Blog #435 Visual Communication (Repost Blog #245)

Female chasing intruding Osprey from nest, courtesy of Cindy Sedlacek.

Raptors are mostly solitary and non-social with few communication behaviors. The few social and colonial raptors, including the Osprey, communicate to live proximate to one another. The frequent contact between individuals in a semi-colonial nesting situation and the lack of feeding territories enables the Osprey to evolve a complex repertoire of ritualized displays. In 1993, Bretagnolle and Thibault catalogued the meaning and function of each Osprey communication behavior—11 visual displays and 5 acoustic signals—that can be deduced from both the context of the displays and their effects on the receiver. The 11 visual displays are described below and the 8 acoustic signals will be addressed in Blog # 434 Verbal Communication.

Osprey mates communicate through sexual displays such as solicitation for food and the courtship sky dance. Pairs also communicate with their rivals through antagonistic behaviors such as nest-protection and defensive postures. When warding off other species (interspecific communication), Ospreys use gradated responses, adjusting the intensity of the alarm displays and calls according to their distance from the threat.

Osprey behavior is individualized among the birds with some being more tolerant of other Ospreys and other species and others more skittish. Nest sites are a key limiting factor for Ospreys, so nest defense is critical and often intense. In dense colonies, aggression must be suppressed, and it is thought that individuals recognize neighbors and tolerate them more than non-neighbors. There is also a sexual dimorphism in some postures as is described in Figures 1 and 3.

Figure 1. Select visual displays of male, female, and juvenile Ospreys (name of display and sexes of birds in red type.) Arrows indicate field marks, such as neck position or wing position, that help distinguish different behavior (Bretagnolle and Thibault 1993, Auk 110:736-751.)

Visual displays include the following:

Resting posture (Figures 1a and b): Adults show subtle but detectable sexual dimorphism. Males rest sitting more upright than females with their wings partly opened. Females pose more horizontally.

Orpheus in Upright pose standing erect with wings held out while Ursula is in more

horizontal resting pose with wings tucked to her sides, Salt Point Osprey Nest Camera.

Upright posture
(Figures 1d and e): An alert posture is more upright than the resting posture. The neck is extended, and the wings are held slightly opened. In a highly motivated version of this display, males erect crest feathers. The upright display is usually an initial reaction to a stimulus and a prelude to a more intense display.

Alarm displays (Figures 1e and f): Alarm display differs from an upright display by a markedly extended crooked neck. These displays are categorized as low and high reactions according to the intensity of an individual’s motivation.

Solicitation (Figures 2a and b): High and low solicitation calls are used by females and older chicks to obtain food from the adult male during the breeding season. The proximate cause is usually to obtain food. During courtship it reinforces the pair-bond. For this display, the body axis is horizontal, the crest feathers slightly erected, and the wings are held close to the body.

Protective and Defensive postures
(Figure 2c, 2e, and f): These similar displays given by females are differentiated by the wing position. In protective display, the shoulder and carpal (elbow) joints are close to the body, and the body axis is usually more or less horizontal. As the display intensifies to be more defensive, wings are opened with the elbow or carpal joint

Figure 2. Visual displays of Ospreys. Arrows indicate important field marks (neck position, wing position, etc.) that distinguish different behaviors. Arrow indicates the movement of the wings during the nest-protection display (Bretagnolle and Thibault 1993, Auk 110:736-751.)

drooped. During a highly intense, motivated display, the bird adopts a more horizontal position with the tail raised and contracted (Figure 2d). Guard Calls are often given during a defensive display, mostly by the female when an intruder flies over a nest. Defensive displays such as mantling prey, is common to raptors and can be regarded as a food-defense posture that generally occurs just after a kill. If, for example, a female obtains a fish from her mate and wants to keep it from him, she will mantle or partially hold out her wings to cover the food.

Wings out and body vertical in a defensive pose, Salt Point Osprey Nest Camera.

Pair defensively guarding the nest, Salt Point Osprey Nest Camera

Ursula (right) mantles her fish with her outspread wings, Salt Point Osprey Nest Camera.

Males also defend their mates and nest from intruding Ospreys, especially early in the mating period and throughout the breeding season. Vigorous aerial chases often ensue if an intruding Osprey approaches a nest too closely, and the defender may try to strike the intruder in the air with its talons, driving it away from the nest site. Ospreys probably less threatened by neighbors and use Guard calls as an aid to recognizing individuals.

Nest protection displays are used by females and directed toward flying intruders, is an extension of a more intense defensive display, characterized by wing-shaking, tail fanning, and holding the body horizontal. Wing-shaking, performed mainly by females at the approach of an intruder, accentuates the white border on the Osprey’s back, perhaps signaling caution to any bird flying overhead. As nest defense modulates from low to high intensity, guard, excited, or screaming calls are issued. Nest protecting is highly antagonistic and often is followed by direct attack if the intruder lands at the nest. When the intruder leaves, nest protection is downgraded to a simple defense or protection display and is not directed toward the intruder.

Nest attacks begin with the female screaming alarm calls, mantling the nest, and spreading its wings defensively. As the intruder approaches, it is met with talons ready to strike. Males, if nearby, respond to intruders by alighting on the nest and then chasing the intruders.

Sky dance is a common, highly conspicuous raptor display and is the main mating display of male Ospreys. This highly visible and auditory display is directed toward both females (advertising the male’s fitness) and other males (excluding them from the nest territory). The sky-dance (https://www.youtube. com/watch?v=E-NoUh3EFIs) is usually performed over the nest as the male partner returns from foraging during the courtship period and early in incubation. In this dramatic sky-dance or fish-flight, the male gives Screaming Calls as it dangles its legs as it holds a fish or nesting material. The slow, undulating flight over the nest site starts

The sky dance with fish above the nest, courtesy of exit63.

Sky dance display with fish, CE Cornell..

The sky dance above nest, courtesy of exit63.

from high overhead (up to 300 yards or more) with shallow dips of 10–20 yards followed by a steep rise at the end of each decline and hovering in place. The sky-dance can last for up to 10 minutes and often it looks like the male loses altitude descending slowly in an undulating staircase fashion to the nest.Osprey behaviors are also sexually dimorphic and demonstrate sexual differences in the number, form, and structure of their displays (Figure 3). Females solicit food from their mate throughout the day and provide round-the-clock nest defense against Osprey intruders and predators. The male provides all food for the female and young from the arrival on the breeding site until the departure of the juveniles. The female stays at the nest for more than five months. Since the male is away from the nest much of the time fishing, defense is left primarily to the female. Since male Ospreys have limited time to defend

Figure 3. Visual displays performed by males and females. Arrow thickness

indicates the probability of the event happening (Bretagnolle and Thibault 1993, Auk 110:736-751.)

their nests and mates, they perform conspicuous displays, such as the sky dance that can be seen for miles around, and flashy antagonistic defensive displays and chases against intruders.

In addition to these active forms of communication, the Osprey’s simple and high-contrast coloration also functions in communication. When perched, the black-and-white contrast may exaggerate the alarm and upright displays by outlining the neck shape and form and indicate an increased level of attention. Wing position is an antagonistic indicator with the white border on the back outlining the position of the wings in the defense, protection, and nest-protecting displays. This contrast is reduced in females owing to their dark breast necklaces and their neck position while resting and upright. In addition, females use a solicitation display to beg for food. They hunch their necks and keep their wings close to body, thereby reducing this contrast in both the neck and back. Conversely, males appear whiter on their backs when perched. In fledglings (Figure 1), the contrasting colors are replaced by camouflaging cream, rufous, and brown.

Other behavioral adaptions revolve around the Ospreys’ large and exposed nests. Osprey nests are highly vulnerable to avian predators and attractive to intruding Ospreys, which may have led to an increased degree of camouflage in the chicks, whose coloration mimic nest material colors. When an alarm is sounded the chicks flatten on their nest bottom. These threats also lead to high intensity of guarding and constant attentiveness of the females. Numerous well-developed motivational signals (alarm, upright, and all calls) indicate that these highly visible and audible displays are intended for both intruding conspecifics and predatory hetero-specifics. This large diversity and ritualization of displays and calls in Ospreys are most likely adaptive responses to living semi-colonially, nesting in the open risking predation on the eggs and chicks, and their prolonged breeding strategy.

Eyes to the sky!

Candace

Candace E. Cornell

Friends of Salt Point, Lansing, NY

Cayuga Lake Osprey Network

cec222@gmail.com