Collectors have long been fascinated by the colours and patterns of bird eggs. Now research has shown that these marks of beauty can also function as indicators of toxic chemicals.
Evidence has been mounting this year that egg markings are connected to the health of the birds that lay them. Studies have shown that egg speckling intensifies when mothers are calcium deficient, and that eggs develop a blue-green hue when maternal immune-system strength is compromised. These discoveries led Andrew Gosler at the University of Oxford and a team of biologists to analyse eggs exposed to DDT (dichloro-diphenyl-trichloroethane).
DDT was discovered to be an effective insecticide during the Second World War, with the capacity to save lives by killing disease-transmitting insects. Later, it was used in agriculture until it was found to be an environmental hazard. In 1972, DDT was banned in the United States.
One of the problems created by the chemical was the thinning of bird eggshells, causing cracking and reproductive failure. Bald eagles, peregrine falcons, California condors and many other species were brought near to extinction.
Gosler was curious whether eggs laid by DDT-contaminated birds showed signs of poor maternal health in their colouration. He expected to see both increased speckling and green hues, because DDT is linked to reduced calcium and immune-system damage.
The study analysed 49 sparrow hawk eggs, collected from abandoned nests around the United Kingdom in 1996, all of which had been previously tested for DDT contamination. These tests revealed a range of chemical concentrations from 10 to 300 parts per million (p.p.m.) by volume, suggesting either continued illegal use of DDT in some areas or a worrying environmental persistence of the insecticide. Previous studies have indicated that a concentration of more than 200 p.p.m. causes shells to become brittle.
Toxic load
The researchers report in the Journal of Applied Ecology 1 a strong positive relationship between speckling intensity, green hue and DDT. Other toxins, such as mercury, are not expected to affect calcium and in fact did not affect speckling; they didn’t seem to affect the egg colour either, which surprised the researchers.
Gosler’s team later compared their sample to eggs laid before 1930. “We were stunned by the difference between healthy and contaminated eggs. I would never have noticed it before, but once we knew what we were looking for it was right there in front of our eyes,” comments Gosler.
The research comes at an opportune time. With increasing malaria resistance, the World Health Organisation (WHO) has endorsed DDT use indoors in areas where the disease has reached epidemic proportions. Although this is vital to stem the tide of malaria, it has many conservationists concerned. “We have no idea how much DDT is getting into waste water or whether it is being used outdoors regardless of the WHO recommendations, but as long as DDT is around we need to monitor ecosystems nearby,” comments Rick Watson, director of international programmes at the World Center for Birds of Prey in Boise, Idaho.
Normally the only way to test for the presence of DDT is to monitor shell thickness and run chemical tests. This is particularly difficult because egg fragments don’t last long in the wild where they are eaten for calcium or quickly dispersed. “If the colour of an egg can really be used instead of these tests, it would make pesticide monitoring a whole lot easier,” adds Watson.
Gosler is attempting to develop a quantitative test for DDT based on these visual factors, and hopes that software might be developed to analyse digital photos in future.
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