[tn-bird] DDT Impacts on Birds

The conventional wisdom is of course that DDT builds up at higher 
concentrations in birds at the top of the food chain.  This includes the 
raptors and wading birds, which suffered major declines after DDT was 
introduced in the 1940's.  Recovery only began after DDT was banned in 1972 in 
the U.S. and in Canada in 1973, and requiring more years while DDT and its 
derivatives dissipated from the environment.   

As an example, records of Reelfoot National Wildlife Refuge list 14 Bald Eagle 
nests for Reelfoot Lake in each of 1954 and 1955 (I estimate that roughly half 
that many nests were occupied throughout the nesting seasons, based on Albert 
Ganier's report during that era).  After only one known STATEWIDE successful 
eagle nest in 1961 (at Lake Isom, 5 miles south of Reelfoot Lake), there were 
no known fledged Bald Eagles in Tennessee until 1983, when one young Bald Eagle 
fledged near Dover, Tennessee.  The release of hacked Bald Eagles (287 in 
Tennessee during 1980 - 2003) and a few other rare raptors has sped up the 
recovery of these species.  Troy Ettel has reported that 76 young Bald Eagles 
have been recorded in 43 Bald Eagle nests in Tennessee during 2002 (compared to 
60 young fledging from 36 nests in 1999).  Late eagle nesting reports have 
usually been received well past this date.

The once huge heronries of Reelfoot Lake also virtually disappeared during 
these periods.  Wading birds have obviously made comebacks since the late 
1970's.  In 1996, Dr. Fred Alsop provided  the following two further examples 
(and more), as related to the Great Blue Heron and Great Egret of Tennessee:

Great Blue Heron - "The breeding colonies in West Tennessee decreased steadily 
since the early part of this century, but their decline has been most dramatic 
from the 1950's to the mid-1970's. At Reelfoot Lake in 1938 there were over 
1000 active nests of 5 species of herons at "Cranetown" of which over 300 
belonged to the Great Blue Heron. 1959 was the last big year for the colony 
with about 200 nests. By 1962 there were only 3 nests, and the following year 
there was none." 

Great Egret - "Much reduced since the late 1930's and early 1940's in 
Tennessee. At Reelfoot Lake in the late 1930's the bird was very common; an 
estimate of 3500 on the lake was thought to be "conservative" and there were 
over 650 nests. By the early 1960's this colony and others had "crashed" and 
many were defunct. For more than a decade Great Egrets were not found nesting 
in Tennessee. They reestablished small breeding colonies in the late 1970's and 
these colonies continue to slowly grow."

Since songbirds are not at the top of the food chain, it would not be 
surprising that they would be relatively unaffected by DDT.  However, the 
evidence is very convincing that DDT had a major negative impact on birds at 
the top of the food chain, at least in the United States.  We can be thankful 
that there are now better alternatives than DDT for fighting mosquitoes and the 
West Nile Virus, which will be a challenge at best.

Bob Hatcher
Brentwood, Tennessee

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