Origin, paleoecology, and extirpation of bluebirds and in the Bahamas across the last glacial–interglacial transition

David W. Steadmana and Janet Franklinb,c,1

aFlorida Museum of Natural History, University of Florida, Gainesville, FL 32611; bSchool of Geographical Sciences and Urban Planning, Arizona State University, Tempe, AZ 85287; and cDepartment of Botany and Plant Sciences, University of California, Riverside, CA 92521

Contributed by Janet Franklin, July 19, 2017 (sent for review May 9, 2017; reviewed by Helen F. James and Joseph M. Wunderle) On low islands or island groups such as the Bahamas, surrounded Sink are the only evidence that either of these species ever re- by shallow oceans, Quaternary glacial–interglacial changes in cli- sided in the Bahamas. We chose these species to compare the mate and sea level had major effects on terrestrial plant and an- paleobiology of two extirpated , one restricted today to imal communities. We examine the paleoecology of two species of continents and the other to an island. Breeding populations of songbirds (Passeriformes) recorded as Late Pleistocene fossils on S. sialis are confined today to continental North and Central the Bahamian island of Abaco—the Eastern bluebird (Sialia sialis) America (Fig. 2). (The population on Bermuda arrived there and Hispaniolan (Loxia megaplaga). Each species lives to- within the past 400 y, with human assistance) (14). Vagrant day only outside of the Bahamian Archipelago, with S. sialis occur- (nonbreeding) records of S. sialis exist from the Bahamas ring in North and Central America and L. megaplaga endemic to (Harbour Island off Eleuthera) (15), Cuba, and St. John in the Hispaniola. Unrecorded in the Holocene fossil record of Abaco, Virgin Islands (16). Loxia megaplaga is endemic to the island of both of these species probably colonized Abaco during the last Hispaniola (Haiti + Dominican Republic) with a possible va- glacial interval but were eliminated when the island became much smaller, warmer, wetter, and more isolated during the last glacial– grant record from Jamaica (16, 17). The two other New World interglacial transition from ∼15 to 9 ka. Today’s warming temper- species of Loxia are continental (Fig. 3 and SI Appendix, Fig. S1). atures and rising sea levels, although not as great in magnitude as Modern populations of any species of Sialia or Loxia are either – those that took place from ∼15 to 9 ka, are occurring rapidly and residents or short-distance diurnal migrants wanderers. Our two may contribute to considerable biotic change on islands by acting focal species differ substantially in their feeding and nesting in synergy with direct human impacts. ecology. The Eastern bluebird occupies open-canopy woodlands with grassy understories, where they are sit-and-wait foragers Bahamas | bluebird | crossbill | extirpation | island biogeography pursuing insects and fruits, primarily on the ground (18–20). Eastern bluebirds are obligate cavity nesters. The Hispaniolan ost of the documented late Quaternary extirpation of in- crossbill, which builds cup nests, requires pine trees (Pinus Msular and other vertebrates took place during the occidentalis) for both foraging and nesting (21, 22). This crossbill Holocene after the arrival of humans (1). While this statement subsists on seeds of the pine tree P. occidentalis (also endemic to holds true on West Indian islands as much as on any other set of Hispaniola), extracting the seeds from cones with its crossed islands (2–4), an ice age (Pleistocene glacial interval; >9 ka, but rostrum and mandible (23). The density of pine trees in the not precisely dated) vertebrate fauna from Sawmill Sink, a blue Hispaniolan crossbill’s montane habitat can vary from a nearly hole on Abaco Island in the northern Bahamas, featured closed-canopy forest to an open woodl