 on May 25th. At the same time we had between 4 000 and 6 000 Red Knot on the New Jersey side of the Bay. The total of around 30 000 was slightly above the previous estimated population for rufa Red Knot in the flyway last year and well above the highest total recorded on any one day in Delaware Bay in recent years. However, we don't think this represents a significant turn around yet in Red Knot population numbers. More, it is the result of the unusual weather and feeding conditions, which meant that this year all the Red Knot had arrived in DB before any had departed. Additionally, we know that the count in Virginia was much reduced this year (3 500 max - versus 7 000 normally) and that even these only stayed there temporarily as feeding conditions were apparently not good there on the shores of the Atlantic coast. We thus, therefore, probably had almost the whole of the rufa Red Knot population present together in Delaware Bay for a couple of days.

Arrival weights of Red Knot, Ruddy Turnstone and Sanderling were all good this year, even on the early arriving birds. Few were below fat free weight. Some of the cohorts arriving around the 20th May were rather low in weight however, because they had had to go through particularly unsettled weather down in Florida. Average weights of nearly all the catch samples of all three species throughout the month have almost always been above the average weights of birds for the previous twelve years of the study. This is in marked contrast with last year when the early arriving birds certainly 'didn't catch the worm', but instead almost starved. As an example of the good situation this year over 50% of the Red Knot we caught on May 26th were above our estimated average departure weight of 180g (last year this figure was 17%). As a consequence there were massive migratory departures of Red Knot that evening with an almost 80% of the population leaving. May 25th-28th was the traditional May departure period and this is the first time that a major part of the population has been able to depart northwards on time for several years. Today (May 31st) we still have a couple of thousand Red Knot and a few thousand Turnstone in the Bay (and probably several thousand Sanderling) but even these later arriving/fattening birds look likely to reach satisfactory departure weights in the next two or three days, given the copious supplies of spilled horseshoe crab eggs on all the shores. So when the team disperses on June 2nd the shores are likely to be almost empty of shorebirds, in marked contrast with most recent years when we have gone home (usually on June 5th) with thousands of shorebirds still remaining.


As the above information implies our catching program has gone very much according to schedule enabling us to monitor weights and arrival cohorts throughout the period from May 8th up to the present time. We have a Sanderling catch scheduled for today, but it is possible that we change that and try for a final Red Knot catch (or try for these tomorrow if any are still left). In 16 cannon net catches on the New Jersey side we have so far had 640 Red Knot, 787 Ruddy Turnstone and 629 Sanderling. Excluded are by-catches of several hundred Dunlin and Semis-palmated Sandpipers (500 of each?) most of which have been released unbanded although we did have the separate Semi-palmated Sandpiper study team with us on one occasion. I don't have figures for the catches on the Delaware side of the Bay (they only operated from the 16th -30 May) but they did have a similar number of Red Knot, fewer Ruddy Turnstones, and one small catch of Sanderling (plus lots of Semi-palmated Sandpiper and Dunlin also).

We put geolocator data loggers on 47 Red Knot. These were the same product that we used recently on Ruddy Turnstone in Victoria. They were all applied via a leg flag on the tibia though a variety of designs was used. The main difference was that darvic spacer rings were placed on the tibia below the flag with the geolocator attached in order to raise this a little above the åkneeπ joint. All birds with data loggers were placed in a specially constructed pen and observed for ten minutes (and compared with banded/flagged birds without geolocators) before release. Great efforts were also made to look for these birds in the field during flag scanning operations and over 30 individuals were relocated (some several times) and some observed for extended periods. Almost all walked, ran, fed and roosted normally. A few seem to find the geolocator attachment slightly inconvenient, but no injuries or serious inhibitions were observed and at least one such bird was later observed not exhibiting any inconvenience. By good fortune two of the nine within-season recaptures were birds carrying data loggers. One had gained 11g in weight in three days and the other 56g in 15 days. These rates of weight gain (3.7g per day) are similar to those for the population as a whole.

All we have to do now is recapture some of these birds so that we can download the data loggers at Delaware Bay next year (or in a later year) or at one of their wintering locations in Florida, Argentina or Chile!

It has been really pleasing and enjoyable to be part of the team which has at last seen some potentially significant benefits from the huge study and conservation efforts which have been put in at Delaware Bay over the last twelve years. Because Red Knots at best only reproduce slowly and Horseshoe Crab eggs take a decade to turn into egg-laying adults recovery to mid 1990s levels of shorebird numbers will inevitably take a long time but it is really good to see that this process has started.