Techniques for Studying Nest Success of Ducks in Upland Habitats in the Prairie Pothole Region
Determining Fate of Nests
Successful Nests
We define a successful nest as one in which at least one egg hatched. The presence of detached shell membranes (Fig. 6) is the best evidence that eggs hatched (Girard 1939). Shell membranes of eggs that are broken during laying or incubation remain firmly attached to the inside of the shell. Other evidence of hatching is the presence of yellowish feather sheaths or small shell fragments without membranes in the nest material.
Detached membranes are not always as conspicuous as shown in the figure; they are often compressed and matted together at the bottom of the nest bowl and may be covered or mixed with egg fragments, nest debris, and down. Membranes are often enclosed by others - something to consider if there is a need to know the number of eggs that hatched. Careful observation and judgment are necessary to correctly determine the fate of successful nests that may have been disturbed by predators or rodents or machinery after the eggs hatched. Sometimes mice or ground squirrels eat or remove some of the membranes. Nests should be visited as soon as possible after the estimated hatch date to better judge nest fate and the number of eggs that hatched.
Destroyed Nests
Nest failure can usually be attributed to predation, farming operations, flooding, or livestock, but sometimes the direct cause cannot be determined. Observer judgment is usually necessary. The appearance of the eggs and the amount of disturbance at the nest vary considerably among destroyed nests. Eggs may be intact, as in flooded nests; more often they are broken and scattered. Predators may remove some or all of the eggs without leaving fragments or disturbing the nest material. Investigators should be aware of predator species on their study areas. In many situations, the identification of predators on the basis of egg remains and nest debris is not reliable. Sometimes more than one predatory species is responsible.
Abandoned Nests
Nests are sometimes abandoned, especially when hens are disturbed during early stages of laying (one to three eggs). Abandoned nests are those containing undisturbed clutches that are no longer tended by a hen. It is usually impossible to determine if a clutch was actually abandoned by a surviving hen or if the nest was untended because the hen was injured or killed. Hens sometimes take extended recesses during incubation and their eggs,. especially during cool and wet weather, may feel cool when handled. Nests that are suspected of being abandoned should be revisited to verify their fate.
Dead Embryos or Infertile Eggs
In some clutches, all of the eggs are infertile or contain dead embryos. When this occurs, hens may continue to incubate after the time of normal hatching. When candled, infertile eggs appear "clear" and those in which the embryo died at an early age have black blotches attached to the inside of the shell. When embryos die after some development, the blood vessels and yolk appear brown (Weller 1966).
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