Northern Prairie Wildlife Research Center
The spring sex ratio in the simulated 11th year was about 126 males per 100 females, consistent with published and unpublished field data. Fall sex ratios and rates of summer mortality were compared to published data; results were generally in agreement, although small samples precluded a definitive conclusion. An examination of other major mortality factors did not indicate any that were severe enough and sufficiently hen-selective to cause an appreciable sex disparity in the mallard population.
We conclude that fox predation (including accumulation of scavenged birds) alone is adequate to distort the sex ratio of a mallard population to the extent observed in the field.