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Temporal Flexibility of Reproduction in
Temperate-breeding Dabbling Ducks

Discussions


Causes of interspecific variation in when dabbling ducks terminate breeding and presumed differences in timing of refractoriness are unknown. However, interspecific differences in timing of cessation of breeding presumably result, in part, because environmental constraints that prevent reproductive success vary among species. Shovelers and pintails are the first to terminate breeding in spring (Table 1) and they seldom nest in summer (Fig. 2), suggesting that environmental conditions that prevent reproductive success are encountered earlier in the season than among Blue-winged Teal, Mallards, and Gadwalls. For shovelers, spatulate mandibles with finely spaced lamellae (Nudds and Bowlby 1984) limit foraging primarily to small crustaceans and snails (Swanson et al. 1979). Cladocera, the principal food of laying females and young shovelers, are among the most abundant invertebrates in spring (Bataille and Baldasarre 1993) but are much less abundant by August (Hall 1964, DuBowy 1988). As a result, shoveler ducklings may have become food limited earlier in the season than other dabbling ducks, leading to evolution of a refractory mechanism that resulted in an early termination of breeding. Breeding female shovelers forage in several types of wetlands including permanent ponds (Poston 1969), suggesting that the early termination of breeding was not linked to seasonal changes in pond abundance. The possibility exists that seasonal declines in food resources prevent female shovelers from nesting in summer, so experiments are needed to establish whether length of breeding can be extended beyond intervals occurring in the wild when appropriate food is provided ad libitum.

Pintails breed early, seeking out temporarily flooded ponds (Smith 1970) that are the first to warm in spring and produce macroinvertebrates sought by females during egg production (Krapu 1974). Pintails also are highly mobile, have weak pair bonds (Derrickson 1978), fledge in as short a period as 42 days in the PPR (Bellrose 1980), and rely the most on endogenous lipids to produce their first clutch of eggs among species of dabbling ducks that have been studied (Esler and Grand 1994). These traits suggest that pintails evolved under environmental conditions that required breeding to begin and end early in order to achieve reproductive success. For pintails, like shovelers, the possibility exists that a seasonal decline in foods required by females for successful breeding, particularly after lipid reserves are exhausted by mid-May (Krapu 1974), accounts for the early termination of breeding. However, female pintails continue to nest after lipid reserves are depleted if wetland conditions are favorable, and during egg production females feed on a variety of animal and plant foods (Krapu 1974), including several that remain plentiful in wet late springs and summers.

Female Mallards, like pintails, start breeding early, rely on endogenous lipids for production of early clutches (Krapu 1981), and prefer temporary and seasonal ponds during egg production (Krapu et al. 1997). Mallard brood survival declines as the availability of seasonal basins with water decreases (Krapu et al. 2000), indicating that seasonal ponds are important to reproductive success. However, other traits of female Mallards, e.g. homing at relatively high rates to sites where they bred successfully the previous year (Lokemoen et al. 1990) and breeding in association with semipermanent ponds and lakes (Krapu et al. 1997), suggest Mallards evolved under more stable environmental conditions than pintails, leading to evolution of a refractory mechanism allowing breeding to continue later in spring.

Blue-winged Teal breed primarily in the PPR (Bellrose 1980), start nesting much later than Mallards (Fig. 1), but also prefer temporary and seasonal ponds (Drewien and Springer 1969). Teal breed successfully in association with temporary and seasonal ponds, despite a relatively late onset of breeding, because they are strong "pioneers." Teal shift their distribution annually, settling in landscapes where high runoff from snowmelt or spring rains results in an abundance of recently flooded ponds (Johnson and Grier 1988). This reproductive strategy enhances reproductive success in years when water is plentiful across part or all of the PPR. The high proportion of teal among broods from summer-initiated nests (Table 3) suggests that teal are particularly responsive to high amounts of rainfall in late spring and summer that cause temporarily flooded wetlands to increase during this period.

Gadwalls are the last of species to initiate nesting (Fig. 1) and continue to nest later in spring, on average, than the other four species (Table 1). Gadwalls commonly breed in association with brackish semipermanent and saline wetlands (Serie and Swanson 1976), suggesting that they evolved in an environment where a late onset of breeding was adaptive, leading to less reliance on temporary and seasonal ponds than pintails, Mallards, and Blue-winged Teal.

Although timing of onset of nesting varied widely among the five studied species, most nesting ended by about the summer solstice (Fig. 1, Table 1). Gadwalls, despite a late onset of breeding, still completed most nesting by early summer (Fig. 1). The fact that most females of all five species, including early and late breeders, terminated breeding by early summer suggests that mechanisms for the timing of refractoriness evolved under conditions where severe environmental constraints lowered reproductive success when nesting continued through summer. Ducklings produced from summer-initiated nests in 1993 experienced high mortality, lending support to the hypothesis that the annual decline in ambient temperature during late summer and fall was a major factor contributing to evolution of photorefractory mechanisms that limited most breeding to spring in temperate-nesting dabbling ducks. In 1993, an estimated 67, 66, and 77%, respectively, of Blue-winged Teal, Mallard, and Gadwall broods from summer-initiated nests were in age classes 1a to 2c, and most of these young disappeared and presumably died after temperatures during the nights of 30 September and 1 October fell to -2.2°C and -3.3°C and shallow ponds froze (G. Krapu, unpubl. data). Declining food resources may have contributed to the evolution of refractory mechanisms that led to an early cessation of breeding in some species (e.g. Norther Shovelers). However, class 2 brood sizes of Mallards, Blue-winged Teal, and Gadwalls when surveyed on transects and plots during 20 to 30 September 1993 were large, i.e. 7.3 SE of 0.7 (n = 12), 6.8 0.4 (n = 32), and 7.9 0.6 (n = 7) ducklings, respectively (G. Krapu, unpublished data), indicating that duckling survival was high during summer and early fall and implying that food remained plentiful into fall after wet summers. Many females that raised late broods in 1993 attempted to molt their primaries during October 1993 and died before or during freeze. This pattern suggests that increased mortality of summer-nesting females also contributed to evolution of refractory mechanisms that limit breeding mostly to spring.

Species-specific differences in timing of cessation of breeding (Fig. 1, Table 1) imply interspecific variation in photosensitivity. Species-specific differences in photosensitivity as day length increases in late spring may explain why reproduction consistently terminates earliest in pintails and shovelers, followed by Mallards and Blue-winged Teal and then Gadwalls. Alternatively, pintails and shovelers possibly evolved lower responsiveness to non-photic cues (e.g. rainfall, wetland habitat conditions) than Mallards, Blue-winged Teal, and Gadwalls, leading to observed interspecific differences in timing of termination of breeding. However, the latter hypothesis is not supported by information from the PPR of Canada, which indicates that the mean length of extension of the nesting interval for pintails and Mallards increased 0.12 and 0.10 days for each 1.0 cm increase in precipitation during May (Greenwood et al. 1995). Controlled experiments will be required to conclusively demonstrate whether interspecific variation in photosensitivity, differing sensitivities to non-photic cues, or other factors are responsible for interspecific differences in the timing of cessation of breeding.

Reproductive mechanisms in the five dabbling duck species studied are sensitive to wetland habitat conditions because of the influence of pond conditions on reproductive success. The proportion of unsuccessful female Mallards, pintails, Blue-winged Teal, and Gadwalls that continue to renest in the PPR as the breeding season advances varies with the number of May ponds (Greenwood et al. 1995), and seasonal ponds account for most of the variation in pond availability in May, June, and July (Krapu et al. 2000). Females of the five species studied rely principally on aquatic invertebrates for protein needs during egg production (Krapu and Reinecke 1992), and the abundance of May ponds influences standing crop and spatial distribution of aquatic invertebrates. Availability of high-quality food influences the number of clutches that can be produced (Eldridge and Krapu 1988).

When temporary and seasonal basins are dry during widespread drought conditions, many dabbling ducks either fail to nest (Smith 1971) or terminate nesting early (Krapu et al. 1983). Under drought conditions in prairie Canada in 1959, female Mallards gathered into flocks by late May, and examination of their reproductive tracts revealed atretic ova but no evidence of ovulated follicles (Dzubin and Gollop 1972), indicating a lack of nesting and an early onset of refractoriness. For female Mallards that attempt to breed under drought conditions, the risk of failure is high. Among Mallards breeding in North Dakota and west-central Minnesota, the risk of brood loss was estimated to be 11 times greater when less than 17% of seasonal wetland basins contained water compared with when more than 59% were filled (Krapu et al. 2000). The influence of water conditions on brood survival results, in part, because Mallard ducklings rely on seasonal ponds for invertebrates that provide protein for growth and survival during the first several weeks posthatching (Cox et al. 1998). Because the availability of temporary and seasonal ponds in spring and summer varies widely among years (Krapu et al. 1997, USFWS 1998), reproductive success varies accordingly for species that are dependent on these pond types. The number of July ponds is highly correlated with the number of May ponds in the PPR (Pospahala et al. 1974), so the level of expected risk to broods is factored into the "decision" by females regarding when to terminate reproduction for the season.

Some Mallards, Blue-winged Teal, and Gadwalls did not become photorefractory under long day lengths and continued to nest until day lengths were no longer stimulatory (Fig. 2), apparently due in part to stimulation from non-photic cues. Summer nesting by Mallards, Blue-winged Teal, and Gadwalls apparently was highest in 1993 (Table 3) when high rainfall led to an abundance of newly flooded wetland habitat in late spring and summer. It probably is more than coincidental that the unusually large mid- and late summer breeding responses by dabbling ducks in 1962 and 1993 in North Dakota immediately followed major three-to-four-year droughts in the PPR (NOAA 1892-1996). Moreover, the strong summer breeding response in both years occurred while ponds were filling during late spring and summer, creating conditions conducive to production of aquatic invertebrates (Euliss et al. 1999). At Jamestown, North Dakota, near the center of the FBSA, annual precipitation during 1988 to 92 was as low as 28.6 cm (1988), and few shallow wetland basins in eastern North Dakota held water in several springs (Krapu et al. 1997). In 1993, when the drought cycle ended, annual precipitation increased to 76.7 cm, with July being the second wettest on record. Precipitation in July 1993, 28.1 cm, was only slightly below the total annual precipitation in 1988 and was 20.1 cm above average. Annual precipitation in 1994 and 1995 was 59.4 and 55.0 cm, respectively. Wetland habitat remained plentiful in 1994 and 1995, but no comparable rainfall events or flooding occurred on the study area in summer; apparently, fewer Mallards, Blue-winged Teal, and Gadwalls nested in mid- and late summer based on the number of broods present during fall (Table 3). Further research is needed to better understand how duck species vary in the way environmental information is transduced into endocrine secretions that regulate length of the breeding cycle (see Bluhm 1992).

Adaptations that allow Blue-winged Teal, Mallards, and Gadwalls to nest later in spring than shovelers and pintails results in a tradeoff because of the higher probability that teal, Mallards, and Gadwalls will breed into summer, potentially leading to high mortality of breeding females and young. Because breeding sufficiently late to cause major losses of young (as in 1993) appears to occur infrequently, the benefits of being able to breed later in years when water is abundant probably outweigh the risks. Among Mallards, the magnitude of renesting effort in spring can determine whether populations are maintained in environments subjected to high rates of nest loss (Cowardin et al. 1985). This relationship reflects the importance to population status of how reproductive mechanisms respond to wetland habitat conditions in spring.

The large size of Mallard, Blue-winged Teal, and Gadwall broods from 20 to 30 September 1993 suggests that males maintain high rates of spermatogenesis for up to two months beyond the summer solstice, which is longer than previously thought. Duration of sperm storage by female Mallards averaged 9.4 days (Elder and Weller 1954), suggesting that the apparent high fertility in summer-initiated clutches on my study area resulted because males remained reproductively active (vs. sperm having been stored by females for protracted periods). Spermatozoa are lost at a more rapid rate from reproductive tracts of female Mallards than among non-waterfowl species that have been studied (Cunningham 1997), which also supports the conclusion that males successfully breed throughout summer in North Dakota. Little is known about how synchrony in reproductive cycles of male and female dabbling ducks is maintained through summer. However, female Song Sparrows (Melospiza melodia) treated with estradiol to keep them in a reproductive state long after the normal breeding season induced their unmanipulated mates to remain reproductively active (Runfeldt and Wingfield 1985). Similarly, male dabbling ducks may be stimulated to remain reproductively active when breeding females are present.

Interspecific differences in timing of termination of breeding in temperate-nesting dabbling ducks, including the capacity of some species to breed through summer when habitat conditions are exceptionally favorable, suggest that refractory mechanisms are more varied and complex than suggested by earlier studies. Interspecific variation in length of breeding affects many aspects of species' biology and thus offers numerous potential avenues for further study. A major gap in information is a lack of understanding of proximate causes of interspecific variation in timing of termination of breeding among temperate-breeding dabbling ducks. Wide variation among dabbling ducks in responsiveness to non-photic cues late in the breeding season suggests that greater attention should be focused on the scale and pathways of involvement of non-photic influences in controlling length of breeding. To date, research on refractory mechanisms in dabbling ducks has focused mostly on Mallards, but future work would benefit from a greater emphasis on comparative studies.


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