Breeding, Morphology, and Growth of the Endangered Dark-Rumped Petrel

Abstract

-Dark-rumped Petrels (Pterodroma phaeopygia phaeopygia) in the Galapagos archipelago breed on several islands. Life-history characteristics such as mate and site fidelity, large egg size, long incubation periods, long incubation bouts, and chick growth patterns are similar to those of other procellarids. Adults nest annually, but the timing of breeding is out-of-phase among the islands of Floreana, Santa Cruz, and Santiago. Banding data for 1,500 adults and juveniles indicated that little or no interbreeding occurs among islands. From adult morphological characteristics measured during five years on Floreana and one year on both Santiago and Santa Cruz, we found that on Floreana, adults were sexually monomorphic in all measured characteristics except bill height, and the adults from Floreana tended to be larger in both size and mass than adults from the other two islands. The possibility that each island's population is distinct should be considered when formulating management plans concerning this species. Received 10 May 1989, accepted 6 November 1989. THE DARK-RUMPED Petrel (Pterodroma phaeopygia phaeopygia) breeds on at least four of the islands in the Galapagos archipelago: Floreana, Santa Cruz, Santiago, and San Crist6bal. They adhere to a rigid, annual breeding schedule; most other Galapagos seabirds breed semiannually or opportunistically (Harris 1977, Valle et al. 1987). Adult petrels return repeatedly to the same island-specific colonies to breed, and they probably disperse over broad areas of the eastern Pacific Ocean between breeding attempts (Murphy 1936). The Dark-rumped Petrel is endangered at all of its breeding sites (King 1981). We studied aspects of petrel breeding biology including mate and site fidelity, egg size, and adult weight loss during incubation bouts. We also compared the breeding chronologies of colonies on Floreana, Santa Cruz, and Santiago; and we examined adult morphological characteristics and chick growth from all three islands. The breeding biology of the Dark-rumped Petrel on Santa Cruz was studied previously by Harris (1970). Tomkins (1985) surveyed the archipelago in 1978-1979 and suggested that breeding was not synchronized among islands. This was supported by Coulter et al. (1981, 1982 1 Present address of author to whom reprint requests should be sent: School of Forestry and Environmental Studies, Duke University, Durham, North Carolina 27706 USA. unpubl.) who found differences in the initiation of breeding among different colonies on Floreana. We studied the breeding biology of the petrel on Floreana as a corollary to a conservation effort that alleviated petrel mortality through control of introduced mammalian predators (Cruz and Cruz 1987a, b; J. B. Cruz and F. Cruz 1987). MATERIALS AND METHODS Study areas. -We collected data from three locations in the Galapagos National Park in 1981, and 19831986 (Fig. 1). The colony on Floreana Island (1?13'S, 90?22'W) was located in a dormant volcano (Cerro Pajas) at 300-640 m above sea level (asl). We sampled petrels from the most accessible and least fragile nests along the interior walls of the volcano to minimize disturbance (J. B. Cruz and F. Cruz 1987). On Santa Cruz Island (0?9'S, 90?21'W), we sampled nests in the Media Luna colonies and their environs at 600-800 m asl. In these colonies, petrels frequently dig their burrows into the soft soil of water gullies (Harris 1970). We also studied petrels in the highlands of Santiago Island (0?8'S, 90?32'W), where colonies were widely scattered. Petrels on this island nest in small groups located in an area of at least 35 km2. We monitored ca. 25 km2. We found most nests deep in caves and other hard-to-reach areas at 500-900 m asl. Field methods.-On Floreana we checked 43 nests (1981), 104 nests (1983), and 100 nests (1984, 1985, and 1986) from January through September. We assessed nests at least weekly, but more frequent checks were made in some years. We banded all adults with 7-mm diameter num317 The Auk 107: 317-326. April 1990 This content downloaded from 157.55.39.132 on Mon, 02 Jan 2017 17:57:26 UTC All use subject to http://about.jstor.org/terms 318 CRUZ AND CRUZ [Auk, Vol. 107