Counts of hybrids used to assess the brood parasitism hypothesis and to compute expected frequencies for Fig 1.

“F1 count” lists genetically identified F1s with known sires. “F1s and others” gives the hybrids used in the computation of expected frequencies. The additional hybrids in this column were identified morphologically but lacked full genetic data and will include a few backcrosses. Mareca americana x M. penelope hybrids were excluded from these analyses because most were target collected.