The longer a female kestrel eats food with toxic chemicals called PCBs, and the later she lays her eggs, the more of those chemicals end up in her eggs.

We analyzed the available evidence and found that 11 studies or assertions support the idea that female kestrels with longer exposure to PCBs in their food, and those that lay eggs later in the season, tend to have higher levels of these chemicals in their eggs. No studies or assertions contradicted this pattern. What we’ve found so far suggests a connection between the duration of a kestrel’s exposure to PCBs and the timing of egg-laying, and the amount of these chemicals passed into the eggs. PCBs are toxic industrial chemicals that can build up in animals over time, especially in those higher up the food chain. The longer a female kestrel consumes prey contaminated with PCBs, the more of these chemicals accumulate in her body. When she lays eggs later in the breeding season, it appears that more of these stored chemicals are transferred to the eggs. This pattern was observed across multiple observations, but we did not find any data showing the opposite — that longer exposure or later laying leads to lower PCB levels. However, we also did not analyze how much this increase matters for the health of the chicks, or whether other factors like diet changes or weather might influence the results. Our current analysis shows a consistent trend, but we don’t yet know how strong or predictable this relationship is across different populations or environments. If you’re tracking kestrel health or pollution levels, this suggests that monitoring both the timing of nesting and how long birds have been living in contaminated areas could help identify which eggs are more likely to carry higher chemical loads.