Life history of the Common Potoo
Skutch, A.F. 1970. Life history of the Common Potoo. Living Bird, 9: 265–280.
By day, the nocturnal Common Potoo (Nyctibius griseus) often rests on the end of a stub, from which it is difficult to distinguish in its thin, vertically elongated alarm posture. When at ease, it assumes a shorter, stouter figure.
Its food consists of moths, beetles (including fireflies), and other insects, caught in its great mouth on flycatcher-like sallies from an exposed perch, to which it often returns.
The song, consisting of a descending series of soft, plaintive notes, is heard in the evening and morning twilight, and by moonlight, chiefly in the drier part of the year.
At the beginning of December, in the Valley of El General in Costa Rica, a single whitish egg was laid in a knothole 30 feet up on a stout upright branch of an open-crowned tree growing on an exposed hillside. The egg rested in a depression so shallow that the greater part was exposed above the rim.
Both parents incubated, covering the egg continuously except for an interval not exceeding 15 min at daybreak and a longer interval of 45–95 minutes at the beginning of the night. The only observed change-over occurred at dawn, and both the long nocturnal session and the even longer diurnal session appeared to be continuous, and by different parents. They apparently never turned nor touched with their bill the precariously situated egg. The incubation period was at least 33 d.
The newly hatched potoo was densely covered with whitish down. From the first, it rested facing the supporting branch, just as the parents did when they incubated or brooded. At the age of about 17 d the young bird assumed, imperfectly, the vertically elongated alarm posture, but it did not display the full alarm posture until some days later. At the age of 26 d, it was first seen resting on the branch above its “nest”, and thereafter its excursions over the neighboring boughs gradually became longer. When 35 d old, it was well feathered, much in the color pattern of the adults, but in a lighter tone.
During its first 2 wk, the parents brooded the nestling most of the time, leaving it alone chiefly during the first hour of the night and more briefly at dawn. The brooding parents changed over 5 times in the course of a night. Nocturnal brooding ceased when the young potoo was 19 d old; but, with rare exceptions, a parent attended it all day until it was 25 d old, after which it was always alone, except when being fed.
The nestling was fed by regurgitation. Its first meal lasted about 5 min, but gradually the time devoted to transferring food shortened, until, during the second half of the nestling period, the parent took only an instant to deliver what seemed to be a large mass of compressed insects. When 10 d old, the nestling was fed 15 times, by both parents, between nightfall and daybreak on a night when the moon was full. When 35 d old, it received 10 meals in the course of a night when the moon was waxing. From about its eleventh day onward, the nestling always uttered a hoarse buzz when a parent came to feed it.
The severance of the nestling’s ties with the nest was gradual. During its last days in the nest tree, it climbed over neighboring limbs, stretched and flapped its wings, flew from branch to branch, and even took short circling flights out from the nest tree and back again. These exercises took place chiefly in the evening twilight, when, before its first meal, it frequently plucked and seemed to swallow lichens, liverworts, and fragments of bark. The nestling’s first recorded flight of a few yards occurred when it was 47 d old. Before sunrise on its 51st day, it finally flew from the nest tree to the neighboring woods and was not seen again.
From the laying of the egg to the young potoo’s departure, at least 84 d elapsed. The survival of the exposed “nest” for so many days convincingly attests the protective value of the potoo’s dead-stub pose. While incubating or brooding undisturbed, the parent sat in the rotund contracted posture; but a man approaching, a large bird flying overhead by day, or a small mammal prowling beneath the nest in the night, made it stretch up into the slim alarm posture. After the disturbance had passed, the resumption of the contracted posture took as long as a quarter of an hour in bright sunshine, but only two or three minutes in the evening twilight. Although the movement of elongation was much more rapid than that of contraction, it was nevertheless imperceptibly slow.