When captive American kestrels eat food contaminated with PCBs at a certain dose, their egg yolks get heavier and the clear egg white part gets smaller, which might mean the baby birds inside don't get the right nutrients to grow properly.

We analyzed the available evidence and found that 11 studies or assertions support the idea that when captive American kestrels eat food contaminated with PCBs at certain levels, their egg yolks become heavier while the egg white becomes smaller. No studies or assertions in our review contradicted this pattern. This change in egg composition may affect how well the developing chick receives nutrients, since the yolk and white serve different roles in early growth — the yolk provides fats and proteins, while the white offers water and additional proteins. A smaller white could mean less available fluid and protein for the embryo. What we’ve found so far suggests a consistent link between PCB exposure in bird food and these specific changes in egg structure. The evidence we’ve reviewed leans toward this being a real effect under the conditions tested — specifically in captive kestrels fed controlled doses of PCBs. We don’t know if this happens in wild birds, or at lower exposure levels, or in other bird species. The studies we reviewed focused only on one type of bird under controlled settings, so we can’t say how broadly this applies. The evidence doesn’t tell us whether the changes harm chick survival or development — only that the egg’s physical makeup shifts. For now, our analysis shows that PCB contamination in bird food is associated with heavier yolks and smaller egg whites in American kestrels. If you feed birds food that may contain these chemicals, it’s worth considering whether the source is clean — especially if you’re caring for birds in captivity.