Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops Fuscatus)

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus) Chapter 6: Survival and Dispersal The pearly-eyed thrasher has a wide geographical distribution, obtains regional and local abundance, and undergoes morphological plasticity on islands, especially at different elevations. It readily adapts to diverse habitats in noncompetitive situations. Its status as an avian supertramp becomes even more evident when one considers its proficiency in dispersing to and colonizing small, often sparsely The pearly-eye is a inhabited islands and disturbed habitats. long-lived species, Although rare in nature, an additional attribute of a supertramp would be a even for a tropical protracted lifetime once colonists become established. The pearly-eye possesses passerine. such an attribute. It is a long-lived species, even for a tropical passerine. This chapter treats adult thrasher survival, longevity, short- and long-range natal dispersal of the young, including the intrinsic and extrinsic characteristics of natal dispersers, and a comparison of the field techniques used in monitoring the spatiotemporal aspects of dispersal, e.g., observations, biotelemetry, and banding. Rounding out the chapter are some of the inherent and ecological factors influencing immature thrashers’ survival and dispersal, e.g., preferred habitat, diet, season, ectoparasites, and the effects of two major hurricanes, which resulted in food shortages following both disturbances. Annual Survival Rates (Rain-Forest Population) In the early 1990s, the tenet that tropical birds survive much longer than their north temperate counterparts, many of which are migratory, came into question (Karr et al. 1990). Whether or not the dogma can survive, however, awaits further empirical evidence from additional studies. To compare annual survival rates of pearly-eyed thrashers with other tropical and north temperate birds, 12 years (1979–90) of mark-recapture (and resight) data from my sampled rain-forest population were analyzed by using program Jolly (Brownie et al. 1986, Jolly 1965, Pollock et al. 1990). Program Jolly provides parameter estimates by using four different capture-recapture models, and at the time of the analyses was considered the best for minimizing the usual biases associated with estimating avian survival. Karr’s group and others (see also Faaborg and Arendt 1995, Johnston et al. 1997) used program Jolly in comparing survival rates of north temperate and tropical birds. The best fit of the thrasher data was generated by model A, the standard model for open populations assuming births, deaths, emigration, and immigration. Very small standard errors and coefficients of variation associated with the survival estimates phi (φ) substantiated that model A gave the best fit. Recently, more sophisticated survival models have been developed to analyze indepth life-history information, for example, survival differences between not only the sexes but also between breeders and nonbreeders (see, for example, 145 GENERAL TECHNICAL REPORT IITF-GTR-27 Sandercock et al. 2000). Unlike systematic mist-net generated survival rates (see following discussion of annual survival in a dry-forest thrasher population), my mark-recapture (and resight) study of a rain-forest thrasher population allowed me to look more closely at the concerns outlined in Sandercock et al. (2000) such as demography, transience, heterogeneous mortality rates between the sexes, reproductive performance, and breeding status (see also chapter 7). Three survival analyses were performed (fig. 6.1a): sexes combined (n = 213), males only (n = 91), and females only (n = 122). For combined sexes, the mean annual rate of local survival was φ = 0.85 (SE = + 0.04; range: 0.40 to 0.98) over a 12-year period. Except for 1990, the first posthurricane year in which survival dropped precipitously for both sexes (see below), the average annual survival for the resultant 11-year period jumped to φ = 0.90 for the combined sexes, mainly owing to the high rate of survival in males. Even with the inclusion of 1990, almost 90 percent of all males survived to the next year (mean = 89 percent; SE = + 0.05; range: 31 to 100 percent). In contrast, however, on average only 82 percent of all females survived to the following year (SE = + 0.04; range: 40 to 98 percent). The mean annual survival rate of males was significantly higher (M-W R S: T = 110; P = 0.02) than that of females because breeding females suffer higher annual mortality, mostly owing to owl predation by the Puerto Rican screech-owl (Megascops nudipes) and dipteran ectoparasitism involving philornid botflies (see Arendt 2000, Loye and Carroll 1995). Effects of a Major Habitat Disturbance on Adult Annual Survival Changes in the environment and adaptive responses to such changes both influence population processes and tend to vary the density and age structure of a population (Ricklefs 2000a). There is a rapidly growing body of information on avian population responses to catastrophic, climatological events (e.g., Faaborg and Arendt 1992, Faaborg et al. 1984, Knopf and Sedgwick 1987, Pagney Bénito- Espinal and Bénito-Espinal 1991, Walker et al. 1991, Wauer and Wunderle 1992, Wiley and Wunderle 1993, Wingfield 1988, Wunderle 1995, Wunderle et al. 1992). Population responses to environmental perturbations differ widely and are highly species specific. “In birds, as in other vertebrates, endocrine secretions regulate morphological, physiological, and behavioral changes in anticipation of future events” (Jacobs and Wingfield 2000, Ricklefs and Wikelski 2002). Proximate responses involve physiological changes within individuals, namely a marked endocrine stress response. Wingfield (1988) showed that stressful events such as severe storms induce an increase in corticosterone that mobilizes energy reserves to combat reduced food intake. Moreover, corticosterone redirects an individual’s 146 Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus) behavior away from reproduction and defense of territory toward survival and increased foraging. In this process, LH (luteinizing hormone) and testosterone are not affected. Thus, the gonad remains functional (or near functional) so that renesting can begin immediately after adverse weather conditions ameliorate. Physiological responses notwithstanding, in general most populations of frugivores (e.g., pigeons and doves) and frugivorous passerines exhibit latent responses to such major habitat disturbances. Many populations of frugivorous birds remain low even 2 to 4 years following disturbance (Knopf and Sedgwick 1987; see also discussion in chapter 8 summarizing forest bird point-count results as part of this research). During the course of this study, the thrasher population inhabiting the Sierra de Luquillo was subjected to two major environmental disturbances, namely Hurricane Hugo’s Hurricane Georges (September 21, 1998) a strong class 31 hurricane (discussed short-term impact on in chapter 7), and Hurricane Hugo, a class 4 hurricane. Hurricane Hugo struck the rain-forest thrasher Puerto Rico on September 18, 1989. Its short-term impact on the rain-forest population was severe. thrasher population was severe. Following the storm, adult annual survival rates plummeted from a 10-year, predisturbance mean for both sexes combined of 0.89 (SE = + 0.02; range: 0.79 to 0.98) to 0.42 (sexes combined: 0.31 for males and 0.52 for females) during the first postdisturbance breeding season (fig. 6.1a). The 10-year mean annual survival rate (fig. 6.1b) for males prior to the storm was 0.94 (SE = + 0.02; range: 0.82 to 1.00). For females it was 0.85 (SE = + 0.03; range: 0.66 to 0.96), a significant difference of 9 percent per annum z( = 3.55; P <0.001; SE of the difference = 0.02; power = 0.94). Similarly, there was a significant gender difference in survival as shown by a comparison of survival rates for the first 7 years following the hurricane (z = 4.05; P = <0.001; SE of the difference = 0.03; power = 0.98). Moreover, a more recent (August 1998) 20-year comparison of annual survival rates for males (mean = 0.87) and females (mean = 0.77) revealed a significant difference between the sexes z( = 4.26; P <0.001; SE of the difference = 0.02; power = 0.98). To quantify the effects of Hurricane Hugo and to further emphasize gender differences in survival potential, annual survival rates from 2 years before to 3 years following the storm were compared (table 6.1). Within each sex, pre- and postdisturbance rates of annual survival were similar. However, for both sexes, survival following the storm dropped significantly, more so in males. In the Saffir/Simpson hurricane scale, there are five classes of hurricanes of progressive intensity (from 1 to 5) based on wind velocity and other physical parameters (e.g., extent of human evacuations, damage to trees, buildings, etc.) in the final designation; for example, Class 3 hurricanes (e.g., Georges, a strong 3.8) produce winds of 180 to 209 km per hour, topple large trees, and cause minor damage to structures. Class 4 hurricanes (e.g., Hugo) produce winds of 210 to 250 km per hour, with extensive damage to forests and human habitations. 147 GENERAL TECHNICAL REPORT IITF-GTR-27 Figure 6.1—Annual survival of 213 adult pearly-eyes calculated by using mark-recapture Program Jolly’s (Brownie et al. 1986) model A for open populations (A). Hurricane Hugo (1989) greatly affected annual survival rates. The 12-year mean annual survival rate (B) of males (89 percent) was significantly higher than that of females (82 percent). In contrast, annual survival of males dropped lower than that of females as a result of the storm (A). Box-plot parameters are defined in fig. 4.6. 148 Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus) Table 6.1—Annual survival rates of pearly-eyed thrashers 2 years before and 3 years following major habitat destruction caused by Hurricane Hugo, a class 4 hurricanea Males Females Combined Breeding seasons n Percentb Pc n Percent P n Percent P 87/88 vs.

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