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DIFFERENTIAL SEX MIGRATION OF MALLARDS IN NEW MEXICO 
By ALDO LEOPOLD 
WENTY 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 sexes. 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 
October 23 ——— 50 15 
November 1 0 50 40 
November 8 42 — 45 
November15 — 65 55 
November 23 50 90 65 
December 1 — 80 62 
December 8 50 —— 60 
December 15 —— —— ——- 
December 23 —— ss ——— 
January 1 —_— meres = 
January 8 —> aaa —— 
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 4 
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. 
