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 182 ol. XX! DIFFERENTIAL SEX MIGRATION OF MALLARDS IN NEW MEXICO By ALDO LEOPOLD Y years ago, in hunting ducks on the bottom lands of the Missis- sippi in Iowa and Illinois, I was taught by my father to "pick" Mallard drakes. It was our custom to measure a day's sport, not merely by the number of ducks bagged, but by the proportion of greenheads to hens. A high percentage of drakes in the day's bag was regarded not only as a special feat of skill, but as proof that the bag had not been attained by "flock-shooting", which was discountenanced as productive of cripples. We shot small-bore guns, used small shot, frowned upon long range shooting, and in general oper- ated on the principle of' ' pick your drake and kill him dead." As a consequence, I have grown up with what might be called a special "eye" for Mallard drakes. Two years ago I started shooting on the Rio Grande in the vicinity of Albuquerque, and naturally tried to adhere to the old custom of shooting the male birds. It soon became apparent, however, that special conditions obtained here with respect to the relative abundance of the two sexea The following table, based on my hunting journal, gives the per- centage of Mallard drakes to hens killed during weekly hunts over the same territory during 1917 and 1918. Percentage of drakes killed Date 1917 1918 Average based on weighted curve October 16 25 10 October 23 50 15 November 1 0 50 40 November 8 42 45 November15 65 55 November 23 ' 50 90 65 December I 80 62 December 8 50 60 December 1 December 23 January 1 January 8 January 15 70 January 23 50 60 January 31 Total number of birds 29 48 77 The short period of time and the small number of birds killed necessarily. renders the foregoing table inconclusive. As supplemented by careful obser- vation, however, I am ready to offer it as evidence in support of the following statement covering the last two years: 1. When the season opened in October, there was a notable preponderance of hen Mallards in the Rio Grande Valley near Albuquerque. 2. By the first week in November this preponderance of hens disappeared, and until the main flight was over, about December 1, there was a preponder- ance of drakes. 3. Among the Mallards wintering here, after the main flight was over, the-pro- portion of drakes to hens was normal.