Blog #393 Body Language
Every unwanted intrusion toward the nest is met with alarms, shrieks, defensive posturing, and counter-attacks if necessary. Ursula stays in the nest protecting her brood as Orpheus does the combat.
It’s a tranquil morning at the Salt Point Natural Area. A merganser swims by with her brood of 12 vying for positions on her back as geese explode from the creek in a raucous explosion of honking. Ursula watches all the happenings from her catbird seat 50 feet above where she dutifully broods her young. Suddenly, Ursula casts her view directly overhead and gives off low-level alarm calls. As the intruder circles, coming closer, Ursula turns up the intensity of her defensive display by mantling and alarming louder. Orpheus. Joins her defense of the nest. Her shrieks become more intense as the distance closes between her and this insistent intruder. She stands, beating her wings and, in a rare move, takes off after the intruder, leaving her young hatchlings behind. The sequence of her behaviors is easy to interpret as all her actions are focused on drawing attention away from her precious hatchlings in the nest.
Ospreys, like other raptors, are primarily solitary living in loose semi-colonial situations. Because of this, they are non-social and use only a few communicatory behaviors. However, they do have frequent contact with neighbors and have developed ritualized displays for when the intruders get too close to their nest. In 1993, Bretagnolle and Thibault cataloged the meaning and function of eleven visual displays which help interpret Osprey behavior.
The majority of Osprey competitors are intruders or floating looking for nests. Mates communicate with these rivals through antagonistic behaviors such as nest protection and defensive postures. Ospreys use gradated responses, adjusting the intensity of the alarm displays and calls according to the threat. Most alarms begin at a low intensity, becoming more forceful as the threat persists and approaches the nest. The highest alarms are given when the intruder violates the nest territory and attempts to land on the nest.
Female Juvenile Osprey mantling in a semi-upright defensive posture, courtesy of Flicker.
Ospreys respond individually to territory intrusions with some being more tolerant of others. Nest sites are a key limiting factor, so nest defense is critical and often intense. In dense colonies, aggression must be suppressed to keep the peace. It is thought that individuals recognize neighbors and tolerate them more than non-neighbors. There also seems to be sexual dimorphism in response to nest intrusions as females are most aggressive to other females and males less aggressive to females.
Visual Displays
Resting posture (Figures 1a and b): Adults show subtle but detectable sexual dimorphism. Males rest sitting more upright than females with their wings partly open. Females pose more horizontally.
Upright posture (Figures 1d and e): An alert posture is more upright than a resting posture. The neck is extended, and the wings are held slightly open. In a highly motivated version of this display, males erect their head 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): An 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.
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 courtesy of Bretagnolle and Thibault 1993.
Solicitation (Figures 2a and b): High and low solicitation or begging calls are used by females and older chicks to obtain food from adult males during the breeding season. The proximate cause is usually to obtain food. During courtship, it reinforces the pair's bond. For this display, the body axis is horizontal, the crest feathers are 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 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. This call is often accompanied by mantling her wings over the nest in defense.
Mantling over the 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.
Ursula screamed in alarm, mantling the nest, and spreading her wings and tail defensively at the intruder.
Figure 2. Visual displays of Ospreys. Arrows indicate important field marks (neck position, wing position, etc.) that distinguish different behaviors, courtesy of Bretagnolle and Thibault 1993.
Males defend their mates and nest from intruding Ospreys throughout the breeding season and are especially diligent early in the mating period. Vigorous aerial chases often ensue. If an intruding Osprey approaches a nest too closely, the defender may try to strike the intruder in the air with its talons, driving it away from the nest site. Ospreys are probably less threatened by neighbors and use Guard Calls as an aid in recognizing individuals.
Nest-protection displays, given exclusively by females and directed toward flying intruders, are an extension of a more intense defensive display, characterized by wing-shaking, tail fanning, and holding the body horizontally. 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 protection is highly antagonistic and often is followed by a direct attack if the intruder lands at the nest. When the intruder leaves, nest protection is downgraded to a simple defense or protection display.
An intruder threatening a nest is typically met with a screaming female mantling the nest and spreading her wings defensively (see photo above.) As the intruder approaches, it is met with her talons ready to strike. Males, if nearby, respond to intruders by alighting on the nest and then chasing the intruders away.
Sky dancing is a highly conspicuous raptor courting display and is the main mating behavior 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 is usually performed over the nest during the courtship period and early in incubation. In this dramatic sky dance or fish flight, the male gives Screaming Calls as he dangles a fish or nesting material in his long legs. Starting high overhead (up to 300 yards or more), the male plunges 30–60 feet headfirst before rising steeply at the end. The male repeats his dive over and over until he reaches the nest up to 10 minutes later.
Male diving to impress female during the Osprey sky dance, courtesy of exit63.
Osprey communicative behaviors are frequently sexually dimorphic with differences in the number, form, and structure of their displays (Figure 3). The female stays in the nest for more than five months and must “beg” or solicit food from its mate throughout the day. The male provides all the food for the female and young until their departure. Since the male is away from the nest much of the time fishing, nest defense against intruders and predators is left primarily to the female. Since male Ospreys have limited time to defend their nests and mates, it is speculated that they advertise their nest ownership by performing conspicuous displays, such as the sky dance that can be seen for miles around, and flashy antagonistic defensive displays and chases against nest intruders.
Figure 3. Visual displays are performed by males and females. Arrow thickness indicates the probability of the event happening Bretagnolle and Thibault 1993).
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 indicating 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 the body, thereby reducing this contrast in both the neck and back. Conversely, males appear whiter on their backs when perched. In fledglings, 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 mimics 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 (same species) and predatory heterospecifics (different species) such as bald eagles. 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
EYES TO THE SKY!
WATCH!
READ!
On Osprey Time: Ospreys of Salt Point
VISIT!
HELP PROTECT OSPREYS:
Eggs will be hatching soon. Stay 300 feet away from nesting sites during the breeding season. If the Osprey vocalizes, you are too close! Back off Immediately.
Carry binoculars to view wildlife from afar.
Help keep local waters clean, healthy, and safe.
Recycle used fishing lines, twine, and nets, which can kill Osprey.
Join the Cayuga Osprey Network and volunteer to help monitor Osprey nests: cec222@gmail.com.
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