[gundog]

Hens eat broken eggs, to be sure. And I had a Buff Orpington break her legs in a high jump. But hens are an exception. They lay nearly every day. Birds of prey will lay two to four eggs a year. Wild birds were destroying their eggs in the process of moving their feet around them and turning them as they incubated.

[Harmless Teddy Bear]

That was probably because of mercury not DDT. In fact the shells began to thin before DDT was used and during the time DDT usage was the highest the shells began to get thicker. This was also the time that there was a reduction in mercury pollution.

The birds they used in the study were quail. Which are fairly close to chicken.

The odd thing is that feeding the birds a regular diet that included DDT did not reduce the number of eggs the bird laid, did not result in thinner shells and actually more eggs hatched then was normal.

It was not until they hit on the low calcium diet that they could "prove" that the problem was DDT