Are we moving in the right direction?
The pesticide DDT causes eggshell thinning in merlin and other birds of prey. This can lead to breakage during incubation and increases evaporation through the eggshell, so that the eggs are more likely to dry out. DDT is still having an impact today, even though Norway brought in a general ban on it in 1970 and a total ban in 1988. The shell index is now coming back to normal values.
In Norway, eggshell thinning has been found in many birds of prey, including the white-tailed eagle, golden eagle, goshawk, osprey, peregrine falcon and merlin. If eggshell thinning exceeds 15 per cent, the population of the species generally declines. For merlins, there is clear evidence that eggshell thickness is lower than before DDT came into widespread use. However, the ban on using DDT and other dangerous chemicals has improved the situation. The number of merlins observed on migration has risen slowly but steadily from 1970 onwards. Unfortunately, we know that concentrations of new hazardous chemicals in merlins are rising. We do not know whether the levels of these chemicals are high enough to cause damage.
Published by Directorate for Nature Management, 01.12.2009, 09:34.