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
Who was “they” and what was their motive?