Bermuda Petrel Pterodroma cahow
Version: 2.0 — Published November 16, 2020
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Authors who wrote about Bermuda Petrel in the early days of European exploration and colonization of the New World used much poetic license, but many accounts point to an abundant, highly vocal, and very confiding bird—and one easily captured. Little was published about the species after 1620 and before the species' rediscovery in 1951, but the few who did write about Bermuda Petrel (e.g., Lefroy 1877, 28, 29) had neither type specimen nor careful descriptions of plumage, voice, or behavior.
At sea, normally detected in flight; travels by sinusoidal arcing flight typical of small gadfly petrels, particularly during brisk winds (> 10 kts).
Swimming and Diving
When not feeding, foraging, or flying, birds often rest on water; on rare occasions, individuals have been seen resting with other small gadfly petrel species. Birds waiting offshore of breeding islands occasionally gather in small rafts (2-6 individuals); these are thought to be nonbreeding subadults.
Nests in burrows in loose colonies; territorial behavior not observed.
Mating System and Operational Sex Ratio
Monogamous but will re-partner in the event of loss of mate or difficulties in reproducing.
Courtship, Copulation, and Pair Bond
Courtship in adults is similar to other gadfly petrels: the male and female duet in flight, calling (often antiphonally) while one pursues the other closely. Typically the male selects and prepares the nest site and entices the female to inspect the site with him. Behavior at the nest is widely observed through public "nest-cams," broadcast via internet, and also by conservation officer and staff through tops of artificial burrows. The pair engages in some allopreening when both are at the nest. The female is also fed by male when preparing to lay an egg.
Social and Interspecific Behavior
Degree of Sociality
Normally solitary at sea, but sometimes small rafts have been observed resting on the water offshore of breeding islands. The birds in these small rafts are thought to be nonbreeding subadults.
Nonpredatory Interspecific Interactions
One of the chief threats to the survival of Bermuda Petrel in the twentieth century, and probably earlier, is its competition for nest sites with White-tailed Tropicbirds (Phaethon lepturus; 26, 30, 31, 32). In Bermuda, the tropicbird returns to nest in early spring, when Bermuda Petrel chicks are still several months away from fledging. When it was discovered that adult tropicbirds were killing and removing petrel chicks in order to utilize the nest site, biologists devised wooden baffles to cover the nest site entrance, with the smaller aperture preventing the slightly larger tropicbirds from accessing the nest sites. At sea, Captain J. B. P. Patteson has also observed apparently antagonistic interactions between White-tailed Tropicbird and small gadfly petrels, including Bermuda Petrels, on several occasions. Such interactions involved stooping and chasing in the air, but it is not clear whether attempted kleptoparasitism or some other behavior was involved (J. B. P. Patteson, in litt.). Tropicbirds and small gadfly petrels take similar prey species, so competition at sites where prey is concentrated may occur.
Kinds of Predators
On Bermuda, Peregrine Falcons (Falco peregrinus) is an annual visitor, mostly in autumn and early winter, and David B. Wingate (in litt.) has documented multiple instances of predation of Bermuda Petrels by Peregrines. In December 1987, a Snowy Owl (Bubo scandiacus) reached Bermuda and took 5 Bermuda Petrels before being collected by Wingate (the heads and wings of these birds are in the collection of the Bermuda Aquarium, Museum, and Zoo). At sea, South Polar Skua (Stercorarius maccormicki) have been observed pursuing and on at least two occasions eating Black-capped Petrel off North Carolina (33, J. B. P. Patteson in litt.); presumably, skuas also take smaller species of gadfly petrel as well.
Response to Predators
Presumably to avoid predation, adults return to their nest burrows only after dark. Subadults also return to breeding islets during hours of darkness, often vocalizing and practicing courtship flights.