Egg colouration helps trace DDT contamination

red - brown speckles on bird-eggs can reveal presence of the insecticide ddt in the environment. A study from the Oxford and Cardiff universities on the eggs of raptors -- a group of carnivorous birds -- has found a link between the speckles, the presence of ddt and calcium deficiency. The insecticide acts as a calcium blocker in eggshells, giving them a speckled look. Researchers say this could be an inexpensive and safe method for monitoring ddt contamination in the environment. Previous studies have explained the link by studying the calcium transport in eggs of another group of birds; passerines.

The study has evolutionary significant for raptors, which eat the bones of their prey and are not normally calcium stressed. The researchers say that 'speckling' to strengthen eggshells seems to have been conserved over evolution, suggesting it is an essential character in maintaining the strength of the shell.

Colouring agents The study says that the two major pigments giving colour to eggs are biliverdin ix and photoporphyrin. Biliverdin ix gives the blue and green colours and photoporphyrin gives the red and brown colours. Biliverdin forms the base colour and is found all over the egg shell and photoporphyrin is deposited in spots.

Photoporphyrin deposits are signs of lack of calcium as well since more speckles are found on thinner patches of the shells--which lack calcium. Both the calcium and photoporphyrin are carried by the same enzyme from the shell gland and deposited on the shell. If calcium supply is inadequate, the enzyme can carry more photoporphyrin, leading to more speckles.

The eggs studied were found to have ddt levels ranging from 10-30 parts per million. The paper says though this link can't be generalised, this could be a useful indicator of ddt contamination for raptors. The study was published in the online edition of the Journal of Applied Ecology and is slated for publication in 2008.