Adaptive Value

Testing the Predator Avoidance Hypothesis

Before the work of ethologist Konrad Lorenz, a friend and collaborator of Nikolaas Tinbergen, the idea that behaviors, not just genes, are acted upon by evolution had not been sufficiently investigated. After spending years of studying animal behavior, however, Lorenz proposed that biologists start to think of each behavior as a unique “organ” that an animal inherits and uses for a specific purpose. This perspective can be applied to the eggshell removal behavior in birds to reveal how valuable this behavior is in ensuring that chicks survive past infancy.

Although we have already discussed Nikolaas Tinbergen’s five hypotheses on eggshell removal, it is important to note that all five explanations discuss egg removal in the context of the survival of the chicks. Behaviors that increase survival rates are considered adaptive because they allow the animal to reach sexual maturity and reproduce. As you can see, all of the hypotheses proposed by Tinbergen explain this behavior in the context of survival:

1) sharp shell edges could injure newly hatched chicks
2) unhatched eggs could be trapped inside hatched shells
3) hatched shells could interfere with brooding (this could reduce the survival of the hatched chick’s nestmates)
4) hatched shells could reduce nest hygiene and increase the risk of bacterial infection
5) hatched shells could reduce nest camouflage, thereby increasing the risk of visually oriented predators.

Nikolaas Tinbergen

Tinbergen mainly investigated the fifth hypothesis and found very compelling evidence for his ideas. In one study, he distributed eggs in an area frequented by predators. He placed sets of gull eggs throughout the area, some alone, and some with empty eggshells four inches away. He then camouflaged the eggshells to varying degrees. Even when the egg shells were better camouflaged than the normal eggs, eggs that were next to eggshells were three times as likely to be found and eaten by predators.  In a second experiment Tinbergen showed that the farther the intact egg is from the eggshell, the less likely it is to be destroyed. In a group of eggs placed within 15 cm of an eggshell, 42% were eaten. When eggshells were placed 100 cm from an egg, 32% of the eggs were eaten, and if the shells were placed 200 cm away from an egg, only 21% of the eggs were eaten. 

Other research has shown that first time breeders will remove shells placed in their nests even before they have laid their first egg. Birds will preferentially remove white shells placed in their nests, a color that compromises nest concealment, but generally fail to remove green shells, since green is a color that increases nest camouflage. Tinbergen hypothesized that if the birds removed green objects around their nest, they might begin removing leaves and reduce the camouflage of their nest. Although birds remove white eggshell-shaped objects most frequently from their nests, if the birds have been given model eggs of unnatural colors to incubate, such as red, they will later remove eggshells of that color as well. A more recent study on Ring Doves found that birds remove eggshells more consistently and more quickly after breeding has taken place and this behavior intensifies as the hatching date approaches.  Birds are therefore primed for eggshell removal at exactly the time it becomes useful for them to conceal their nests from predators.

The Black-Headed Gull (Larus ridibundus) studied by Tinbergen

Testing the Other Hypotheses

Tinbergen and his team of researchers found ample support for the predator avoidance hypothesis, but never tested in detail their other hypotheses. Since, researchers have looked at these other hypotheses to determine if there are multiple evolutionary pressures which make egg removal a selectional advantage.

Researcher Todd Arnold tested the capped egg hypothesis using American coots. He decided to study this species because they have very low rates of chick fatality due to predation (0.2% in some areas). Furthermore, most of these loses are due to predators that hunt by scent rather than vision, making it unlikely that they would need to remove eggshells for this reason.  With coots, nests are generally crowded, offspring hatch at different times, and egg size also declines in laying sequence, all factors that could cause one egg to get caught under a hatched egg’s shell, and prevent it from hatching. Arnold set up an experiment in which half of the eggs were capped throughout development, and another half, the control, were left uncapped. However, he found no significant difference between the number of eggs that hatched in the control group and the number of eggs that hatched in the capped group. The capping hypothesis does not seem to be a reason for the egg removal behavior. There are other studies on egg-capping that challenge this result, but egg-capping is probably not the central reason for eggshell removal in most birds. For an experiment that did find slight effects of egg capping on survival, see Derickson, K.C. and I.G. Warkentin, “The role of egg-capping in the evolution of eggshell removal.”

    American coot and chicks

Most of the other hypotheses have not been tested as extensively as the egg-capping and predator avoidance theories. But the observations of Tex A. Sordhal in a study on Mountain Plover eggshell removal have led researchers to believe that the other hypotheses (risk of diseases, injury, and blocked brooding) are probably not central factors in causing birds to remove eggshells. Sordhal found that adult Mountain Plovers would often move their eggshells slightly out of their nest immediately after their chicks hatched, and then later come back and move them farther away. This pattern of egg removal is exhibited in many species. In the experiment, Sordhal placed eggshells at varying distances from the plovers’ nests, and found that if they were within 3 meters of the nest, the adult plovers would remove them further. This would suggest that at least in this species, egg removal is not due primarily to hypotheses 1-4, because if it were, the birds would have no reason to move the eggs further away from the nest.

Although we can conclude that in general, eggshell removal behaviors aid in camouflaging the nest and consequently avoiding predators, there remain cases in which avoiding predation seems an unlikely reason for this behavior. For example, there are hole nesting species that dispose of eggshells.  Shell disposal in these cases could prevent problems such as bacterial infestation in the nest. In species where the eggshell removing behavior serves primarily to reduce predation, it is still possible that the behavior does have the added benefit of preventing disease, injury, and other deleterious factors.

Species-Specific Adaptations of the Eggshell removal Behavior

Although a wide variety of birds, and nearly all shorebirds exhibit the eggshell removing behavior, many species have developed specific variations on this behavior that are best suited for their environment. When comparing the egg removal behavior of different species of birds, Tinbergen noticed that oystercatchers and ringed plovers removed eggshells from their nests much faster than gulls did. He suggested that the slowness of the gulls stems from their habit of nesting in groups. Experimentation proved this hypothesis correct—neighboring gulls will eat a neighbor’s freshly hatched chicks if they are still wet and easy to consume. New parents wait until the chicks are dry and fluffy, foiling the cannibal gulls’ hopes of a meal, before leaving to dispose of the shells. Since oystercatchers and ringed plovers are solitary nesters, they leave earlier to dispose of the shells. Researchers have found that birds whose chicks are not threatened by intraspecific predation generally show immediate-egg-removing behavior.  The behavior of the adult gulls after their chicks have hatched balances two different predation pressures—from within and without the colony. Although it seems like a dangerous risk to leave chicks unprotected in order to remove eggs, experiments like this one show that the adults are constantly weighing competing threats to their young. Other studies have reported that birds will not leave their nest to remove eggs if there are predators nearby, choosing to protect their young from the direct threat of predation before protecting them from the less pressing threat of reduced camouflage.

Although many birds that abandon the nest after their young are born do not bother with egg removal, some do, such as the American Avocets and Black-Necked Stilts. These birds generally leave the nest about 24 hours after their young are born, and eggshells could attract predators during that time. Researcher Tex A. Sordahl found that adults of these species tend to submerge their eggshells, rather than carrying them great distances. This behavior probably keeps predators from finding the shells, and allows the parents to leave their chicks unattended for shorter periods of time.

Kittiwakes are a rare species of shorebird in that they do no perform the shell removing behavior that is nearly universal in the Laridae family. In fact, Kittiwakes differ from other gulls in many other behavior aspects. Unlike their ground-nesting relatives, they do not swarm predators that attack, and they have fewer violent competitions for food.  Tinbergen and E. Cullen have suggested that this is due to the extremely narrow ledges that these gulls live on. The nesting spots of the Kittiwakes serve as protective devices against invasive predators, making it unnecessary for adults to leave their nest to remove eggshells since the nests are already well camouflaged and difficult to reach. It is thought that Kittiwakes once removed eggshells like their close relatives, but lost this behavior when they moved to a new environment under different selective pressures.

    Kittiwakes in their cliff homes

Similar Adaptive Behaviors

A behavior similar to eggshell removal has evolved in many birds—fecal sac removal. Researcher Kenneth E. Petit worked off of the findings from Tinbergen’s research on the importance of removing conspicuous objects from around the nest, hypothesizing that the presence of fecal sacs near the nest increases fatality in those nests due to predation. He found that eggs close to fecal residue suffered greater predation than those farther away, confirming his hypothesis.  Both the eggshell and fecal sac removing behaviors involve ridding the nest of predator-attracting elements and ultimately help offspring survive better.

Although there are many explanations for the role of eggshell removal, all of the experimental evidence suggests that eggshell removal increases the likelihood that chicks will survive and go on to reproduce, making the evolutionary value of this behavior to birds very high. Most shorebirds are ground-nesting, so many of the behaviors common among the shorebird order (Charadriiformes) involve protecting chicks from invaders. Tree nesting or cliff nesting birds are not under the same sorts of selectional pressures from predation, and this is clearly evident when comparing the behavior of these animals to ground-nesting species. Shorebirds eggs are cryptically colored, and chicks even beg for food at a different frequency than their tree-nesting counterparts, in order to avoid detection. Ground-nesting adults have also evolved a mobbing behavior to drive off predators when they approach newborn chicks. The eggshell removing behavior studied by Tinbergen and more recent researchers clearly reflects the predation pressure on ground nesting shorebirds, and is just one of a whole battery of behaviors shorebirds have developed to keep their young alive.