March through May, 2009 - New Jersey Audubon Society

Figure 4. Mean peak counts (bars) with 95% confidence intervals (lines) of
shorebirds observed on Delaware Bay beaches in aerial surveys, spring 1986-
2008. Years were separated into groups of 6 years (except 1998-2002, 5
years) to better portray general trends.
over for shorebirds. Peak single day counts averaged
nearly 180,000 shorebirds, with a high of over 426,000
shorebirds counted on one survey in 1986, and a low
of 100,000 in 2003. Although we cannot estimate
how many individual shorebirds these counts represent
without knowing length of stay, these counts are
minimum numbers that place Delaware Bay among
the ten largest staging sites in the Western Hemisphere
(Senner and Howe 1984, WHSRN 2009). Moreover,
the number of shorebirds using the vast marshes and
mudflats of the bayshore were sampled in transect
surveys conducted in the early 1990s, and were
estimated to be more than double the number of
birds counted on beaches (Clark unpubl. data). However,
the actual number of total migrant shorebirds
using Delaware Bay cannot be estimated without a
better estimate of shorebirds using bay marshes and
nearby Atlantic coastal habitats concurrent with
shoreline surveys. The number of total shorebirds
may be useful in the future as an index of shorebirds
using the Delaware Bay shoreline, particularly in
the context of measurable changes in habitats and
sea level.
Another element of stopover importance is the
portion of a species population in one location at one
time (WHSRN 1990). Our data show that a major
portion of the Red Knot population uses Delaware Bay
during the spring migration. In recent years the Red
Knot population has been the subject of intense study,
resulting in a well-informed estimate of population
size of 18,000 to 33,000 in 2007-2008 (Niles et al.
2008). Thus the Delaware Bay stopover supports the
overwhelming majority of the hemispheric population
of Red Knots each spring.
Delaware Bay also hosts a large portion of the
Ruddy Turnstone population: there is low confidence
in the U.S. population estimate of 180,000 turnstones
(Morrison et al. 2006), but if it correct, then the
average peak count of 55,000 represents 26% of their
population, and high counts of 100,000 (observed in
1989 and 1998) are 47% of their population. Average
and high counts of Sanderlings in Delaware Bay
may represent 4% and 11%, respectively, of the total
U.S.–hemispheric Sanderling population estimated
at 300,000 (Morrison et al. 2006). The population of
Semipalmated Sandpipers is estimated at 2 million,
although there is low confidence in that number.
The number of semipalmateds counted on Delaware
Bay beaches is between 4% and 13% of that figure
(Morrison et al. 2006), although many more were
certainly uncounted in the salt marsh mudflats. Red
Knots, turnstones and Sanderlings were all classified
by Andres et al. (2006) as species of high conservation
concern, mostly due to declining populations as
measured in Delaware Bay and elsewhere.
90 — March through May, 2009