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Weights of Wild Mallard Anas platyrynchos, Gadwall A. strepera, and Blue-winged Teal A. discors During the Breeding Season

Discussion


Weights of eggs and hatchlings

The average of unincubated Mallard eggs we weighed (49.3 g) was similar to the 46 g modal weight reported for eggs of wild birds (46.0) in Russia (Dement'ev & Gladkov 1967) and the 50.0-51.3 g range reported for eggs from captive birds in Manitoba (Batt & Prince 1978). The 34% weight decline we noted between fresh eggs and day-old young was similar to the 37-40% loss noted by Prince et al. (1970). The 13.1 g loss between pipped eggs and day-old young closely compared with the 12 g loss recorded by Kear (1965). In her study she noted that this weight was regained after about 48 hours.

The average weight we calculated for fresh Gadwall eggs (43.1 g) was similar to the average of 42.1 g observed by Dement'ev & Gladkov (1967) for eggs from wild Gadwall. The 29% weight decline we documented between fresh Gadwall eggs and one-day-old young was more than the 22 to 24% noted by Dement'ev & Gladkov (1967) for the same period.

In our study, one-day-old Blue-winged Teal weighed 33 % less than pipped eggs. This weight decline was the largest of the three species, probably because all Blue-winged Teal nestlings were dry when weighed, whereas some Mallard and Gadwall were wet. The average weight of Blue-winged Teal nestlings we recorded (18 g) was the same as the average calculated by Southwick (1953) for one-day-old young.

Growth of ducklings

Weights of ducklings we captured varied greatly within feather-age class. At least part of this variation was due to true variation in weight among individuals of the same age. Dzubin (1959) alluded to wide variation in weight among Mallard and Canvasback Aythya valisineria within a feather-age class. Also, Southwick (1953) noted considerable variation in weights and plumages of hand-reared Blue-winged Teal of the same age.

Mallard and Gadwall in our study weighed about the same as hand-reared birds observed by Prince et al. (1970) and Oring (1968), respectively, up to about two weeks of age. After two weeks of age the females in our study weighed less than captive females studied by Prince et al. (1970) and Oring (1968). In contrast, body weights of young wild Mallard captured in Russia (Dement'ev & Gladkov 1967) were similar to ours. It appears that captive birds gain weight faster and have less variability in development compared with wild birds. These differences probably reflect greater energy expenditures of birds in the wild. Owen (1969) calculated that wild birds needed to expend 13% more energy during maturation than captive birds.

Mallard and Gadwall young gained weight faster in years with good wetland conditions, presumably because of the superior feeding habitat in those years. Favourable wetland habitat can aid duckling growth and survival in two primary ways. First, it provides increased invertebrate food for nesting hens, which in turn may lay larger eggs (Eldridge & Krapu 1988). Larger eggs yield larger young (Pehrsson 1982), which may be better able to survive harsh environmental conditions (Koskimies & Lahti 1964). Second, a larger invertebrate food base will enhance duckling weights directly. Little published information exists on the direct effects of food supplies on duckling weight changes, but Street (1977) found decreased duckling survival on infertile ponds and Oring (1968) noted that well-fed ducklings attained flight sooner.

Flying young

The mean weights of flying young Mallard and Gadwall for both sexes weighed less than the younger Class III ducklings and less than values predicted from the growth curves. Although samples were small and the overall test only approached significance, we suspect that there is an actual weight loss associated with the onset of flight. The period when young birds are making trial flights and finishing primary feather growth may be the time of highest energy demand. Although weight loss at fledging is not easily documented in either wild or pen-reared waterfowl, other authors have noted weight losses during this period. Veselovsky (1953) recorded weight declines of Mallard ducklings when flight feathers began growing and when young began to fly. Sugden et al. (1981) found weight declines in Lesser Scaup Aythya affinis and Mallard reared in captivity and Weller (1959) noted a weight loss in captive Redhead A. americana at the time of first flight.

Variation in adult weights by sex and age

Breeding-season weights of males were greater than females for all three species, the differences ranging from 14% of the female average weight in Blue-winged Teal to 21% in Mallard. This well-known sexual dimorphism (e.g. Kear 1970, Johnsgard 1975) is accentuated during the egg-laying and incubation phases of the breeding season, when females encounter heavy demands on their energy reserves.

We found weight differences between SY and ASY females only for Mallard. Batt & Prince (1978) and Krapu & Doty (1979) also found age-related weight differences in female Mallard. For female Gadwall, we suspect that weight differences existed but were undetected because the ageing method we used incorrectly classified an unknown portion of ASY females as SY females (Lokemoen et al. 1990).

Seasonal change in adult weights

Weights of all three species changed through the breeding season. The onset of weight changes varied by species in accordance with the phenology of breeding. For Mallard, which begin nesting shortly after arrival, the weight loss began immediately in April. For Blue-winged Teal, which are mid-season nesters, the decline began in late May. For Gadwall, which are late nesters, the loss was not apparent until June. Typically 10% of the nests were initiated in our study area by 26 April for Mallard, 8 May for Blue-winged Teal, and 20 May for Gadwall. The weight loss of female Mallard began some two weeks before the 10% nest initiation date whereas female Gadwall and Blue-winged Teal weight loss began two to three weeks after the 10% nest initiation date. Similarly, Krapu (1981, Fig. 1) did not find weight gains by female Mallard in spring but he recorded stable weights followed by weight loss during nesting. Mallard females may lose weight earlier in the breeding cycle than other species because they have less time between arrival and nesting to acquire nutrients and fewer nutrients are available.

From spring peaks to post-nesting lows, Mallard females in our study averaged a 19% drop in weight, Gadwall a 17% drop, and Blue-winged Teal 14%. These declines compare to weight losses during incubation of 18% for Mallard, 16% for Gadwall, and 12% for Blue-winged Teal found by Gatti (1983). The consistency between his figures and ours for each species suggests that percent weight loss is less in smaller-bodied ducks. The energy demands of nesting were emphasized by the weight differences between females on and off nests: 6% for Mallard, 8% for Gadwall, and 9% for Blue-winged Teal.

Males showed significant weight changes during the breeding season but followed a pattern somewhat different from females. Losses began one 10-day period earlier than females, except for the Mallard, in which the change in both sexes was apparent from the beginning. The declines also ended earlier for males than for females. Male Mallard began gaining weight by the end of May and male Blue-winged Teal by early July. We did not detect an increase by male Gadwall, perhaps because we captured too few of them after mid-June.

Earlier weight declines for males are not surprising as males spend more time than females from spring arrival to incubation defending a space for mating, feeding, and possibly nesting (McKinney 1965, Siegfried 1974). At this time males spend more time in movement and mate-guarding activities and less in feeding than do females (Dwyer 1975, Stewart & Titman 1980, Titman 1981). When females begin incubating, males become less active and begin to gain weight. Oring (1969) found that male Gadwall gained weight rapidly after deserting their mates. Females continue to lose weight until late summer because their breeding-season activities - including incubation, possibly renesting, and brood rearing - continue longer than males.

Annual variation in adult weights

Annual variation in weight was apparent in all species and sexes. In general, the highest average weights occurred in 1976 and 1978, lowest weights in 1977, and intermediate values in the other years. Sample sizes were sometimes small, however, and in 1976 no Blue-winged Teal were captured. Some of the annual variation could be attributed to wetland area in a particular year or to the change in wetland area from the previous year. Female Blue-winged Teal weights were heavier in years with greater wetland area. This result seems to fit well with the strategy of this species not to home but to seek good wetland habitat each spring (Lokemoen et al. 1990). We found higher weights for male Mallard and female Gadwall in years when the wetland area had increased from the previous year. We suspect that favourable wetland conditions help ducks gain weight because they can feed in the newly flooded feather-edge where food is highly available. We found heavier weights in years with good wetland conditions, despite the fact that energy-expensive breeding activities were probably greater in those years (Cowardin et al. 1985).


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