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Effects of Grazing and Burning on Densities and
Habitats of Breeding Ducks in North Dakota

Discussion


Nest Density and Vegetation Structure

Annual nest densities of mallard gradually declined throughout the years of the study on the control fields and on all treated fields. There are several possible reasons for the decline in mallard density on control fields, all of which may have contributed to the decline. One is the gradual decline of wetlands with water and total surface area with water, which began in 1985 (see Methods). The driest year during the study was 1988. A second possible cause for the decline in nest density on the control fields is the poor reproductive success of mallard in 1982 (see Nest Success). In the early years of the study, mallard nest densities were highest and nests built in the same nest bowl that had been used the previous year were not unusual. Such behavior is characteristic of mallard when females return to breed in the same place where they had previously nested successfully (Duebbert et al.1983). In 1982 and 1985, mallard suffered low reproductive success and higher predation from red fox (A. B. Sargeant, pers. comm.). If resident females were killed by fox or had their nests destroyed and did not return to breed the following year, the local density of mallard may have declined. A third reason for the decline in nest density is that the continental populations of mallard declined during the 1980s (Cowardin and Blohm 1992). If a decline in the local population occurred after poor reproductive success, new recruits may not have replaced them if the pool of potential breeders was low.

Nest density declined on the control fields for only the mallard. American wigeon, the only other species at Lostwood with significant variation among years in nest density, had 1 year (1981) higher than the others on all treatments. In the remaining species at Lostwood, density fluctuated among years, but there was no pattern of density change on the control fields. Thus, lack of burning and grazing of the vegetation during this 9-year study, which might be expected to have a detrimental effect on duck nesting densities (Kadlec and Smith 1992), had no effect.

Gadwall had lower (P = 0.016) nest densities on the spring burn fields in post-treatment years than in pre-treatment years. Vegetation changes on those fields are probably responsible for much of the density change. Before burning, 10 and 23% of the vegetation available in April and June had visual obstruction readings >/= 2.5 dm. After burning, none of the April readings and less than 5% of the June readings were in this highest class. The proportion of vegetation >/=2.5 dm did not increase much during the study. Thus, there was little tall vegetation available for gadwall nesting in the post-treatment years. This visual obstruction reading class was preferred nesting cover for gadwall and held 81% of gadwall nests. Lokemoen et al. (1990) also found the highest nest densities of mallard and gadwall in habitats with the highest visual obstruction readings. Most mallard and gadwall nests were in brush (primarily western snowberry) or brush/grass, whose aboveground growth was killed by fire. The resprouting plants had not regrown to a height that was preferred by either species after the third growing season following treatment. Lower than average precipitation in 1987-88 may have retarded recovery of the vegetation. Our study did not last long enough to determine whether the vegetation -- and the density of nesting gadwall -- would return to pre-treatment levels.

Spring grazing was detrimental to gadwall and blue-winged teal in the years when cattle were present. In the spring grazing treatment, gadwall had significantly lower nest densities during the 3 consecutive years of spring grazing than during the post-treatment years. Grazing started each year in early May. At the time of the June visual obstruction reading sampling, the spring graze fields had only 8% of vegetation >/= 2.5 dm, the preferred class, compared with 10% in that class on the control fields during 1983-86. Nest densities of blue-winged teal were lower in the treatment years than in the pretreatment years. Grazing also reduced cover for blue-winged teal; 69% of the vegetation was <0.5 dm, which was avoided by this species. In addition to the reduced cover, the presence of cattle during the peak of nest initiation in gadwall and blue-winged teal may have led to lower nest densities. Mallard, which nested earlier, did not show a reduction in nest density on grazed fields during the treatment years. Cattle present during nest initiation are known to reduce nest densities of another grassland species, the upland sandpiper (Bartramia longicauda) (Bowen and Kruse 1993).

In the years after spring grazing, annual nest density of gadwall increased, which paralleled the increase in visual obstruction readings of the vegetation in the years after treatment. Nest densities of mallard and blue-winged teal were reduced on spring graze fields in post-treatment years compared with pre-treatment years. These results probably occurred because there was less water available in wetland basins during the post-treatment years (Lokemoen et al. 1990), and not due to long-term changes in the vegetation following spring grazing. The visual obstruction readings of vegetation on the spring graze fields were similar to control fields within 1 year after grazing ended. Adequate precipitation in the years immediately following grazing (1985-86) probably enhanced recovery of the vegetation.

For lesser scaup, northern shoveler, and northern pintail, there were not enough nests to test for differences among years within treatments. Visual inspection of the data suggests that nest densities of lesser scaup may have been reduced on spring graze fields, but northern shoveler and northern pintail were not affected.

Our analysis of total nest density was conducted in an attempt to ascertain the overall effect of the treatments during the treatment and post-treatment years. For all species, we found no difference among treatments in the total density of nests during the treatment and post-treatment years. Thus, during these 7 years, the grazing and burning treatments did not have a negative or positive effect on the total number of waterfowl nests. Although this finding could be interpreted as indicating that the treatments were neutral, in fact, we had wide confidence intervals on the maximum differences between treatments, indicating we had low power to detect differences among treatments.


Nest Success

Nest success was high most years at Lostwood. Through all years of the study, mallard averaged 34.5% success, gadwall 44.7%, and blue-winged teal 31.3%. This is considerably higher than found in central North Dakota by Lokemoen et al. (1990) and Klett et al. (1988) and in unmanaged cover in North Dakota for mallard (Cowardin and Johnson 1979, Cowardin et al. 1985).

We found no differences in nest success among treatments, but the power of our tests was low. The differences we measured in Mayfield estimates of nest success appeared large, but due to the small number of replicates (3) and large amount of variation, the differences were not statistically significant. Many more replicates would be needed to demonstrate different nest success rates with the range of variation we found at Lostwood. Conducting such a study would be more expensive and time-consuming than most resource agencies could afford.


Vegetation Types

Vegetation types changed during the study; brush and brush/grass decreased in area and grass/brush increased in area. Although there was no difference among treatments in the vegetation type changes, there was a suggestion that the spring burn and summer burn/spring graze treatment had more dramatic decreases in brush and brush/grass and increases in grass/brush than the control and spring graze treatments. Burning reduced the height of brush significantly and grass tended to compete with resprouting brush for the first several years following treatment, thereby providing a higher percentage of the canopy coverage during those years.

Most of the vegetation type affinities we observed are similar to those found in other studies. Brush was the preferred vegetation type for nesting mallard and gadwall, with 57% of the mallard and 33% of the gadwall nests in brush. Brush was also a preferred nesting cover in other studies conducted in central North Dakota (Duebbert et al. 1986) and Saskatchewan (Dzubin and Gollop 1972, Hines and Mitchell 1983). Brush/grass, the most common vegetation type early in the study (1981), was the most commonly used nesting habitat of gadwall, northern pintail, American wigeon, and lesser scaup. Brush/grass was the second most frequently used vegetation type of mallard. Grass/brush, the most common vegetation type late in the study (1987), was the most commonly used nesting habitat of blue-winged teal and northern shovelers. Grass/ brush was avoided by mallard and gadwall. Grass/forb was the second most common nesting habitat of blue-winged teal and it was avoided by mallard and gadwall.

The 2 species with the highest frequency of nesting use of shallow marsh emergent vegetation were the lesser scaup and northern shoveler. Mallard avoided this type. Other studies have found considerable nesting by mallard in marshes, which include shallow marsh and deep marsh emergent vegetation (Evans and Black 1956, Krapu et al. 1979).


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