The biogeography of Striated Caracaras Phalcoboenus australis

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2006-05

Authors

Meiburg, Jonathan A.

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Abstract

This thesis defines and offers explanations for the distribution, range, and behavior of Striated Caracaras, coastal raptors of far southern South America. Like other caracara species, Striated Caracaras are relatively social, nest in dense aggregations where food is abundant, and feed largely by scavenging and opportunism. Unlike other caracaras, however, their distribution and range are severely restricted; they live only on the outermost islands of the Falklands/Malvinas archipelago and the Fuego-Patagonian islands of southernmost Argentina and Chile. Striated Caracaras' morphological differences from their closest relatives in the genus Phalcoboenus (which inhabit alpine habitats in the Andes) suggest a period of isolation from their sister species, resulting in speciation by vicariance. A possible mechanism for this isolation is the Patagonian Ice Sheet, which spread over much of southern South America during glacial epochs. During these periods, the sub-Antarctic Falkland archipelago and islands to the south and east of Tierra del Fuego appear to have remained largely ice-free, and probably acted as glacial refuges for colonial seabirds and pinnipeds, as well as for the ancestors of Striated Caracaras. On these islands, Striated Caracaras developed a dependence on the food resources that seabird and pinniped colonies provide, and a preference for their habitat - seacoasts fringed with the giant grass Paradiochloa flabellata. In these coastal environments, selection probably favored the tools that Striated Caracaras use today to exploit the seabird colonies' resources. These tools include large size (to handle large prey species), strong talons and bills, ease of movement on the ground (both for foraging in seabird colonies and precise mobility in near-constant strong winds), and good night vision (to capture burrowing petrels as they return to colonies at night). The caracaras' curious, opportunistic nature (to which they are ancestrally disposed) would have been preserved and perhaps enhanced, due to the necessity of investigating any potential resource during winter months when seabird colonies are vacant. A philopatric tendency might also have developed, as outside of seabird colonies food is scarce and chances of breeding are slim. Isolation in the islands' relatively simple ecosystems probably had another evolutionary effect, typical among island species - it deprived caracaras of defensive adaptations they might have once possessed, including wariness toward potential predators and nesting in inaccessible locations. The result was "ecological naivete", a phenomenon in which species that evolve in simple ecosystems lose (or simply lack) behavioral and morphological traits necessary for survival in more complex environments. Striated Caracaras demonstrated this naivete in encounters with humans in the 19th and 20th centuries, whom the birds approached without fear and by whom they were heavily persecuted. Even after decades of persecution by farmers in the Falklands, Striated Caracaras "had not learnt that man is dangerous" (J Hamilton 1922). They remain so today. A tourist guidebook refers to the birds as "charmingly tame"; islanders are more likely to call them "cheeky" but are increasingly tolerant of them, as wildlife tourism has become a major source of income. The caracaras' ecological naivete also probably restricts them from the more complex South American mainland, where mammalian predators (including humans) and other caracara species are common and food sources are not as concentrated or dependable as the coastal seabirds and pinnipeds. Thus, Striated Caracaras' preference for a habitat and resource set to which they are well-adapted, combined with the loss of defensive behaviors that might protect them from the hazards of the mainland, may leave them essentially "trapped" within their current range. Present-day threats to Striated Caracaras include habitat destruction due to exotic predators and browsers, threats from fisheries and global climate change to the seabirds and marine ecosystems on which the caracaras depend, and the intrinsic genetic pressures of the caracaras' small population size.

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