Landscape Movements of Migratory Birds and Bats Reveal an Expanded Scale of Stopover

et al. (2011) Landscape Movements of Migratory Birds and Bats Reveal an Expanded Scale of Stopover. PLoS ONE 6(11): e27054. doi:10.1371/journal.pone.0027054 Landscape Movements of Migratory Birds and Bats Reveal an Expanded Scale of Stopover Philip D. Taylor 0 Stuart A. Mackenzie 0 Bethany G. Thurber 0 Anna M. Calvert 0 Alex M. Mills 0 Liam P. McGuire 0 Christopher G. Guglielmo 0 Tapio Mappes, University of Jyvaskyla, Finland 0 1 Acadia University , Wolfville , Canada , 2 Bird Studies Canada , Port Rowan , Canada , 3 University of Western Ontario , London , Canada Many species of birds and bats undertake seasonal migrations between breeding and over-wintering sites. En-route, migrants alternate periods of flight with time spent at stopover - the time and space where individuals rest and refuel for subsequent flights. We assessed the spatial scale of movements made by migrants during stopover by using an array of automated telemetry receivers with multiple antennae to track the daily location of individuals over a geographic area ,20640 km. We tracked the movements of 322 individuals of seven migratory vertebrate species (5 passerines, 1 owl and 1 bat) during spring and fall migratory stopover on and adjacent to a large lake peninsula. Our results show that many individuals leaving their capture site relocate within the same landscape at some point during stopover, moving as much as 30 km distant from their site of initial capture. We show that many apparent nocturnal departures from stopover sites are not a resumption of migration in the strictest sense, but are instead relocations that represent continued stopover at a broader spatial scale. - Funding: Funding for the study was provided by grants from the National Sciences and Engineering Research Council (Discovery Grants to PDT, CGG; Scholarships to SAM, BGT, LPM), Environment Canada (PDT) and the Ontario Ministry of Natural Resources (CGG). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Billions of birds and bats undertake seasonal, nocturnal migrations between over-wintering and breeding grounds. During migration, periods of flight alternate with time spent at stopover the time and space where individuals rest and refuel for a subsequent migratory flight. Because time and energy costs are elevated during stopover relative to flight [1], most migration mortality probably occurs as a result of decisions made during that period [2]. As a consequence, the importance of stopover for the ecology and conservation of migrants is an area of active research [3,4]. A stopover bout can be thought of as the time spent between migratory flights [5]. On a fine scale, the site where an individual is found at any particular time during a stopover bout can be thought of as its stopover site. More broadly, the space occupied during a stopover bout can be considered its stopover landscape. A stopover landscape can be spatially and temporally equal to or larger than a stopover site [6]. The difficulty with these definitions is in determining whether an individual has undertaken a true migratory flight as opposed to a flight that only involves changing stopover sites within the stopover landscape. For some species, notably waders and ducks, individuals may stop over at well-defined geographical locations where it may be more reasonable to assume that a flight away from the site represents a continuation of migration (e.g. Western Sandpiper [7], but see [8]). However, for many other species (such as most passerines and bats), the initiation of a migratory flight is more difficult to determine so the spatial and temporal scales of stopover are not as well defined. Because of this difficulty, most researchers appear to assume that when an individual leaves a stopover site, it is continuing migration. While this assumption may hold at islands or habitat patches with clear boundaries [911], in other studies, at sites where individuals have been observed to move beyond small monitored areas prior to initiating a migratory flight [1215] this assumption appears to be violated. Permanent local emigration out of a monitored study site has been recognized as a confounding factor in studies of stopover, but the frequency and spatial extent of such relocations within the stopover landscape, and the mechanisms involved, are not well known [10,15,16]. Some relocations may occur via small-scale diurnal foraging and exploratory movements [14,17], while others occur via relocation flights [6,18]. At least some of these relocation flights are likely reverse migrations, where individuals are thought to change migration direction upon encountering adverse weather conditions or ecological barriers [1922]. While such movements may not represent continuation of migration, they are thought to occur shortly (,24 h) after the end of a migratory flight, implying that these individuals have not yet settled at a particular stopover site [10,15,16,23]. Although it has been recommended that inference be drawn only from individuals whose arrival and departure timing is known [24], identifying arrival and departure states with certainty is difficult. In previous work [6] we showed that some individuals of two passerine species, Swainsons Thrush (Catharus ustulatus) and Hermit Thrush (C. guttatus) undertake nocturnal flights that result in landscape-scale relocations of up to 30 km from the site of initial capture. We have subsequently conducted a series of follow-up studies at different seasons, using different taxa, designed in part to clarify the frequency, true spatial extent and taxonomic breadth of these landscape-scale relocations. Using an automated telemetry system, we monitored the daily location of seven species of smallto medium-sized, nocturnal vertebrate migrants at a broader spatial scale than is typically employed (,20640 km). The continuous and simultaneous use of multiple antennae on multiple towers allowed us to distinguish probable migratory departure flights from flights where individuals simply changed stopover sites within the stopover landscape. We studied the movements of radio-tagged individuals of five passerines, one owl, and one bat, some during both spring and fall migration. These provide a comprehensive set of data with which to view the spatial and temporal scale of migratory stopover within a diverse group of taxa, during stopover bouts that have been more comprehensively observed than in most previous studies. Materials and Methods Study site and the digital array The study was conducted in and around Long Point, Ontario, Canada (42u349 N, 80u139 W). Long Point is a largely uninhabited sand peninsula that extends ,40 km into Lake Erie from the north shore of the lake. The surrounding mainland comprises a mixture of deciduous forest and agricultural land (see Fig (...truncated)