Pathological Complexity Theory and the Piping Plover

Males Establish Breeding and Feeding Territories

Male PIPLs arrive at the breeding area before the females and begin establishing territories by early April. The territories are usually contiguous: the breeding area is in or near the dunes and the feeding site is below that territory up to the low tide line (Cairns, 1982, p. 532). The feeding area is used exclusively by the mated pair and offspring. There are often additional, more communal feeding areas behind the dunes, which is especially important during high tide when much of the private territorial feeding grounds are covered and during human disturbance (e.g. summertime beach use, including walkers and all-terrain vehicles). Studies indicate worse hatch outcomes across popular beach going regions (e.g. Guild et al., 2024).

Males Make Several Nest Scrapes, One of Which Will Be Chosen by a Female

The scrapes are potential nest sites and may be covered with pebbles or shells placed by both the male and the female.

Attract, Court, and Mate with Female

With calls and aerial and ground displays, the male seeks to attract a female and publicize his territorial claim to other males. Once created, the pair remains monogamous over that breeding season and a few continue the association in the next or another season.

Maintain Territory

The area’s size may enlarge or decrease throughout the season. Parallel-run displays are the most frequently used actions for boundary maintenance and are exhibited by both sexes, though the male plays a greater role. (Each bird runs along the boundary, often next to each other, on either side of the boundary line.) Sometimes, more aggressive actions occur.

Egg Laying and Incubation and Predator Defense

Females typically lay four eggs, one each 48 hours, though nests of three and sometimes only two eggs also occur. Hatching of all eggs within 2–3 hours of each other occurs after about 26–28 days. Both parents take turns incubating the eggs. If a nest is destroyed within the first half of the season, a second nest may be attempted, though it is not often successful. Single eggs may be lost to mice or rats, while all eggs are usually taken by the common crow, Red Fox, or raccoons and other terrestrial mammals or destroyed by off road vehicles or pet dogs (Wilcox, 1959, p. 140). Gulls are also common predators of eggs and chicks.

The parent plovers engage in a variety of behaviors to protect the eggs, and later the young, from predators. The parents, nest, eggs, and young, are all very well camouflaged against the sand. Some of the parents’ behaviors entail camouflage, such as sitting silently in a hollow or doing a low run, preferentially in any tire ruts. More conspicuous protective behaviors include a “rodent run” during which the plover wiggles its rump, appearing to some like a rodent. False incubation occurs when the parent sits as though incubating, but far from the actual nest, averaging 40 m. away in one study (Viola, 1995).

A far more energy-expensive behavior is the “Broken Wing Display” (BWD) aka “Injury Feigning,” a compelling, intense activity. It is often made when a predator/intruder is close to the nest/young and to the parent bird, a dangerous situation for both the eggs/young and the displaying parent. During the egg stage, PIPLs are more likely to make the display during the last week before hatching. I will discuss later in detail an experimental analysis of the plover’s use of the display (“The Broken Wing Experiments”) (Ristau, 1991). I will explore whether the BWD is simply an innate behavior such as a “Fixed Action Pattern,” emitted in random directions or whether the plover is able to use the innate behavior strategically.

I will also discuss separately the “Safe-Dangerous” Experiments that investigated the plovers’ ability to learn to discriminate potentially dangerous intruders from those that are not and the “Gaze Experiments” investigating the plover’s response to an intruder’s attention.

Hatching/Birth

Hatching day is preceded by several days during which the young may make sounds within the eggs, audible to the parents. This seems to add to the general state of arousal/anxiety of the parents during hatching when the movements at the nest and the white of the insides of the broken eggshells may increase the likelihood of detection by a predator.

The young are precocial, leaving the nest after 2–3 hours, and beginning to feed, but they generally remain within about 400–500 feet of the nest until able to fly.

Care of Chicks

The parents appear to “keep tabs” on the young, huddling over them with physical contact and protecting and warming them during harsh weather and while sleeping. Both the parents and young have calls, both soft and louder, that appear to help maintain contact and/or alert the parent to a chick’s distress.

The parents engage in various protective behaviors, including the “Broken Wing Display”; the displays are performed more frequently and intensely during the chick stage than during the egg stage.

Build Weight and Strength for Migration

Both young and parents must avoid predation and injury, gain sufficient weight, and increase stamina to ensure a safe migration later in the summer.

Migration to Wintering Grounds

Migration is a dangerous aspect of a plover’s life cycle. Like all avian migrators, they suffer significant loss of life during such times, through predation, severe weather, and challenges to their energy resources, for the flights require high energy consumption. The southern migration begins in late July and extends through September.

Overwinter in Same General Area as Their Parents

During overwintering, piping plovers establish seasonal home ranges and smaller cores areas; they show site fidelity. However, data from radio-marked PIPLs in a southern Texas site indicated that the area sizes change with larger areas in winter and smaller in fall. Plovers’ movements were due primarily to seasonal high tides during fall and spring and seiches that inundated barrier island flats during the winter. Movements were usually less than 10 km, less than most wintering shorebirds. At least on this coast, the juxtaposition of heterogeneous habitats permitted the plover to survive this period without energy consuming lengthy flights.

A second factor influencing plover and other shorebird survival is predation by raptors. Some shorebirds take flight upon a falcon’s approach, while plovers crouch. Plovers thereby tend to survive, for falcons tend to capture prey from the air.

Wintering in warm southern regions above freezing temperatures also contributes to the plovers’ survival. These authors conclude that declining plover populations are not due to death during overwintering, but climate change and continuing human development will constrain plovers’ available habitat and probably impact survival rates (Drake et al., 2001).

No studies have investigated the possible formation of preferred companions and social groups.

Chicks’ First Flight to a Breeding Site

Again, similar dangers apply to this migration north as to the flights southward. A very few first-year plovers (<5%, Wilcox, 1959) return to the hatching area, but most do not (Cairns, 1982; Lenington & Mace, 1975).

The Cycle Begins Again

Though only 1 year old, the young plover begins the reproductive cycle of territorial acquisition, courting and finding a mate.

The “Safe-Dangerous” Experiments

The world of the PIPL contains both predators and non-predators. Humans may pose a threat simply by walking near the nest, which typically causes the incubating parent to leave the nest, thus exposing the eggs to predators or possibly harmful weather conditions. (A nest without a parent on it is less conspicuous than one with the sitting parent.) Although a plover may be innately aroused by certain terrestrial and aerial species, might a plover also be able to learn to distinguish between potentially dangerous intruders and those that are not?

To test this hypothesis, I conducted field experiments, with humans as “intruders.” In the experiment, conducted during the incubation stage, each of two intruders walked past the nest at a far distance (12–32 m. from the dunes where the nest is located). The intruders were dressed differently to aid the discrimination and walked at similar rates along the same path during each pair of trials. Pretests were usually conducted to ascertain that the plover was not biased in its response to either intruder. Then, for about two trials (range 1–4), the “Dangerous” intruder paused before she was just opposite the nest and then walked directly toward the nest, pausing and hovering over it. The “Dangerous” intruder then continued walking over the dunes and left the immediate area.

On the test trials, each intruder walked identical pairs of paths ranging from 12 to 32 m. from the dunes and parallel to the dunes. Two observers recorded the parent plovers’ behavior and the intruder’s location during the walk-bys, using video and acoustic recordings. The plovers were more aroused by the “dangerous” intruder as each walked “safely” by the nest at the relatively far distance from the nest (X² = 31.23, p < .001). (For more details about the experiment, see Ristau, 1991.) I have thus established very rapid learning of a significant discrimination by plovers between threatening intruders and “safe” ones.

Note that we do not know whether the plovers employ concepts like “threatening” or “dangerous.” Nor do we know what characteristics of the “Dangerous” intruder are causing the plover’s alarm. Is it that the “Dangerous” intruder knows the nest location? Is it conditioned fear from the close proximity of that intruder to the plover’s eggs and/or the plover. Both the presumed knowledge and fear? Other?

Some of the plover’s discriminatory abilities and habituation to non-threatening persons were evident in our study sites. Plovers nesting on the remote Metompkin Island, Virginia, were much more easily aroused by our presence than at our popular New York Long Island beaches. Metompkin was seldom visited except by a passing fisherman or the occasional human who had arrived by private boat. The Long Island sites were frequented by persons on walks, sometimes with a dog, and, during the summertime, by beachgoers. The plovers seldom left their nest for any of these intruders unless they were close to the dunes.

However, one plover who had experienced partial nest predation from an unknown cause behaved in a highly aroused fashion to a person walking a dog down by the shoreline. Upon seeing the dog, she immediately flew from the nest, flying just in front of the dog. The dog avidly followed until the plover led it into the water; the dog got completely wet and the plover flew in a great circle back to her nest. The same human and dog returned another day, and the plover did the same thing, though the dog didn’t go all the way into the water the next time.

In other study sites by Lake Manitoba, Canada, Haig (1983, p. 37) reports that “piping plovers will not perform courtship displays, copulate or incubate if humans are within 100 meters of their territories.” Clearly, PIPLs can learn to respond differently to human intruders depending on their prior interactions with them.

The Broken Wing Display Experiments

Broken Wing Displays or “Injury Feigning” is widespread among ground-nesting birds, particularly shorebirds (Ristau, 1991). It is present in 52 different families, encompassing 285 species, suggesting that “it has independently evolved multiple times” (de Framond et al., 2022). The researchers explored the literature to discern the habitat features underlying the BWD’s evolution. The underlying determinant for BWD’s presence in a species appeared to be predation pressure. An example is that BWDs were associated with higher maximal latitude, affording more daylight hours and thus more activity by diurnal predators. Temperate regions appear to have more diurnal terrestrial nest predators than tropical areas with more nocturnal ones (Armstrong, 1954). BWDs are used in encounters with diurnal terrestrial predators. The authors acknowledge that the lack of expected correlation between BWD occurrence and certain habitat features might have been that judgments (e.g. of nest conspicuousness) were based on human perceptions and not those of nest predators (de Framond et al., 2022, p. 9). It is noteworthy that the researchers are cognizant of likely different perceptions. The essential message for my discussion is that the BWD is an evolved behavior, not conjured up by each individual piping plover.

When I began my research, many scientists believed that the BWD was merely an innate “Fixed Action Pattern” or, similarly, a convulsive action that resulted from the conflict between a drive to escape and to act aggressively. It was thought to be emitted in random directions when a plover first detected a potential intruder/predator (Skutch, 1976, p. 403). (Many scientists still maintain these views.)

My interest in the problem was sparked by a philosopher, Daniel C. Dennett, who remarked, during a workshop on Communication, something like those “injury feigning” birds are simply performing innate responses, not purposeful, no deception involved.” I was intrigued and so began my series of experiments.

To investigate whether the plover’s use of the BWDs entailed some strategic use, whether it was purposeful, I sought to determine what philosophers and experimental psychologists/ethologists considered to be attributes of purposeful behavior.

Inspired both by the ideas of Donald R. Griffin (1976, 2001) and Daniel Dennett’s discussion of the “Intentional Stance” (1983), I attempted to apply an “Intentional Stance” to a plover’s behavior. Specifically, I examined the responses to a human intruder who walked near a plover’s nest with eggs or its offspring, varying her direction toward and away from the eggs/young. I asked what evidence would support the hypothesis that the plover was exhibiting a First Order Intentional behavior, namely, that “The plover wants to lead the intruder away from the nest/young.” This is distinguished from a Zero-Order level by which the BWD would simply be a reflexive behavior provoked by the stimulus of an intruder’s presence, entailing no mental states of the plover. (For further information on the influence of Griffin’s ideas and those of philosophers, including Daniel Dennett, on this and other related cognitive ethological field research, please refer to Bats, Birds and Minds: Tales of a Revolutionary Scientist, Donald R. Griffin, particularly Volume 3 (Ristau, 2024)).

I used the following criteria to support my interpretation of a First Order intentional stance:

A. The direction of BWDs should usually be adequate to lead an intruder away from the nest.

I will briefly review the BWD research which was conducted with a human intruder on a beach (Metompkin Island). Ideally, we would observe naturally occurring interactions between a parent plover and a local predator, such as a raccoon, but such occasions are rare and highly unlikely to be observed when they do occur. Our remote-controlled, mobile stuffed raccoon encountered technical difficulties in the beach environment.

Both the intruder and the observer communicated with each other via walkie-talkies and recorded their observations on audiotape, while the observer supplemented hers with video recording. The intruder moved around near the plover’s offspring, occasionally following the plover during a BWD and sometimes not.

I asked several questions:

1. Was the plover strategic about where it began its BWD or did it simply start displaying upon detecting an intruder/predator. We found that the plover repositioned itself by walking, running, or flying when it detected the approaching intruder, before it began a BWD. Contrary to the conflict hypothesis, it did not attempt to escape from the intruder, but in 13 of the 13 cases when the plover flew, it flew to be closer to the intruder and, in most instances (11/13), closer to the center of the intruder’s visual field. Flying provides specific information about the location change and suggests the urgency of the PIPL’s actions.

However, if the intruder follows the displaying plover, it does not reapproach the intruder (100% = 5/5 cases).

3. Is the direction of BWDs appropriate to lead intruders away from the nest/young? Yes. If the intruder had followed the displaying bird and had reached the final location at which the bird was displaying, the intruder would be further away from the nest/young than it was at the beginning (98% = 44/45).

I take these behaviors as evidence supporting a First Level Intentional stance, that the bird wants to lead an intruder away from its nest/young. It should be noted that the parent plover exhibits flexibility in its use of the BWD beyond that described above. It has other forementioned protective behaviors within its repertoire and does not always make a BWD upon the approach of an intruder to its nest/young. During the “Safe-Dangerous” experiments, when an intruder approached a nest with eggs, BWDs were made in only 40% of the approaches.

Field observations report that some avian species, for instance, the killdeer (Charadrius vociferous), a close relative of the PIPL, behave differently depending on the type of danger the intruder poses to the eggs. Only rarely does a killdeer perform a BWD at the approach of a grazing animal such as a cow, which does not eat the eggs but may accidentally trample the nest. Instead, the killdeer may lunge at the cow’s face, startling it and causing it to veer from its path (Amstrong, 1947; Graul, 1975; Walker, 1955).

These instances all exemplify the plovers’ flexibility and ability to adapt its protective behaviors as needed, lending further support to an interpretation of purposeful, specifically, conscious, control of its actions.

The “Gaze” Experiments

This research in which plovers can read gaze direction of an intruder suggests that the plover may be capable of “perspective taking,” a foundation for a theory of mind or “mind-reading” (Ristau, 1991). Very briefly, two intruders, dressed differently, walked, one at a time, on the same paths parallel to the nest at a far distance from it (15–25 m. from the dunes). One intruder scanned the dunes where the plover’s nest was located, while the other gazed in the opposite direction, toward the sea. Just before each was directly opposite the nest, each sat down and continued gazing either toward the dunes or the sea. They then continued on the parallel path. The plover was more aroused by the person gazing toward the dunes than away as measured by a longer time off the nest.

Note that the plover has cues other than direction of intruder’s gaze, namely, orientation of the head and the back or front of the intruder’s body. Also, the plover is initially on its nest and thus within the gaze of the person looking toward the dunes and may continue to be during all or part of the continuing trial. Despite these caveats, the plover has been shown to be sensitive to the attention of an intruder to the location of its eggs and/or itself.

Chick Distress Calls

In a series of experiments, pre-recorded distress calls of different chicks were played back to parent plovers during the egg or chick stage of their reproductive cycle. The parents were far more responsive during their chick stage than during incubation. The astonishing aspect of the parents’ behavior was the great variability in their responses, stressing the significance of researchers recording detailed observations, particularly of few or one-time occurrences of animals’ behaviors. Such cases can suggest complex abilities that might not otherwise be evident. One parent made repeated, searching flights for over 15 minutes in the area over the sound source, but then abruptly stopped the search flight and flew directly to another location and began searching there. That location was the site of a sound source (SS) in a playback experiment conducted a few days earlier.

In many other instances, the plover continued search flights after the sound source was silent, as occurred between trials. Plovers also tended to search longer in contexts when the SS was at least partially hidden in grasses than when out on the open sand. Responses also differed depending on whether the parent(s) were with their chicks as the recorded calls began (Ristau, 2025; Ristau et al., 1997).