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Fig. 1.   Cumulative proportion of nest initiations completed by Julian date among Mallards, Blue-winged Teal, Gadwalls, Northern Pintails, and Northern Shovelers during spring and early summer, 1993 to 1995, in the Prairie Pothole Region of North Dakota and north-central South Dakota.

Interspecific differences in breeding frequency and distribution during summer. — Of broods seen on transects and plots during September and estimated to have hatched from nests initiated after 10 July, 81 of 82 (99%) were Mallards, Blue-winged Teal, and Gadwalls (Table 3, Fig. 2). Mallards, Blue-winged Teal, and Gadwalls continued to nest an estimated 58, 56, and 47 days, respectively, beyond the summer solstice (Fig. 2). The last nest initiations for Blue-winged Teal, Mallards, and Gadwalls, based on fall brood surveys, were estimated to be 18, 16, and 7 August, respectively. The latest estimated nest-initiation date for a dabbling duck species, based on an opportunistic sighting, was 3 September for Mallards (based on a class 1 brood sighted on the FBSA during late October 1993). The last Mallard, Blue-winged Teal, and Gadwall nests known to have been initiated in 1993 were near the low end of day lengths that stimulated breeding in spring (Fig. 2). Blue-winged Teal accounted for 70 and 83% of dabbling duck broods observed from nests initiated after 10 July 1993 and 1994, respectively. Mid- and late summer nesting was most frequent during 1993 (Table 3). The number of broods seen on transects from nests initiated after 10 July declined sharply from 1993 to 1994 and 1995 (Table 3), as did sightings of dabbling duck broods across the FBSA.

Fig. 2. Breeding intervals for Mallards, Northern Pintails, Northern Shovelers, Blue-winged Teal, and Gadwalls by day length during spring and summer of 1993 in eastern North Dakota. White circles denote one or more nest initiations based on nest surveys conducted from early May through early July 1993 in North Dakota and north-central South Dakota. Black circles denote dates or one or more nest initiations based on broods sighted on transects and plots surveyed during 20 to 30 September 1993 in Barnes and Stutsman counties, eastern North Dakota. Summer nest-initiation dates were estimated by backdating broods of known age class. Some records denoted by white and black circles are hidden from view because of multiple nest initiations on the same date.

Ninety-eight percent (58 of 59) of flightless young of the five species shot on or after 1 October 1993 to 1995 by duck hunters across the United States were Mallards, Blue-winged Teal, and Gadwalls. Only one flightless juvenile shoveler, and no flightless juvenile pintails, were in the sample of wings. Flightless young Mallards, Blue-winged Teal, and Gadwalls reported by hunters were from 7, 5, and 4 states in the western and central United States (Table 4). The number of shot flightless juveniles that came from nests initiated in mid- or late summer declined from 35 in 1993 to 12 each in 1994 and 1995.