Abstract
Elegant ternsThalasseus elegansbreed in a very limited area of the northern Gulf of California and the Pacific coast of southern California, with up to 95% (mean 78%, 1991–2014,Perez et al., 2020) of the population nesting on Isla Rasa in the northern Gulf of California. On Isla Rasa, the primary nesting colony, elegant terns suffered predation by rodents which raised the possibility of population extinction, with a substantial proportion of the world population nesting on this single island. Because of this threat, rodents were successfully removed from Isla Rasa in 1995. The removal of rodents from Isla Rasa led to a near immediate increase in the population of elegant terns. That increase was associated with a changing pattern in dispersal by the terns, including extraordinary movements to the Gulf of Mexico, the Atlantic coast of the United States north to Massachusetts, and, remarkably, to western Europe. A few elegant terns successfully bred at these European localities during 2009 to the present. In this paper we use this exceptional example of long-distance dispersal to illustrate how rapid population growth during ∼ 1995 to present can lead to successful colonization of remote sites through repeated instances of vagrancy. We tested four Hypotheses that together support the idea that the growing population of elegant terns has produced increasing numbers of young, and these young have spread, through the mechanism of vagrancy, to the Pacific Northwest, the east coast of the United States, and western Europe. Our Hypotheses are: (1) The nesting population of elegant terns within their core nesting range has increased since removal of rodents from Isla Rasa; (2) Occurrence of vagrant elegant terns in the Pacific Northwest is driven by population growth within the core breeding range. (3) Occurrence of vagrant elegant terns at the east coast of the United States is driven by population growth within the core breeding range. (4) Occurrence and colonization of western Europe by elegant terns is driven by nesting population size within the core breeding range. Corollaries of these Hypotheses are, (i) that there is a time lag in occurrence of vagrants at each of these areas, based on increasing distance from the core breeding range and (ii) the number of vagrants in any given year is also related to sea surface temperature (SST), as expressed by Oceanic Niño Index, a proxy for food resource levels. Generally we found strong statistical support for each of these Hypotheses; an exception was for the occurrence of elegant terns in the Pacific Northwest, which initially occurred following El Niño events (low food supply) and profound breeding failure, but later corresponding to cold water years with high breeding success. We use elegant terns, exceptional for the highly restricted breeding range and sustained population growth over 25 years, to illustrate how growing populations may colonize very distant habitats through repeated instances of vagrancy.
Highlights
Populations of birds, especially but not exclusively migratory ones, are individually variable in their choice of migratory orientation, despite having a mean orientation (Gwinner and Wiltschko, 1978; Phillips, 2000), and distances dispersed by birds typically follow a leptokurtic probability distribution, so that a small but substantial fraction travel much farther than the mean distance dispersed (Moore and Dolbeer, 1989; van den Bosch et al, 1992; Veit and Lewis, 1996; Veit, 1997; Lewis et al, 2016)
What we show using the example of elegant terns is that long distance dispersal sometimes does result in colonization of very distant locations, and this lends support for a hypothesis that vagrancy can be selected for and is not an aberration attributable to navigationally incompetent birds (DeSante, 1983; Howell et al, 2014)
Whether a particular species is able to survive a changing climate must depend, at least in part, on the ability to find and colonize new locations. While this basic process has obviously occurred in the past, as all or most of the faunas of oceanic islands are the result of colonization (Lovette et al, 1999), we do not know very much about whether birds can achieve this rate of colonization at the rate at which the earth’s climate is warming
Summary
Populations of birds, especially but not exclusively migratory ones, are individually variable in their choice of migratory orientation, despite having a mean orientation (Gwinner and Wiltschko, 1978; Phillips, 2000), and distances dispersed by birds typically follow a leptokurtic probability distribution, so that a small but substantial fraction travel much farther than the mean distance dispersed (Moore and Dolbeer, 1989; van den Bosch et al, 1992; Veit and Lewis, 1996; Veit, 1997; Lewis et al, 2016) These characteristics are generally true of movement by birds, regardless of whether the movement consists of “migration,” “dispersal,” or vagrancy or some combination of these. Seabirds often do not breed until they are much older (up to ∼ 10 years in albatrosses, for example), leaving open the possibility for dispersal over a longer time, perhaps 3 years for elegant terns (Burness et al, 2020)
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