Birds exposed to a certain chemical have eggs that vary more in size from one to another in the same nest, but the average size of the eggs is about the same as in birds not exposed to the chemical.

We analyzed the available evidence and found that American kestrels exposed to PCBs tend to lay eggs with more variation in size within the same nest compared to unexposed birds, while the average egg size remains similar [1]. This pattern was observed across 11 studies or assertions, with none contradicting it. The variation means that in a single nest, some eggs may be noticeably larger or smaller than others when PCB exposure is present, even though the overall middle value of egg size doesn’t shift much. We don’t know why this happens, but it could relate to how the birds’ bodies manage energy or hormones under chemical stress. This kind of inconsistency in egg size might affect how well chicks develop, since egg size often links to the nutrients available to the growing embryo. However, we haven’t seen data showing whether this variation leads to lower hatching rates or weaker chicks — only that the size differences exist. Our current analysis shows this pattern consistently across multiple observations, but we still don’t know how common it is in wild populations or whether other factors might also cause similar changes. What we’ve found so far suggests PCB exposure may disrupt the consistency of egg production in kestrels, but more research would be needed to understand the full impact. For now, if you’re monitoring kestrel nests, noticing uneven egg sizes could be a sign worth tracking — especially in areas with known PCB contamination.