Reasons Why Predators Cannot Be Inferred from Nest Remains

1. What they did

Researchers often decide which animal raided a bird’s nest by looking at what is left behind: the state of the eggshells, the nest bowl and other signs. The method grew out of watching how captive animals broke eggs in the 1930s and 1940s, and it stayed popular because it needs no equipment and can be used even on nests found already destroyed. Several researchers had questioned whether it is reliable, yet it was still being used and tested.

In this short commentary, the author reviewed the published evidence, together with his own work on striped skunks and duck nests in Saskatchewan, Canada, to ask whether identifying nest predators from nest remains is supported by science.

2. What they found

The author set out a series of problems, any of which can mislead someone reading nest remains, and which may act at the same time.

Confusing predators

• Different species leave the same signs. Red foxes, Franklin ground squirrels, raccoons, weasels, mink, crows, rats, snapping turtles and gulls can all take whole eggs, so an empty nest bowl does not point to any one species. Published photos of eggs broken by raccoons and striped skunks look extremely similar, and other photos show overlap between gray fox and coyote, and between bobcat and coyote. In South America, eight of nine mammal species left egg remains that could not be told apart.
• One species leaves different signs. Early authors described striped skunks as “invariably” crushing shells completely, while another described them as “invariably” opening the end of the egg. Skunks have since been reported to leave many different patterns. Raccoons and gray foxes are also variable. Patterns may differ with a predator’s size, sex, age, hunger or even race.
• Unexpected predators. Coots, armadillos, badgers, night herons, bobcats, cattle, ravens, dogs, hogs, moorhens, muskrats, turtles, ungulates, weasels and wild turkeys have been recorded destroying eggs. Passerine eggs have even more: porcupines, black bears, feral cats, fishers, a woodpecker, snowshoe hares, white-tailed deer and woodchucks. Because no one expects them and their patterns are not described, they are seldom reported.
• Assumptions about access. Researchers may rule out predators that “cannot” reach a nest, yet mink and red foxes sometimes climb trees, badgers and skunks forage in water, and mink swim long distances to islands.
• Egg size. Crows usually carry eggs away but eat eggs that are too large on the spot. The smaller ground squirrels do not eat duck eggs, while the larger Franklin ground squirrel often does. Small mammals may not break the large quail eggs used in many experiments, so their predation goes unnoticed. Eggs may also be destroyed by competitors, birds of the same species or nest parasites.
• Stage of development. Eggs with advanced embryos often break into smaller pieces than fresh eggs, so a different pattern may reflect egg age rather than a different predator.
• Observer experience. Nest checks are often done by inexperienced students and summer assistants. For example, the smell of red fox urine (which foxes may leave on raided nests as a “no-food” signal) can be mistaken for striped skunk musk, although skunks only spray when threatened and never use musk to scent-mark.

Partial predation and nest visit intervals

• Nests are usually checked every 5 to 14 days. Once a nest is raided, broken eggs give off sights and smells that draw scavengers and other predators.
• In Virginia, 43% of 114 raided nests were visited by 2 to 5 species within a 7-day interval. In Texas, hairs of both skunks and raccoons were found at nests raided within 7 days. In Saskatchewan, over 30% of nests visited by striped skunks had already been partly or fully raided by other animals. In the Ozarks, 6.7% of 326 clay eggs showed marks of more than one predator species after 5 days. In one study, 2% of raided nests were visited by scavengers within a single night. Two predators may even raid a nest at the same time.
• Missing eggs were reported as common at nests raided by striped skunks, but skunks do not cache or remove eggs; scavengers were probably responsible. In Saskatchewan, nests partly raided by skunks were often cleared of all eggshells by scavengers within 4 days.
• Leftover yolk has been used to identify predators, but scavengers, including carrion beetles, eat it, so a nest checked 6 days later may hold only clean shells.
• Nesting hens may tidy up after a partial raid, removing eggshells or damaged eggs, which can look like a predator carrying eggs away.

3. What they concluded

• Blaming the wrong species can have serious and expensive consequences for wildlife management, and can lead researchers to wrong conclusions about the behaviour and ecology of nesting birds.
• Which approach is best depends on the question. For management, landscape approaches that reduce nest predation by many species may matter more than naming one predator, and are less affected by a wrong identification.
• When the true predator does matter, the author points to simulated nests and predator identification devices such as hair catchers and nest cameras. Neither is free of bias, but any device is more objective than a guess based on eggshell remains.
• The author suggests that further attempts to validate the method be abandoned, calling it a procedure “made into a law through verbal repetition”, and states that basing management on it is unacceptable.

4. How tracking was or was not used

No new tracking was done. The whole paper is about the reliability of one kind of sign interpretation: reading egg and nest remains to identify a predator. It also mentions track counts, sightings and live-trapping as ways researchers learn which predators are present, hair collected at raided nests, and scent (fox urine versus skunk musk) as a sign that can be misread.

6. Other technologies, skills or innovations

• Predator identification devices, such as hair catchers and nest cameras, are suggested as more objective alternatives.
• Simulated (artificial) nests with quail or clay eggs are discussed, with the caution that they may not experience the same survival as natural nests and that small mammals may not break large quail eggs.

7. Implications for conservation (from the paper)

• Misidentifying nest predators can send management effort and money after the wrong species.
• Multi-species, landscape-level approaches have benefits for many species, are more likely to be supported by the public, and are less affected by mistakes in naming the “most important” predator.