Factors Influencing Summer Breeding In Prairie Ducks

Krapu, Gary.  1997.  Factors Influencing Summer Breeding in Prairie Ducks.
     Waterfowl 2000 10(1):11.

Krapu, Gary.  1997.  Factors Influencing Summer Breeding in Prairie Ducks.
     Waterfowl 2000 10(1):11.  Northern Prairie Wildlife Research Center Online.  
     http://www.npwrc.usgs.gov/resource/birds/sumbred/index.htm
     (Version 16JUL97).

Knowledge of factors controlling when waterfowl breed and duration of nesting are important when attempting to accurately predict production and to develop effective strategies for managing waterfowl populations. In the Prairie Pothole Region (PPR), the length of the nesting effort increases in wet years due to greater renesting as foraging conditions improve. Breeding in most temperate-nesting ducks is thought to end spontaneously at about the summer solstice (21 June) with limited variation among species.

Among diving ducks, ruddy ducks and redheads (pictured) were the most frequent late breeders.

Reproduction in duck populations inhabiting semi-arid regions such as the PPR is often severely limited by drought, resulting in marked declines in populations. I hypothesized that there may be an adaptive advantage for some species to continue breeding into summer under wet conditions. With the assistance of colleagues during 1993-95, I measured extent of late breeding by surveying broods on randomly selected transects during September. Backdating from brood age to obtain date of nest initiation, I measured characteristics of late breeding with the data set restricted to broods hatched from nests initiated after 10 July.

My results revealed that several species of ducks continue to nest for up to 2 months beyond the summer solstice in some years. In 1993, an average of about 1.0 flightless brood/mi² were sighted during the mid-September survey on my 1,500-mi² study area. Five species (mallard, blue-winged teal, gadwall, redhead, and ruddy duck) accounted for most successful nests initiated after 10 July. Blue-winged teal formed 70% of the late dabbling duck broods, a pattern probably linked to teal young being able to fly within 35 days after hatch, about 3-4 weeks shorter than in most other species. Northern pintails and northern shovelers were absent from the late breeding effort and related analyses indicated few nests were initiated after the summer solstice in 1993-95 despite these species being common breeders in eastern North Dakota. Among diving ducks, ruddy ducks and redheads were the most frequent late breeders, and in fact, nested so late in 1993 that a major part of their broods perished while still flightless at freezeup in late October. In 1994 and 1995, the number of late dabbling duck broods decreased by 56 and 93% from 1993, respectively, while diving duck broods increased by 47 and 93%, mostly due to a marked increase in ruddy ducks.

What are the implications of these findings to waterfowl management? First, these results indicate some species of ducks are capable of breeding successfully much later in the season than generally recognized. Second, wetland habitat conditions have a major influence on extent of summer breeding, an important consideration because nesting effort is a key determinant of reproductive output. Third, in wet years particularly following drought, significant production occurs from nests initiated after what is generally considered the end of the nesting season and from broods that hatch after July brood surveys are completed. Fourth, the shorter duration of breeding by pintails than most other dabbling ducks combined with the species' exceptionally high early nest loss, helps explain why the prairie pintail population has not rebounded as have other dabbling ducks during the current wet cycle.

For more information, write to Gary Krapu, U.S. Geological Survey, Northern Prairie Science Center, 8711 37th Street SE, Jamestown, North Dakota, or call (701) 253-5500, or fax (701) 253-5553.