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Browsing by Author "Dwyer, James F."

Now showing 1 - 5 of 5
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    Correlation of Cere Color with Intra- and Interspecific Agonistic Interactions of Crested Caracaras
    Dwyer, James F. (Raptor Research Foundation, 2014)
    Bright coloration in birds is an important indicator of individual quality often used in social displays. Structural, carotenoid-, and melanin-based colors are long-lasting, widespread, and widely studied. Hemoglobin-based colors are ephemeral, rare, and less studied. Hemoglobin-based displays occur when an individual facultatively enhances or restricts blood flow through caruncles, combs, wattles, or other highly vascularized un-feathered skin patches. In Crested Caracaras (Caracara cheriway; hereafter "caracara") highly vascularized ceres facultatively undergo immediately reversible hemoglobin-based color changes, hypothesized to correlate with status during contests. I predicted aggressors in contests would consistently display hemoglobin-deprived ceres (hereafter "light"), and receivers would display hemoglobin-enhanced ceres (hereafter "dark"), or vice versa. To test this hypothesis, I conducted 149 30-min group observations during which I recorded outcomes of all observed intra- and interspecific agonistic interactions involving caracaras in groups including up to 46 caracaras (x = 13.4, SD = 6.9). I recorded 2586 agonistic interactions in which I could identify cere colors and ages of both caracaras involved in an intraspecific interaction (n = 1160), or of one caracara involved in an interspecific interaction (n = 1426). Cere colors of caracaras were consistently light when acting as aggressors in intra- and interspecific agonistic interactions, and dark when acting as receivers. Within age classes, caracaras displaying light-colored ceres were consistently aggressors toward caracaras displaying dark ceres, and between age classes, adults with light-colored ceres were aggressors toward younger birds with dark ceres. Caracaras displaying light-colored ceres were aggressors toward Black Vultures (Coragyps atratus) and Turkey Vultures (Cathartes aura) and caracaras with dark ceres were receivers of aggression from these species. Regardless of the cere color, caracaras were subordinate to much larger Bald Eagles (Haliaeetus leucocephalus) and dominant over much smaller American Crows (Corvus brachyrhynchos). My observations support the hypothesis that cere color is correlated with agonistic behaviors and support the signaling hypothesis by correlating specific cere colors displayed with individual roles in intra- and interspecific interactions.
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    Ecology Of Non-Breeding And Breeding Crested Caracaras (Caracara cheriway) In Florida
    Dwyer, James F. (Virginia Tech, 2010-05-06)
    Like many species, Florida's population of Northern Crested Caracaras (Caracara cheriway, hereafter "caracara") is likely declining due to loss of breeding habitat. Consequently, management-oriented restrictions on landscape modification are applied where breeding occurs, but management rarely is extended beyond breeding areas. Focusing management on breeding areas can be effective if all caracaras occupy breeding areas, all breeding areas are detected, and no intermittent breeding occurs. Management may not operate as intended if any of these criteria are unmet. To explore this possibility, I investigated the movement, habitat, survival, and social biology of non-breeding caracaras. I also investigated long-term occupancy of breeding habitat, and factors contributing to detection of breeding. Non-breeding caracaras occupy areas much larger than individual breeding territories, particularly during breeding seasons. Pastures occupied by cattle were the most used habitat, but non-breeding caracaras also occupied habitats atypical of breeding areas. Specifically, citrus groves were occupied extensively, and row crops were used particularly during breeding seasons. Non-breeding caracaras also shared communal roosts, sometimes with hundreds of conspecifics, and roosts were occupied year-round. Survival of non-breeding caracaras was lowest during breeding seasons. Adult non-breeding caracaras persisted in groups for multiple years without establishing breeding territories. This implies that breeding habitat is limited and saturated. Given the proportion of adults in groups, adults also were the first to find carrion more often than expected. Apparently, young caracaras benefit from grouping by following adults. I found caracaras at all sampled breeding areas where nests were originally documented during the 1990s, and found nests at 83% of territories where nests likely existed. I also found that observer experience, visit start time, and weather affected the probability that a nest would be detected. Thus, not all caracaras occupy breeding areas, and not all breeding attempts are likely to be detected. Long-term occupancy of breeding areas should render annual verification of nesting unnecessary as a trigger for maintaining management actions. Rather management should persist even without confirmation of annual breeding. Caracara management also may be optimized through supporting the non-breeding population by maintaining a matrix of cattle pasture and citrus groves, particularly around roosts.
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    Electric Shock Injuries in a Harris's Hawk Population
    Dwyer, James F. (Raptor Research Foundation, 2006)
    Electrocution may be an important agent of mortality in many raptor populations, and has been implicated as a contributing factor in the endangerment of some species. In Tucson, Arizona U.S.A. the electrocution of Harris's Hawks (Parabuteo unicinctus) was reported in both the 1980s and 1990s. The latter report also described Harris's Hawks that survived electric shock injuries. From February 2003-May 2004, I captured and examined wild Harris's Hawks in Tucson to investigate whether electric shock injuries might be present in individuals that appeared healthy from a distance. I trapped 85 birds; seven exhibited definite electric-shock injuries, and seven exhibited suspected electric-shock injuries. One individual exhibited injuries not consistent with electric shock. I captured an equal number of males and females, but only one of the injured birds was a male. Females were significantly more likely to have injuries. Whether this difference reflects patterns of initial incurrence of injury, or of higher probability of survival after electric shock remains unresolved. As medium- and large-bodied raptors colonize urban areas where overhead electric systems tend to occur in high densities, those species may also incur increased risk of electrocution.
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    Preventing Raptor Electrocutions in an Urban Environment
    Dwyer, James F.; Mannan, R. William (Raptor Research Foundation, 2007)
    Electrocution of raptors on poles supporting overhead electric lines is a cause of concern in the United States. Techniques for modifying (i.e., retrofitting) potentially lethal poles to reduce electrocutions have been applied in rural areas to poles most likely to be used by raptors. However, raptors also live in urban areas, and criteria for selecting poles to retrofit in towns and cities may differ from those in rural areas. We assessed the effectiveness of using nest sites of Harris's Hawks (Parabuteo unicinctus) in Tucson, Arizona, U.S.A., as a means to focus proactive efforts to retrofit potentially lethal poles. Specifically, we investigated: (1) whether poles close to Harris's Hawk nests were more likely to cause electrocutions than more distant poles with the same configuration; and (2) whether retrofitting poles within 300 m of nests reduced electrocutions in treated areas. We assessed the number of electrocutions before and after retrofitting by searching for electrocuted hawks at a sample of poles. In 2003, we found 23 electrocuted Harris's Hawks within 300 m of nests. The proportion of poles that electrocuted a juvenile Harris's Hawk remained relatively constant from 0 to 300 m from nests. Poles 201-300 m from nests were more likely to electrocute subadult and adult hawks than were poles <200 m from nests. Prior to retrofitting poles, we detected 1.4 electrocutions per monitored nest. After about half of the potentially lethal poles within 300 m of nests were retrofitted, we detected 0.2 electrocutions per nest. For Harris's Hawks in Tucson, risk of electrocution was at least partially related to the proximity of nests to potentially lethal poles. This relationship also may hold for other medium- to large-bodied raptors nesting in urban environments. We recommend that all potentially lethal poles within 300 m of the nests of urban-nesting raptors be retrofitted through the addition of insulation, or through increased spacing between conductors.
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    Return Rates of Aluminum Versus Plastic Leg Bands from Electrocuted Harris's Hawks (Parabuteo Unicinctus)
    Dwyer, James F.; Mannan, R. William (Raptor Research Foundation, 2009)
    Ecological research programs investigating dispersal, migration, habitat selection, social structure, survival, reproductive success, and population size depend on the ability to identify individual study animals on multiple occasions (Bibby et al. 2000). Leg bands often are used to facilitate the identification of individual birds and more than one million birds, including numerous raptors, are marked annually in North America with aluminum leg bands (USGS 2008). In the United States, aluminum is also used in overhead electric systems because of its strength, low resistance, long life, and relatively low cost (APLIC 2006).