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Temporal Flexibility of Reproduction in
Temperate-breeding Dabbling Ducks

Introduction


Reproductive fitness of individuals is enhanced by timing reproductive efforts to coincide with environmental conditions that maximize survival of young and parents (Farner et al. 1983). Termination of breeding cycles in temperate-nesting waterfowl in the Northern Hemisphere is accomplished through photorefractoriness, a process by which levels of plasma gonadotropins decline under day lengths that otherwise induce or maintain high levels (Hahn et al. 1997). In the waterfowl genus Anas, environmental conditions under which species evolved, and thus length of the period when breeding could occur successfully, presumably varied widely among species. As a result, refractory mechanisms may reflect interspecific variation in the duration of successful reproduction. However, quantitative evidence of variation in timing of refractoriness among temperate-nesting Anas is lacking.

Photorefractoriness does not occur under long day lengths in late spring among species of dabbling ducks that breed in temperate regions of the Southern Hemisphere (Murton and Kear 1976). Species confined mostly to the tropics or otherwise distributed in the Southern Hemisphere have breeding seasons that are long and symmetrically positioned in relation to the summer solstice ("Type A" species of Murton and Kear [1976]). Type A species potentially can continue to breed until day length shortens to lengths that no longer are stimulatory. Conversely, species that nest in temperate regions of the Northern Hemisphere have distinct short breeding seasons that end at long day lengths around the summer solstice ("Type B" species). Among the Type B species, long day length in late spring and early summer is generally considered to be the decisive factor that controls the end of breeding for the season. Type B species have photorefractory mechanisms that cause breeding to cease spontaneously when day lengths are still stimulatory (Murton and Kear 1976). Among Mallards (Anas platyrhynchos), the most studied species of the Type B group, laying dates, testicular mass changes, and histological investigations of the testes suggest that germ cell production is restricted to the period from March through June in the wild and in captivity (Hohn 1947; Johnson 1961, 1966; Donham 1979). Verification of absolute photorefractoriness in Mallards by late June under long day lengths has been demonstrated experimentally (Lofts and Coombs 1965, Haase 1983, Haase et al. 1985).

Despite much evidence that temperate-nesting ducks are absolutely photorefractory under long day lengths in late spring and early summer (clearly fitting within the Type B category), I observed major breeding activity by dabbling ducks, including Mallards, throughout summer of 1962 in southeastern North Dakota (Bluhm 1992:343). Summer breeding in 1962 occurred in one of the wettest recorded summers for southeastern North Dakota (NOAA 1892-1996). Rainfall from May through July totaled 13.5, 11.4, and 22.9 cm at Fullerton (where observations of summer breeding occurred), and July 1962 was the wettest on record. Over the next three decades, no comparable patterns of precipitation occurred during late spring and summer in southeastern North Dakota, nor were similar patterns of flooding observed. From 1963 to 1992, neither I nor colleagues at the Northern Prairie Wildlife Research Center saw evidence of large-scale mid- and late summer nesting by dabbling ducks in eastern North Dakota. The lack of conspicuous summer breeding by dabbling ducks in the years following 1962 led me to hypothesize that the extended nesting season in 1962 was due to stimulation resulting from the wet summer.

In 1993, record amounts of rain fell from late spring through summer across a large area of the midwestern United States (USACE 1994), producing extensive flooding of wetland habitats across the region. With water conditions similar to 1962 in eastern North Dakota, I initiated studies to assess whether length of breeding varied among temperate-nesting dabbling ducks, and if so, to evaluate possible causes for the variation. Specifically, I focused on five common species (Mallard, Northern Pintail [Anas Acuta], Northern Shoveler [Anas clypeata], Blue-winged Teal [Anas discors], Gadwall [Anas strepera]) that breed in the Prairie Pothole Region (PPR), the principal breeding area of dabbling ducks in North America (Bellrose 1979), to determine whether differences existed among species in timing of cessation of breeding and to use this information to infer whether differences occur in timing of photorefractoriness and / or in responsiveness to cues delaying its onset.


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