The PBMS expanded its monitoring range of compounds to reflect contemporary conservation and regulatory concerns. Monitoring of the assimilation of industrial polychlorinated biphenyls (PCBs) began in 1966 after these contaminants were identified as pollutants that were potentially toxic to birds.
In terms of long-term trends, there has been a decline in congener sum PCB contamination in the eggs of most of the species that have been monitored, except for coastal nesting golden eagles. In contrast, there has been no significant decline over time in PCB concentrations in sparrowhawk livers.