Monitoring chlorinated hydrocarbon exposure in Great Blue Herons by using mixed-function oxidases : J. E. ELLIOTT,∗ P. E. WHITEHEAD,∗ R. J. NORSTROM,‡ S. M. BANDIERA,§ L. E. HART,§ K. M. CHENG§ & G. D. BELLWARD,§∗Canadian Wildlife Service, Environment Canada, Delta, British Columbia, V4K 3Y3; ‡Canadian Wildlife Service, Environment Canada, Ottawa, Ontario, K1A 3H0 & §University of British Columbia, Vancouver, British Columbia, V6T 1W5, Canada