High Nest Predation by Brown Tree Snakes on Guam

A great deal of attention has recently been focused on the drastic decline of the native avifauna of Guam since the 1960s (Ensbrina 1983: Jenkins 1983: Enebrine and Ramsey 1984,Savidge 1984, 1986, 1987).Pipulations of once common native forest birds have declined and are now thought to be locally extinct or restricted to the northernmost part of the island (Jenkins 1983; Engbring and Ramsey 1984; U.S. Fish and Wildlife Service, in press).Savidge (1986, 1987) investigated the role of disease and predation in the decline and concluded that the introduced brown tree snake (Boiga irregularis) was responsible for the extinction or decline of 10 species of native forest birds on Guam.My report provides direct evidence of the significant impact that snake predation is having on the nesting success and population dynamics ofthe Philippine Turtle-Dove (Streptopelia bitorquata), one of the few wild birds on Guam that remains sufficiently abundant for such a study.The Philippine Turtle-Dove was probably introduced to the Mariana Islands by the Spanish in the late 1700s (Baker 195 1).The species became one of the most abundant birds on Guam and was made a legal game bird (Baker 1947, 1951: Conrv 1987).More recently, however, numbers have declined in a pattern similar to that of the native forest birds, though not as severely, and the hunting season for the species has been closed (Engbring and Ramsey 1984, Conry 1987).The turtle-dove is presently uncommon to rare throughout southern Guam, common in localized areas of central and northern Guam, and abundant on Cocos Island, a small snake-free islet 2 km off southern Guam (Conry 1987).The turtle-dove also occurs on the neighboring southern Mariana Islands of Rota, Aguijan, Tinian, and Saipan (Pratt et al. 1987) where its numbers appear stable (T.K. Pratt, pers.comm.) and the brown tree snake does not occur.STUDY AREA AND METHODS Nesting success of the Philippine Turtle-Dove was studied in forest and urban habitats on Andersen Air 1 Received 25 April 1987.Final acceptance 28 October 1987.Force Base (AAFB), Guam, from February 1984 to October 1985 and in urban habitat only during May to Julv 1986.The forest studv site is located in the Northwest Field area of AAFB.Northwest Field is an abandoned World War II airfield now occasionally used for training purposes and recreational hunting.The area contains patches of secondary vegetation located inside a network of asphalt runways, taxiways, and roads.The airfield complex is surrounded by primary and secondary limestone forest and scrub forest.This patchy habitat offers a mosaic of grass, shrubs, and trees that provide ample nesting and feeding sites for turtle-doves.Engbring and Ramsey (1984) categorized the area as being 43% primary limestone forest, 50% scrub forest, and 6% urban.The urban study site is located in the interior of the present-day airfield complex on AAFB.Turtle-doves nest on the corrugated surface of steel-sided, earthenfilled revetments constructed to shelter parked aircraft.Most nesting occurs on one row of revetments in a 24ha area along the center taxiway of the airfield.Mowed grass, asphalt runways, and roads completely surround the revetments and the closest forest is 0.5 km away.Secondary and primary limestone forest and scrub forest surround the northern two-thirds of the airfield and the remainder of the general area is urban.Primary limestone forest has a dense canopy 10 to 20 m high and an understory of varying density.Common species include Ficus, Aglaia, Guamia, Cycas, Neisosperma, Elaeocarpus, Pisonia, Pandanus, Wikstroemia, and Triphasia.Secondary limestone forest has a shorter, broken canopy with a dense understory and contains many of the same species as primary forest plus Hibiscus, Leucaena, Casuarina, Cocos, Carica, and Morinda.Scrub forest has similar species composition as secondary forest but is shorter and more open with small grassy patches interspersed with scattered trees and shrubs.The vegetation types of the study areas are described in detail by Stone (1970) Jenkins (1983), and Engbring and Ramsey (1984).Turtle-doves build shallow, flimsy, platform nests made of small twigs.Nests are typically built in a variety of small trees and shrubs and average 1.8 m in height and 0.9 m distance from the center of the tree (Conry 1987).Nests in forest habitat were located by flushing nesting adults while walking or driving through forested areas, or by searching vegetation.Nests in urban habitat were located by driving the outside perimeter of the U-shaped revetments and observing birds nesting on the structures.All nests on revetments were