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Duck Nest Success in the Prairie Pothole Region

Discussion


The threshold level of nest success that will sustain a population depends on the survival rates of females and ducklings and the renesting rate, all of which are subject to species, regional, and temporal effects. Based on a model developed by Cowardin and Johnson (1979), Cowardin et al. (1985) concluded that nest success of about 15% will maintain a mallard population for a long term in NDC. Maintenance of populations with nest success rates <15% requires immigration of breeding pairs from other regions. Similar estimates for other species and regions are lacking. In assessing this model, Cowardin and Johnson (1979) found that adult survival rates, nest success, and the number of renests are the most crucial parameters affecting population stability. Given roughly similar survival rates among species, the nest success rate necessary to maintain a population will depend largely on renesting potential. The threshold levels for mallards and pintails are likely similar because they nest early and are persistent renesters. The threshold for shovelers, blue-winged teal, and gadwalls is probably >15% because they are mid- to late-season nesters and have less potential for renesting than mallards or pintails. Deferred nesting by some yearling gadwalls (J. T. Lokemoen et al., NPWRC, unpubl. data) also suggests a higher threshold level. To put our nest success estimates in perspective, we used the threshold level of 15% for mallards suggested by Cowardin et al. (1985) and assumed a level of 15% for pintails and 20% for the other species.

Based on the application of these criteria, nest success was inadequate to maintain populations in most regions. Exceptions were SDC for all species; NDC (1975-79) for gadwalls, blue-winged teal, and shovelers; and SDE for gadwalls (1980-84) and blue-winged teal (1966-74, 1980-84).

In NDE and NDC, no consistent increase or decrease in nest success was apparent between 1966-74 and 1980-84. In general, nest success was lowest in 1966-74 and highest in 1975-79. Differences among periods were usually <4 percentage points for all species, but ranged from 7 to 10 points for gadwalls, blue-winged teal, and shovelers in NDC. Gadwalls and blue-winged teal had consistently higher nest success rates than pintails and mallards. Rates for shovelers were higher than those for the other species in NDC and SDC but were similar to those for mallards in the other regions.

Nest success varied considerably among the 8 habitat classes in North Dakota (NDE and NDC combined). Success rates consistently approached or exceeded hypothetical threshold levels only in idle grassland and in grassland in 1975-79. Grassland was of major importance to all species studied because it was plentiful and nest success was relatively high. Idle grassland was of minor importance regionally because of its scarcity but may have been important locally because nest success was usually high. Wetland, odd area, and planted cover accounted for about 25% of the nest initiations by all species except pintails. Planted cover was by far the most preferred nesting habitat for all 5 species. Use was highest in 1966-74 because it was more common than in later periods. Because planted cover is highly preferred, it has a great potential for producing ducks if nest success can be increased. Nest success was usually lowest in cropland, hayland, and right-of-way. Use of these 3 habitats was generally low, but pintails initiated >50% of their nests in cropland and mallards and gadwalls initiated between 9 and 13% of their nests in hayland. Losses of grassland, wetland, odd area, and planted cover habitats due to intensive farming practices may cause more ducks to nest in cropland and hayland where nests are exposed to increased risk by predation and farming operations.

Predators were the most important cause of nest losses in all regions and in all habitats. Farming operations caused appreciable losses in cropland and hayland. The most important egg predators common to all regions studied were red fox (Vulpes vulpes), striped skunk (Mephitis mephitis), mink (Mustela vison), raccoon (Procyon lotor), badger (Taxidea taxus), and Franklin's ground squirrel (Spermophilus franklinii) (Sargeant and Arnold 1984). Coyotes (Canis latrans) were present locally, most often in the western parts of NDC and SDC, and reduce red fox predation on female ducks and eggs (Sargeant and Arnold 1984). The same authors thought the red fox had the greatest impact on nest success of upland nesting ducks. Variation in nest success rates among regions was most likely a result of differences in the size and composition of predator populations and differences in the abundance and distribution of their alternative foods.


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