Northern Prairie Wildlife Research Center
Duck eggs are oblong and usually lie on their side in nests or outside nests when displaced by predators. Consequently, predators open most duck eggs on the side, unless the predator has special egg-holding (e.g., raccoon [Jones et al. 1983]) or egg-opening (e.g., Franklin's ground squirrel [Sowls 1948]) behaviors.
Contents of duck eggs change from liquid albumen and yolk to a fully-developed embryo during the approximate 25-day incubation period (Bellrose 1980). The change is especially rapid during the last 10 days of incubation (Weller 1956). Also, eggshells become brittle late in incubation (Terres 1980). Thus, the greatest damage to shells is likely to occur when eggs are depredated late in incubation.
Size of duck eggs and shell thickness vary. For example, the average mallard egg is 1.9 times heavier and the shell is 1.2 times thicker than the average blue-winged teal egg (Mallory and Weatherhead 1990). Egg size influences ability of a predator (especially small species) to transport and/or open the egg, the number of eggs taken and amount of egg contents eaten, and the probability that eggs will be removed from the nest site. Montevecchi (1976) found that American crows were less likely to transport, and more likely to open at acquisition sites, large rather than small poultry eggs. Thickness of the shell affects ability of predators to open eggs, as well as the amount of fracturing of shell during consumption of contents. Fleskes (1988) found that weasels had greater difficulty opening eggs of wood ducks (Aix sponsa) and mallards than opening the smaller and thinner-shelled eggs of blue-winged teals.
Most duck nests in uplands change from a shallow depression with little nest material and no down when the first egg is laid, to a well-constructed bowl of vegetation, down, and other small feathers when incubation starts (Sowls 1955). Down and other small feathers are gradually added after a few eggs are laid but may not be abundant until incubation starts (Bellrose 1980). Hens have more down to pluck during their initial nesting attempt than when renesting. Thus, potential for obtaining information about predator displacement of nest material is greater for nests destroyed after most eggs are laid, and greater for first nests than renests.
Number of eggs present when a duck nest is destroyed affects amount and quality of evidence likely to be found at the nest. As each egg is depredated, the pattern of depredation becomes more evident and chances of finding evidence to implicate the predator species increases.
Location of a nest affects vulnerability of the nest to individual predator species. For example, Franklin's ground squirrels seldom venture into areas of short or sparse vegetation or into water (Choromanski-Norris et al. 1989). Location of a nest also affects amount and types of evidence likely to be found at the nest. Eggs and eggshells from nests over water may be pushed into the water and sink or float away. Moreover, eggshells at over-water nests are more likely to be trampled than are eggshells at nests in uplands, because predators are usually on the platform of an over-water nest when eating at the site.
Vegetation at duck nests may influence amount and types of evidence of depredation left at nests, because predators can maneuver around nests more easily in short, fine-stemmed vegetation than in tall, robust vegetation. Also, density and structure of vegetation at the nest may result in predators choosing different places to eat eggs (e.g., in nest, along trail to nest). These factors may affect number and arrangement of eggshells at nests, distance of eggshells from nests, and degree of trampling of eggshells and vegetation at nests.
Hens often continue to attend nests after a clutch has been partially depredated (Fleskes 1988). This behavior can affect number and location of the eggshells, as well as appearance of the nest after a clutch is destroyed. Sowls (1955) reported instances of northern pintails and northern shovelers carrying eggshells from nests partially depredated by Franklin's ground squirrels. A blue-winged teal was observed carrying an eggshell from a partially depredated nest at Audubon National Wildlife Refuge, North Dakota (T. D. Kostinec, Audubon National Wildlife Refuge, Coleharbor, ND, pers. commun.). Also, whole eggs (Sowls 1955) or nest material (unpubl. data) displaced during partial depredation of a nest may be returned to the nest by the hen.
Variation exists within predator species in how individuals depredate nests. Our tests with captive striped skunks (discussed later) showed that a skunk opened half of the eggs on an end. This occurrence was about 3 times more often than the 3 other skunks tested. Variation of this type complicates correct identification of offending predators.
Evidence found at nests with eggs destroyed by juvenile predators may differ from that found for eggs destroyed by adults, because juvenile predators are smaller and less experienced in handling eggs. By early to mid-June, juvenile red foxes have potential to destroy nests near their dens, especially in the southern portion of the region where whelping dates are earliest (Sargeant 1978, Sargeant et al. 1981). Juvenile American crows have potential to depredate nests in late June, when they begin to fledge (Ignatiuk and Clark 1991). Juvenile striped skunks and juvenile American badgers have potential to depredate duck nests in early July, when they begin to travel independently (unpubl. data and pers. observations).
Sometimes a predator will leave scent, urine, and/or feces at a nest with eggs destroyed. Such evidence shows a particular predator species visited the site, but it is not proof that species depredated any eggs. For example, red foxes often mark prey remains (e.g., eggshells) with urine and/or feces (pers. observations).
Franklin's ground squirrels cannot spread their jaws widely enough to easily open duck eggs (Sowls 1948). They rely on repeated biting and gnawing to open duck eggs, which often results in finely serrated edges of openings. Avian predators usually transport duck eggs by pecking a hole and then inserting a mandible to grasp the shell. However, some avian individuals can carry a whole duck egg using their beak without puncturing the shell (Odin 1957 [California gull], Montevecchi 1976 [American crow], pers. observations [black-billed magpie, American crow, gulls]). Except for gulls, avian predators open eggs by pecking. Gulls, having slightly hooked beaks, occasionally open eggs by biting (pers. observation) or dropping eggs (Odin 1957 [California gull]). Avian predators may peck an egg in several places to gain entry, thereby leaving scattered peck marks on the eggshell and/or ≥2 openings in the eggshell.
A predator's size influences extent of egg contents consumed during a feeding bout. Large carnivores customarily eat all contents of an egg (pers. observations). Also, they can eat an entire clutch of duck eggs during a feeding bout (e.g., coyotes [Sooter 1946]), but may not do so (e.g., raccoons [Rearden 1951]). Small carnivores (except possibly large minks), Franklin's ground squirrels, and avian predators can eat only 1 or a few duck eggs during a feeding bout. Fleskes (1988) reported individual captive ermines ate 0.5-2.0 duck eggs/day. Thus, likelihood of finding whole and/or partially eaten eggs at a destroyed duck nest is greater if the offending predator was small.
The feet of a predator affect how it treats duck eggs, nests, and nest sites. The raccoon, with its handlike front feet (Jones et al. 1983), is the only predator in the Prairie Pothole Region adept at holding duck eggs upright to eat, although American badgers and striped skunks sometimes prop duck eggs upright. Corvids may hold a duck egg in place on the ground with a foot while pecking into an end (pers. observations of black-billed magpie). We never observed gulls use their webbed feet to open duck eggs in this manner.
American badgers, and to a lesser extent striped skunks, are the only predators of duck nests in the Prairie Pothole Region that have long-clawed feet (Jones et al. 1983). Thus, they are species most likely to dig at nests.
Duck nests with clutches partially or completely depredated may be subsequently visited by another predator or other animals. Baker (1978) used hair-catchers to study depredation patterns of artificial nests containing poultry eggs and found that 20% of 45 nests were visited by both raccoons and striped skunks before he rechecked the nests at weekly intervals. Effect of subsequent visits by animals other than the offending predator on evidence of depredation probably varies greatly. If whole and/or partially eaten eggs are left by the first predator, another predator may depredate those eggs. Some birds, small mammals, and insects may peck or chew on eggshells, and/or may consume uneaten portions of eggs (see Jones  regarding deer mice [Peromyscus maniculatus], Greenwood et al.  regarding carrion beetles [Silpha sp.] and possibly other insects).