In an attempt to
solve the baffler maintenance problem, Wingate decided to reverse the historic
chain of events which forced the cahow to compete with the longtail. He built
artificial burrows near the crowns of the islets, where there is vegetation and
a bit of soil, although not enough to enable the cahows to dig. The burrows are
lined with cement, and the nest chambers have removable lids to facilitate
observation. To date Wingate has made 14 burrows, of which five are occupied.
None has a baffler and none has been entered by longtails.
It was Wingate's
dream to build up the cahow population until a natural overflow was created,
and the birds would move to neighboring islets more closely resembling their
ancestral habitat—like Nonsuch, which he estimates could support 25,000 pairs.
To hasten the emigration, he had planned to record their call and play it back
on the slopes of Nonsuch. Then, as he somewhat facetiously proposed, the
program could have been put on a paying basis by selling surplus cahows to
hotels for gourmet dishes.
But although the
breeding population has gradually increased to its present level of 22 pairs,
the pairs that have raised chicks have declined during the past decade from
more than 60% to 25%. In the beginning Wingate attributed this to senility and
inbreeding, but the adults faithfully incubated the eggs and cared for the
chicks, which would be unlikely with senile birds; even more significantly, no
deformed chicks were observed, and if senility or inbreeding were a factor it
should be declining with the burgeoning population, the formation of new,
vigorous pairs and an expanding gene pool.
production of young may be due in part to a reduction in the percentage of
pairs laying an egg, but the primary cause is undoubtedly the failure of eggs
to hatch. In particular, there has been a growing mortality of chick embryos
either in the egg or at hatching.
In 1966 these
phenomena had become so prevalent that they ranked in significance with the
earlier mortality from longtails. The record of one breeding islet which
supported six pairs in 1967 is typical: of five fertile eggs laid, no less than
three failed to produce young. One chick died within a day after hatching and
the other two while pipping the egg. In general, this mortality has been
random, affecting different pairs in different years. However, a few of the
oldest pairs with an earlier record of breeding success have failed
consistently since 1961.
prompted to examine the possibility of pesticide residues in the cahow by the
extreme similarity of its plight to that of certain birds of prey, where the
correlation between pesticide residues and reduced hatching success has been
At first glance
it seemed inconceivable that the cahows could have become contaminated with
DDT. Pesticides are used on Bermuda, but the cahow islets have never been
treated, the birds spend most of their time underground and they feed far at
sea. However, in recent years it has become evident that DDT is present in most
of the world's animals, including birds and seals whose entire lives are spent
in the Antarctic. Man is not immune: for example, Americans average 11 parts
per million (ppm) of DDT in their fatty tissue. An organism doesn't have to be
sprayed with DDT to become contaminated. DDT is dispersed over the globe by
wind and water in much the same manner as radioactive debris. When DDT is
sprayed aerially only about half may reach the ground; the rest is dispersed in
the air, where it may circle the globe in a few weeks. Oceanic currents
distribute it too. Because DDT has a low solubility in water and a high
solubility in fatty tissue, it becomes concentrated in marine organisms, which
also act as carriers.
DDT residues are
very persistent chemicals, sometimes retaining their toxicity for decades. Due
to their solubility they accumulate in food chains and reach their highest
concentration in the final link. Some carnivorous birds carry residues at a
concentration more than a million times greater than their environment. In a
Long Island marsh sprayed with DDT for 20 years for mosquito control, the
plankton contained 0.04 ppm, small fish 0.25 to 1 ppm, larger fish nearly 2 ppm
and cormorants and mergansers about 25 ppm.
The cahow is the
terminal carnivore of a five-stage food chain consisting of phytoplankton,
zooplankton, small fish and squid. As Wingate and Dr. Charles F. Wurster,
assistant professor of biological sciences at the State University of New York
in Stony Brook, pointed out in a recent issue of Science, the cahow feeds
exclusively in the open ocean and is therefore "an ideal environmental
monitor for detection of insecticide contamination as a general oceanic
pollutant, rather than contamination resulting directly from treatment of a
specific land area."
In March 1967
Wurster analyzed two unhatched cahow eggs and three dead chicks and found DDT
residues averaging 6.4 ppm. Although this coincidence in itself does not
establish a causal relationship, there are convincing parallels to other birds.
Ospreys normally produce 2.2 to 2.5 chicks per nest, but a Maryland colony with
residues of 3.0 ppm in its eggs yielded 1.1 young per nest, and a Connecticut
colony containing 5.1 ppm produced only 0.5 offspring. (This colony declined
from 200 pairs in 1938 to 12 pairs in 1965.) In Britain breeding success in
five species of birds of prey with residues averaging 5.2 ppm in their eggs has
declined, while in five crow-like species, which are mainly herbivorous,
residues averaged 0.9 ppm and reproduction has been unaffected. Moreover, in
the last decade peregrine falcons have ceased to breed in the eastern U.S. and
the bald eagle, whose eggs contain an average of 10.6 ppm, may suffer a similar