Within-year movements and site fidelity of Purple Sandpipers during the nonbreeding season

ABSTRACT Although within-year site fidelity to specific wintering sites allows shorebirds to use prior knowledge of resources and microhabitats, such fidelity may also make populations more vulnerable to extirpation in the event of increased predation pressure, habitat loss, or disturbance. In the eastern Atlantic, Purple Sandpipers (Calidris maritima) have been found to be highly faithful to specific sites in wintering areas. However, little is known about the use of wintering areas by these sandpipers along the coast of Maine. We quantified movements of 60 radio-marked Purple Sandpipers in a bay near the mainland and on an offshore cluster of islands along the mid-coast of Maine during two winters (2005–2006 and 2006–2007). Birds marked in early- and mid-December remained until spring migration, with no evidence of onward migration. Mean maximum distances moved did not differ significantly between either males (8.6 ± 1.0 [SE] km; N= 30) and females (7.4 ± 0.8 km; N= 30) or juveniles (9.9 ± 1.6 km; N= 9) and adults (7.8 ± 1.1 km; N= 26). We also detected no monthly (January–May) differences in maximum distances moved. Sixty percent of marked individuals moved ≤5 km between the two most distant relocations and no birds moved >25 km during the 2- to 4-month tracking period. We attribute the high site fidelity primarily to the plentiful prey base in the study area. During a 2-d period with severe cold, feeding areas at locations protected from wave action became encased in ice and birds at these locations moved up to 10 km offshore to sites with less ice. Species with strong site fidelity, like wintering Purple Sandpipers, may be at higher risk in the event of large-scale changes in their food base, increased predation pressure, habitat loss, or disturbance. However, the short-distance movements made when intertidal feeding areas became encased in ice suggest that Purple Sandpipers could potentially move greater distances in response to changing conditions in their wintering areas. Movimientos a través del año y fidelidad a localidades de parte de Calidris maritima Aunque la fidelidad anual entre lugares invernales específicos le permite a los playeros utilizar conocimientos previos sobre recursos y microhábitats, dicha fidelidad pudiera hacer a estas poblaciones más vulnerables a su eliminación en el evento de un incremento en la presión de depredación, perdida de hábitat o disturbio. En el Atlántico este, se ha encontrado alta fidelidad a lugares específicos invernales de parte de Calidris maritima. Sin embargo, poco se sabe sobre el uso de aéreas invernales por estos playeros, a lo largo de la costa de Maine. Cuantificamos los movimientos de 60 individuos de Calidris maritima, con radiotransmisores, en una bahía cerca de tierra firme y en un grupo de islas cerca de la costa, a lo largo de Maine durante dos inviernos (2005–2006 y 2006–2007). Aves que fueron marcadas temprano y a mitad de diciembre, se mantuvieron en sus lugares hasta la migración primaveral, sin evidencia de migración hacia otros lugares. Las distancias máximas de movimiento no difirieron significativamente entre machos (8.6 ± 1.0)(ES) km; N = 30) y hembras (7.4 ± 0.8 km; N = 30), juveniles (9.9 ± 1.6 km; N = 9) y adultos (7.8 ± 1.1 km; N = 26). Tampoco detectamos diferencias mensuales (enero – mayo) en distancias máximas de movimiento. El 60% de los individuos marcados se movieron ≤5 km entre las dos distancias más distantes de relocalización y ningún ave se movió >25 km durante los 2 a 4 meses del periodo de monitoreo. Atribuimos la alta fidelidad, primeramente a la abundancia de presas en las aéreas de estudio. Durante un periodo de dos días con frio severo, las aéreas de alimentación en localidades protegidas del embate de las olas, se cubrieron con hielo y las aves, de dichos lugares, se movieron hasta 10 km dentro de la playa a localidades con mucho menos hielo. Especies con alta fidelidad a lugares, como Calidris maritima, pudieran estar en mayor riesgo que otras especies, en el caso de cambios significativos en la base de sus alimentos, incremento en la presión de depredación, perdida de hábitat o disturbio. Sin embargo, el movimiento a distancias cortas, cuando algunos lugares se cubrieron con hielo, sugiere que estos playeros, potencialmente, pudieran moverse a lugares más distantes en respuestas a cambios en las condiciones de los lugares en donde pasan el invierno. The ability of shorebirds (suborder Charadrii) to make long-distance movements on their wintering grounds allows some species, such as Bar-tailed Godwits (Limosa lapponica) and Dunlins (Calidris alpina; Symonds et al. 1984, Sanzenbacher and Haig 2002, Rehfisch et al. 2003), to rely on unpredictable food resources and cope with changes in predation pressure, habitat loss, and disturbance (Gadgil 1971, Sherry and Holmes 1996). However, where food is plentiful and predictable and there is little predation pressure and disturbance, species such as Ruddy Turnstones (Arenaria interpres) and Western Sandpipers (Calidris mauri) make short movements and are faithful to specific wintering areas (Warnock and Takekawa 1996, Burton and Evans 1997, Rehfisch et al. 2003). Because the abundance and predictability of the food base, predation pressure, habitat availability, and levels of disturbance in the intertidal zone where many of these shorebirds feed can vary from region to region, some species, such as Red Knots (Calidris canutus) and Sanderlings (Calidris alba), show variation in within-year movements and site fidelity in their wintering areas (Zeeuw 1990, Rehfisch et al. 2003, Leyrer et al. 2006). Purple Sandpipers (Calidris maritima) have the most northerly wintering distribution of any shorebird (Cramp and Simmons 1983, Hayman et al. 1986) in the northern hemisphere and, in contrast to most other shorebirds, feed along offshore, wave-exposed, rocky shorelines (Summers et al. 2002). In the eastern Atlantic, their prey, e.g., mollusks (Littorina, Lacuna, Margarites, Nucella, and Mytilus) and crustaceans (Gammarus and Idotea; Feare 1966, Summers et al. 1990a, Payne and Pierce 2002), stay firmly attached to rocks or seaweed and are less influenced by severe cold, wind, or erosion than the prey of shorebirds that feed on sandy shores and mudflats (Myers 1984, Metcalfe and Furness 1985). With this predictable food base, Purple Sandpipers are highly faithful to specific sites in wintering areas in the eastern Atlantic (Atkinson et al. 1978, Summers et al. 1990b, 2001). Although we suspect that similar site faithfulness is likely in the western Atlantic, predation pressures and disturbance levels may vary regionally and locally (Gadgil 1971) and these factors can influence site fidelity across the range of a species. Assessing patterns of movements and site fidelity in Purple Sandpipers in eastern North America is essential for understanding their ecology, population dynamics, and conservation. Coastal Maine, with its numerous offshore islands and ledges, is an important wintering site for thousands of Purple Sandpipers, with the first fall migrants arriving in late November and the last birds departing for breeding grounds in early June (Mittelhauser et al. 2006), yet little is known about their use of and winter movements in this region. Thus, our objectives were to quantify the movements and site fidelity of Purple Sandpipers during the nonbreeding season along the mid-coast of Maine. Study area We studied Purple Sandpipers during the nonbreeding seasons (December–May) of 2005–2006 and 2006–2007 at two areas in the mid-coast region of Maine: Frenchman Bay with an estimated winter population of 1200 Purple Sandpipers, and the Isle au Haut area with an estimated winter population of 1800 Purple Sandpipers (Maine Department of Inland Fisheries and Wildlife, unpubl. data; Fig. 1). We chose these study areas because of the high numbers of Purple Sandpipers and because shorelines in these areas were relatively accessible. Habitat in the nearshore Frenchman Bay study area is diverse, ranging from mud flats to rugged granite shores with precipitous cliffs, high headlands, and numerous islands and ledges. The inner bay had less water circulation and wave energy (Procter 1933) and more ice than the outer bay. The offshore Isle au Haut study area ranged from 10 to 30 km offshore, had numerous islands and ledges, and was more exposed to wind and waves. Rugged granite shores were the main feature of this region with occasional small pockets of boulder and cobble. Tidal range in both areas averaged ∼3 m (Procter 1933). Location of the Frenchman Bay and Isle au Haut study areas along the mid-coast of Maine. The regional climate was characterized by cold and windy winters, with frequent winter storms. At Matinicus Rock, an island 30 km southwest of the Isle au Haut study area, winter winds were predominantly from the northwest (40%) and exceeded 10 m/s ∼40% of the time (National Oceanic and Atmospheric Administration 1998). Capture We captured Purple Sandpipers in the intertidal zone during daylight hours with a .308 caliber, four-barrel net gun (Coda Enterprises, Mesa, AZ). Nets measured 4 × 4 m or 5 × 5 m with a 5-cm mesh. Weights attached to each corner of the net serve as projectiles that, when fired, propel the net ∼7–10 m. To ease recovery of nets that missed their mark, we attached 5 m of nylon twine with a trailing float to the center of each net. Most capture sessions were conducted from a 5.5-m skiff; occasionally, if conditions were appropriate, we worked from shore. At Frenchman Bay, we captured 19 birds during December 2005 and 27 birds during late February and early March 2006. We captured 14 birds at Isle au Haut during December 2006 and, because of the difficulty of working at this site, this was the only capture effort for the winter. During our study, we netted 68 birds and 60 were banded and outfitted with radio transmitters; one bird died and the leg of one bird was injured during a capture attempt. Age and sex determination We aged birds by plumage characteristics (Atkinson et al. 1981, Boer et al. 1984). Sex was determined using morphological measurements and predictive models (Engelmoer and Roselaar 1998). We assigned age as either first winter (HY) or adult (AHY) for December captures based on coloration of median and tertial coverts (Prater et al. 1977); feather wear prevented age determination for late winter and early spring captures (Payne and Pierce 2002). Linear measurements, following methods described by Engelmoer and Roselaar (1998), recorded for all birds included culmen, tarsus, middle toe, maximum flattened wing chord, and first secondary. We measured wing chord and first secondary length with a stopped wing ruler (± 0.5 mm) and all other measurements were made with digital calipers (± 0.01 mm). Birds were weighed with a spring scale (± 1 g). All birds were measured by GHM. Purple Sandpipers are sexually dimorphic with respect to morphometric measurements, especially bill length (Payne and Pierce 2002). Therefore, we used the computer program “POSCON” (Engelmoer and Roselaar 1998) to estimate the probability that captured birds were male or female based on the above measurements. This method correctly classified the sex of ∼85–90% of birds captured, based on a subsample of 62 males and 47 females whose sex were also determined by genetic analysis (GHM, unpubl. data). Radio tracking We fitted sandpipers with 1.6 g transmitters (model BD–2, Holohil Systems Ltd., Carp, ON, Canada) with a 7-week expected lifespan for the transmitter batteries. We attached transmitters with a single thread leg-loop harness, as described by Sanzenbacher et al. (2000). The transmitter and harness together weighed 1.8 g and were placed on birds ranging in mass from 64 to 98 g. All radio-tagged birds were returned to their original capture area; most birds were released within 20 min, but, when multiple birds were captured, holding time increased to a maximum of 40 min. We determined the locations of radio-tagged birds between sunrise and sunset using a four-element, handheld, Yagi antenna. One person estimated the location of radio-marked birds based on the intersection of signal bearings taken from two or more locations in quick succession or by direct observation. Birds typically ranged from 10 m to 5 km away from tracking locations. Signal detection range was ∼5 km under ideal conditions (i.e., with direct line of sight over water), and detection range increased when tracking from elevated locations. We estimated the exact location of each bird during our fieldwork and these estimated positions were used for all distance calculations. Because signal attenuation from nearby islands and shorelines and our inability to approach birds on foot often limited our ability to determine exact locations of birds, we later grouped all estimated positions into whole islands (N= 77) and subdivided the mainland shoreline and shoreline of Mount Desert Island and Isle au Haut into ∼2 km segments (N= 22) to calculate the most frequently visited sites. We tested the accuracy of our ground tracking efforts in a series of blind tests (N= 49) by placing a test transmitter at typical high tide roost locations with a history of shorebird use. All test transmitters were placed by above the high tide line along mainland and island shorelines and the location of the test transmitter was two to a no to the when transmitters were were or if were on foot or were on an The was to estimate the location of the test transmitters during all efforts using the of effort used for We their estimated locations based on our tracking efforts with their locations recorded with a We used and to radio-tagged We at from December We birds from from m above with of offshore the we most radio-marked birds each we to radio-marked birds for we or we conducted either to or up to 40 km from capture locations. We conducted two 2006 in the Frenchman Bay region and March in the Isle au Haut of km of from a with on the et al. In the Frenchman Bay region, the included the region from to the of The all to the In the Isle au Haut region, the included the region from Island to and to Matinicus During by we to all sessions were birds time to their We included only one location bird tracking location for that in our We birds the tide that locations included both feeding and sites. analysis each bird we the maximum distance moved from their of capture and the line distance between the two most distant locations. We tested for differences between or with either the or the with We quantified fidelity of birds to specific islands or shorelines by the of sites where each bird was To determine the most frequently visited we only included birds with locations and sites with of all locations of an To the of severe cold on bird we used and wind taken at the Maine km of the Frenchman Bay study area and km of the Isle au Haut study area. We all as ± We birds females and in the Frenchman Bay area for from December 2005 to June and 14 birds females and in the Isle au Haut area for from December 2006 to birds captured, were were and could not be assigned to an age Birds were for an of ± 60 birds one was not at Isle au Haut on December until the signal was detected on March in the of 5 km from its original we were not to determine if this was a radio or a We the transmitter signal to the of a of on the of the we were not to the transmitter or This was not included in our At the of transmitter we were not to between a transmitter and a transmitter of transmitters in we birds from to and averaged transmitters in February and birds were last detected 40 to with an of Although spring for a birds could be many working transmitters in early spring were the of their expected as birds were and could not be male birds were last recorded on 4 or 5 when their transmitter were under and this likely early of birds from the study area. the from the study area was on when the transmitter was We recorded locations of radio-marked birds ± range = at Frenchman Bay and locations ± range = at Isle au The distance between estimated locations and based on of was ± km = km; N= 49) and of these test transmitters were correctly assigned to a whole island or shoreline We all marked birds at The maximum distance between locations of radio-marked individuals was ± km 1). Most birds moved ≤5 km between most distant locations and no birds moved more than km We found no between study areas in maximum distances moved 1). maximum distances moved did not differ between females and males or between adults and juveniles distance between for radio-marked Purple Sandpipers along the mid-coast of Maine, To determine if the bird we bird movements at Frenchman Bay on and 27 February 2006. in Maine, to the study area, a of during this the for that winter. at an offshore where winter are than the during this 2-d period averaged and winds averaged m/s with up to 19 for the of February averaged and winds averaged m/s at this offshore During this cold birds at inner Frenchman Bay moved greater distances ± N= than birds in outer Frenchman Bay ± N= and also greater distances during in the ± N= the intertidal shorelines in the inner bay most protected from wave action during this cold more shorelines in the outer bay remained relatively the birds in the inner bay with an of ± locations bird = and recorded at ± 1.6 sites bird = the cold two moved to sites where had been and one to use these sites birds were detected at 98 sites and at an of ± sites We found no in the of sites visited at Frenchman Bay and Isle au Haut between males and females or between adults and juveniles of ± of each (N= locations were at their most visited and these sites averaged ± km = In 62 ± of all locations of Purple Sandpipers were at the most visited site fidelity by Purple Sandpipers in Maine was with 60% of individuals ≤5 km and no birds >25 km over to tracking In the most visited sites of all averaged less than 2 km and over 60% of all locations of sandpipers were at the most visited site fidelity by Purple Sandpipers in the eastern Atlantic, based on and maximum movements of km with of individuals ≤5 km during the nonbreeding season (Atkinson et al. 1978, Summers et al. 1990b, 2001). One banded on (Summers et al. was km away from the location in March Although many birds banded in late at this location the winter (Summers et al. the of this one bird is likely a of onward migration, in this species, than an Ruddy a species that also rocky shorelines may be more site faithful and have winter movements than Purple Sandpipers and Evans 1997, et al. 2003), radio tracking are to One for high fidelity to specific at in Maine, is prey as was by Burton and Evans to site fidelity by Purple Sandpipers in During we Purple Sandpipers feeding in the of the intertidal primarily in an area with two species of and and only in other areas in the intertidal zone In a study, prey species to these two species of and and and and were in the Isle au Haut study area and averaged individuals (Mittelhauser Most prey species were unpubl. and of the typically by Purple Sandpipers (Summers et al. Because by and of Purple Sandpipers were study is to the between Purple site fidelity and prey We found that Purple Sandpipers wintering in Maine areas because of severe cold movements were likely not for because Purple Sandpipers are to cold, with plumage most other shorebirds and little in the winter et al. 1981, Summers et al. However, wintering birds moved to shorelines up to 10 km away when intertidal feeding areas in more sites became encased in ice during a period of cold offshore and shorelines are typically during severe cold because of the influence of the of Maine on and the of the shoreline by wave action and During the shorelines in Maine are often encased in ice from the intertidal zone and in intertidal areas where sandpipers typically feed by Purple Sandpipers in response to severe cold and of ice over feeding areas were also in the eastern Atlantic, especially at the northern of their wintering range and Evans 1997, 1998). site fidelity allows shorebirds to use prior knowledge of resources and in a area. site fidelity make populations vulnerable to extirpation in the event of large-scale changes in the food base, increased predation pressure, habitat loss, or disturbance (Gadgil 1971, Sherry and Holmes 1996). Although our and of other (Atkinson et al. 1978, Summers et al. 1990b, that Purple Sandpipers high site fidelity, we also found that made short-distance movements when intertidal feeding areas became encased in ice. suggest that Purple Sandpipers could potentially move greater distances in response to changing conditions in their wintering areas. study is to the factors that to fidelity to wintering sites by these sandpipers and to changes in these with tracking birds and analysis during the study, we and We on the of our for often during and we and for their with genetic of birds was by and This was made from Maine Department of Inland Fisheries and Wildlife, and the We for and on of this and also and for their

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