Erect Posture of the Young Black-billed Cuckoo: an Adaptation for Early Mobility in a Nomadic Species

The vertical or perpendicular pose of the young Black-billed Cuckoo (Coccyzus erythropthalmus) has been suggested to function in concealing the young, implicitly from predators (e.g.Herrick 1910, Bent 1940).F. H. Kennard (in Bent 1940: 75-76) gives the following description of a young Black-billed Cuckoo in this pose (see also Fig. 1): "When I parted the branches a trifle, and finding out that he was discovered, he promptly assumed an almost perpendicular position, with his neck stretched out almost unbelievably and his bill almost straight in the air; and there he sat, immovable, with his bill in the air like a bittern, only oscillating a trifle when the branch on which he was sitting was disturbed a little by the breeze."It should be added that the bird's eyes remain open (Fig. 1).This pose differs from the "hunting and alert (to man) posture," which Hamilton and Hamilton (1965: 413) described in adult Yellow-billed Cuckoos (C.americanus).In this posture the adult's body is not held perpendicularly and the bill does not point upward.The importance of the erect posture that young cuckoos assume is discussed in the present paper in relation to the mode of post-hatching development exhibited by the genus Coccyzus.I twice observed, in 1977, young Black-billed Cuckoos assuming an erect posture on the forested dune ridge (described by Sealy 1980a, MacKenzie 1982) that separates Lake Manitoba and Delta Marsh, Manitoba.One observation, on 11 June, was of a banded, flightless young about 2 m from the nest it had left 3 days earlier.The second observation was of an approximately 6-week-old young, which could fly, that I mistnetted on 23 July.I banded this young and returned it to willows near the net site.The bird abruptly assumed an erect posture (Fig. 1), which it held until I had backed away more than 10 m from it.It assumed the pose again, this time gradually, when I moved slowly to within 2 m of it.I repeated this sequence 3 more times before the bird flew away.Each time that the bird stood erect, it remained motionless and was concealed.When it relaxed, although somewhat concealed by foliage, it assumed a pose similar to the "alert" posture of adult Yellow-billed Cuckoos (Hamilton and Hamilton 1965).In 1979 a single 9day-old cuckoo gradually assumed an erect posture, with its eyes open, as it stood on the edge of its nest before departing later in the day.It responded similarly that day to my earlier approach.This observation implies that this behavior develops before the young leave the nest.Do young cuckoos conceal themselves when they assume this erect pose, i.e. do they visually deceive an observer (Hailman 1977), such as a predator?Curio (1975) pointed out that when a function is ascribed to a particular trait of an organism, e.g.concealment by posing erectly, implicitly the trait enhances fitness.If the erect posture of young cuckoos does indeed reduce their risk of predation, their fitness should be enhanced.Alcock (1979) pointed out that animals that seek concealment should avoid moving abruptly, because predators readily detect movement.The gradual assumption of the erect pose by young Black-billed Cuckoos suggests that it does function to conceal, although it might be expected that its eyes would be closed while adopting the pose, as they are in some adult owls (Bent 1938, Ligon 1968, Catling 1972, Bondrup-Nielsen 1983) and potoos (Borrero H. 1970, Perry 1979).Bondrup-Nielsen (1983) noted four features common to the so-called concealing poses described for several species of nocturnal owls: (1) the posture was upright, (2) the wing nearest the intruder was raised to the bill, (3) the body plumage was appressed, and (4) the lateral crown feathers were expanded.However, he considered that the open eyes, increased exposure of white feathers around the eyes, and the abrupt manner in which some poses were adopted, particularly by Aegolius owls, weaken the hypothesis that this pose functions to conceal the owl.James and Nash (1983) reported fledged but essentially flightless Northern Hawk-Owls (Surnia ulula) that assumed an upright posture, with their eyes open.These authors believed, as Bondrup-Nielsen did with Aegolius owls, that the widely open eyes of the hawk-owls suggested that this pose is not entirely concealing, but perhaps is a "fright" response to a disturbance.They compared this response to screech-owls (Otus spp.) that close their eyes to narrow slits when apparently concealing themselves (Walker 1974).Interestingly, James and Nash noted that the upright posture was assumed only by hawk-owls under one month of age, and not by the adults.They raised the possibility that this behavior has been lost among the adult hawkowls, which are largely diurnal in their habits, in contrast to the nocturnal owls that Bondrup-Nielsen (1983) discussed.James and Nash (1983) believed that the loss of such behavior might be expected if it served to conceal the bird, but that if it were only a fright reaction, the adults also might be expected to assume it.More observations of young cuckoos are needed to establish whether their dark eyes are always open when the erect posture is adopted.The juvenal and alternate plumages of the Black-billed Cuckoo are in a new population in Manitoba.Can.Field-

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.