Mallards were caught at a duck decoy located near Oud Alblas in the Alblasserwaard, The Netherlands, from March 2010 until February 2011. Each captured mallard was marked with a metal ring, sexed and aged. Cloacal and oropharyngeal samples were taken for detection of current LPAIV infection, and blood samples collected for detection of antibodies to AIV. The cycle threshold (Ct) value, which is the first real-time amplification cycle in which matrix gene amplification was detected, was used to assess the degree of viral shedding from the cloaca and oropharynx. The Ct-value is inversely proportional to the number of virus particles in a sample. Feather samples were taken to assess the origin of birds (resident, local migrant, distant migrant) using stable hydrogen isotope analysis. For more details see Appendix 2.