United States Department of Agriculture Adaptations of an Avian Forest Service Supertramp: Distribution, International Institute of Tropical Forestry Ecology, and Life History of General Technical Report the Pearly-Eyed Thrasher IITF-GTR-27 August 2006 (Margarops fuscatus) Wayne J. Arendt The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of multiple use management of the Nation’s forest resources for sustained yields of wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with the States and private forest owners, and management of the National Forests and National Grasslands, it strives—as directed by Congress—to provide increasingly greater service to a growing Nation. The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or part of an individual’s income is derived from any public assistance program. (Not all prohibited bases apply to all pro- grams.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, 1400 Independence Avenue, SW, Washington, DC 20250-9410 or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal opportunity provider and employer. Author Wayne J. Arendt is a research wildlife biologist, USDA Forest Service, International Institute of Tropical Forestry, Sabana Research Field Station, HC 2 Box 6205, Luquillo, PR 00773. Cover photograph: Male pearly-eyed thrasher (Margarops fuscatus) singing from its favorite song perch in an avocado tree. Photo taken by Jerry Bauer in May 2006 from the second-story balcony of the author’s home in Pitahaya Ward in northeastern Puerto Rico. Abstract Arendt, Wayne J. 2006. Adaptations of an avian supertramp: distribution, ecology, and life history of the pearly-eyed thrasher (Margarops fuscatus). Gen. Tech. Rep. 27. San Juan, PR: U.S. Department of Agriculture, Forest Service, International Institute of Tropical Forestry. 404 p. The pearly-eyed thrasher is a major nest predator and competitor for nest sites of the endangered Puerto Rican Parrot. In all aspects of its distribution and ecology, Margarops fuscatus is a classic example of an avian “supertramp.” It is a pugnacious, highly vagile species, i.e., a good disperser, with a propensity to fill vacant or underexploited niches at all elevations by adopting generalized nesting and foraging strategies, as well as a varied diet. Having evolved superior coloniz- ing traits at the expense of strong competitive characters, it is confined to about 80 generally small, often disturbed, species-poor islands and habitats throughout the Caribbean. The pearly-eye’s future depends on its ability to adapt to the ever- changing conditions in the region’s natural and anthropogenic environments. Keywords: Amazona vittata, life history, Margarops fuscatus, pearly-eyed thrasher, Puerto Rican parrot, supertramp. Preface This volume attempts to explain the current distribution of West Indian birds from an ecological perspective rather than from a physical or geological stand- point. Its primary objective is to present to the reader, for the first time, indepth information on the life history of a unique and widely distributed Caribbean mimid, one which exemplifies many of the ecological and reproductive traits recognized more than three decades ago by the human physiologist and avian ecologist Dr. Jared M. Diamond of the University of California in Los Angeles. Contents 1 Chapter 1: Introduction 1 Study Area and Methods 3 Nest-Box Study 6 Natural Habitat Disturbances 9 The Effects of Dipteran Ectoparasitism 18 Supplemental Nest Sites and Thrasher Densities 20 Effects of Artificial Nest Sites (Boxes) on Study Results 22 Lack of Experimentation in This Study 23 Banding 23 Statistical Analyses and Graphics Software Programs 27 Chapter-Specific Information 34 Distribution of West Indian Landbirds 34 Subjectivity of West Indian Avifaunal Analyses 35 Chapter 2: Supertramps in the West Indian Avifauna 35 What Is an Avian Supertramp? 36 Incidence Functions 36 Intertaxonomic Analogues of Avian Tramps 37 Species-Area Curves and Incidence Functions of West Indian Birds 37 Comparison of Pacific and Caribbean Island Landbirds 41 Species-Specific Incidence Functions 42 Competitors Versus Colonizers Among Taxonomic Groups 48 The Pearly-Eyed Thrasher as a Classic Supertramp 50 Tramp Species Research 50 Summary: Supertramps in the West Indian Avifauna 51 Chapter 3: Origins, Classification, and Taxonomy 51 Colonization and Dispersal of West Indian Fauna 52 History of Classification, Affiliations, and Distribution of the Mimidae 53 Origin of the Genus Margarops 55 Taxonomy of the Genus Margarops 55 Pearly-Eyed Thrasher 57 Scaly-Breasted Thrasher 61 Summary: Origins, Classification, and Taxonomy 63 Chapter 4: Distribution and Abundance 63 Current Range 64 Historical Range 66 Range Contractions 67 Range Expansions 67 Bahamian Archipelago 68 Puerto Rico 69 Montserrat 69 Bonaire 73 History of Regional Abundance 73 Virgin Islands 74 Sombrero Island and Anguilla 74 Netherlands Antilles 75 St. Kitts and Nevis 75 Need for More Standardized Census Methods 75 Comparisons of Regional Abundance 75 Mist Netting 78 Variable-Width Line Transects 80 Point Counts 83 Density Compensation in Mona Island Birds 89 Extreme Temporal Fluctuations in Mona Island’s Thrasher Population 89 Seasonal Fluctuations in Mona Island’s Thrasher Population 91 Environmental Factors Influencing Range and Abundance 91 Climate 92 Habitat Destruction 93 Hunting 94 Ecological Correlates of Range and Abundance 94 Evaluation of Ecological Correlates Via Categorical Modeling 95 Island Size 95 Number of Species 98 Geographical Distribution of the Red-Legged Thrush 100 Island Size 101 Number of Species 101 Thrasher-Thrush Distribution Analysis 103 Competition Within West Indian Bird Communities 104 Summary: Distribution and Abundance 107 Chapter 5: Morphology 107 Statistical Analyses 108 Sources of Intraspecific Variation in Avian Body Size 108 Geographic Effects 110 Latitudinal Effects (Museum Specimens) 111 Landmass and Elevational Effects (Field Measurements) 112 Climatic Effects 114 Effects of Dispersal on Local Body Size 114 Effects of Investigator Bias, Genes, Age, Gender, and Season 124 Extent of Regional Sexual Dimorphism 132 Terrestrial Vs. Arboreal Adaptations (Museum Specimens) 134 Sources of Interspecific Variation in Avian Body Size 134 Effects of Competition on Body Size 135 Potential for Intrageneric Competition 136 Combination of Uni- and Multivariate Analyses (Museum Specimens) 139 Combination of Uni- and Multivariate Analyses (Field Measurements) 140 Intergeneric Competition 143 Conclusion 143 Summary: Morphology 145 Chapter 6: Survival and Dispersal 145 Annual Survival Rates (Rain-Forest Population) 146 Effects of a Major Habitat Disturbance on Adult Annual Survival 148 Effects of Dipteran Ectoparasitism on Adult Survival 151 Annual Survival Rates (Dry-Forest Population) 152 Annual Survival of North Temperate Vs. Tropical Birds 152 Pearly-Eyed Thrasher Longevity 152 Rain-Forest Population 153 Dry-Forest Population 154 Pearly-Eyed Thrasher Longevity Compared to Other Landbirds 156 Dispersal 158 Short-Range Natal Dispersal Within the Sierra de Luquillo 164 Factors Affecting Natal Dispersers Within the Sierra de Luquillo 171 Short-Range Natal Dispersal Within Puerto Rico 171 Long-Range Natal Dispersal Among Islands 172 Homing Experiments on Adult Males 174 Conclusions 175 Summary: Survival and Dispersal 177 Chapter 7: Reproduction 177 Return Rates of Resident Breeders 180 Nest-Site Persistence and Mate Fidelity 182 Pair Bonding and Survival Rates 182 Number of Mates 183 Courtship Behavior 184 Cuckoldry and Egg “Dumping” 185 Comparison of Survival, Nest-Site, and Mate Fidelity With Other Species 186 Reproductive Dispersal 187 Males Changing Nest Boxes 190 Females Changing Nest Boxes 190 Prehurricane Disturbance Nest-Box Changes 190 Propensity for Posthurricane Nest-Box Changes 193 Interseasonal Nest-Box Changes 193 Early-Season Final Nestings in Original Boxes 193 Late-Season Final Nestings in Original Boxes 194 Interseasonal Reproductive Success of First Nestings in Postmove Boxes 195 Intraseasonal Nest-Box Changes 195 Intraseasonal Reproductive Success of First Nestings in Postmove Boxes 196 Phenology of Reproduction 196 Length of Breeding Seasons 199 Length of Nonbreeding Seasons 199 Incubation Period in Relation to Adult Body Mass 200 Number and Timing of Clutches and Egg Deposition Dates 209 Age of Female Breeders and the Onset of Egg Laying 210 Reproductive Recycling 210 Comparison of Clutch Types 212 Temporal Aspects of Recycling (Clutch to Clutch) 214 Temporal Aspects of Recycling (Interclutch Recovery Period) 219 Extremities in Interclutch Recovery Periods 221 Females Recycling in Fewer Than Two Weeks 223 Females Recycling in Less Than a Week 223 Annual Reproductive Success and Influential Factors 224 Reproductive Yield 229 Egg Mortality 232 Nestling Mortality and Number of Fledglings 235 Egg Deposition Strategy and Reproductive Response to Hurricane Hugo 238 Lifetime Reproductive Success