FACULTATIVE ALTITUDINAL MOVEMENTS BY MOUNTAIN WHITE-CROWNED SPARROWS (ZONOTRICHIA LEUCOPHRYS ORIANTHA) IN THE SIERRA NEVADA

Auk, The, Oct 2004 by Hahn, Thomas P, Sockman, Keith W, Breuner, Creagh W, Morton, Martin L

ABSTRACT.-

Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) winter in Mexico and often arrive in the vicinity of their breeding grounds in the Sierra Nevada well before nesting is possible. Arrival at Tioga Pass, California (elevation 3,030 m), usually occurs in early May, but residual winter snow and adverse weather can delay nesting for weeks. We used radiotelemetry to determine whether prebreeding Mountain White-crowned Sparrows engaged in weather-related altitudinal movements during the waiting period between the end of spring migration and onset of breeding during 1995-2001, with a range of residual winter snowpacks. Interannual variation in arrival date and onset of egg laying was 18 and 41 days, respectively. We tracked females for two years and males for all seven years. During spring snowstorms (which occurred in four years), radiomarked individuals moved to lower elevation sites, where they often remained for several days. Departing birds left Tioga Pass by early afternoon and returned early in the morning after storms. More frequent storms during tracking increased the likelihood of facultative altitudinal movements, but heavier residual winter snowpack did not. Warm days increased the likelihood of birds returning to Tioga Pass from low elevation. This study demonstrates that facultative altitudinal movement behavior can be a common feature of spring arrival biology in montane-breeding birds. Received 1 November 2002, accepted 30 June 2004.

RESUMEN.-Zonotrichia leucophrys oriantha pasa el invierno en México y usualmente llega a las cercanias de sus sitios reproductivos en Ia Sierra Nevada bastante antes de que sea posible anidar. La llegada a Tioga Pass, California (elevación 3030 m) ocurre usualmente a principios de mayo, pero la presencia de nieve residual del invierno y las condiciones adversas del clima pueden demorar el anidamiento por semanas. Usamos radio-telemetría para determinar si los individuos pre-reproductivos de Z. l. oriantha realizaron movimientos altitudinales inducidos por el clima durante el periode de espera entre el final de la migración de primavera y el comienzo de la reproduccion durante 1995-2001, considerando un rango de niveles de acumulación de nieve residual del invierno. La variación interanual en la fecha de llegada y en el comienzo de la puesta de huevos fue de 18 y 41 días, respectivamente. Seguimos a las hembras durante dos anos y a los machos durante los siete años del estudio. Durante las tormentas de nieve de la primavera (que tuvieron lugar en cuatro años), los individuos marcados con radios se desplazaron a sitios a elevaciones más bajas, donde permanecieron por lo general por varios días. Las aves se marcharon de Tioga Pass a comienzos de la tarde y regresaron temprano en la manana luego de las tormentas. Una mayor frecuencia de tormentas durante el periodo de seguimiento de las aves incrementó la probabilidad de que los individuos realizaran movimientos facultativos, pero mayores acumulaciones de nieve residual no tuvieron el mismo efecto. Los días cálidos incrementaron la probabilidad de que las aves regresaran a Tioga Pass desde elevaciones bajas, lo cual demuestra que el comportamiento de movimientos facultativos de los individuos puede ser un rasgo caracteristico de la biologia de la llegada de primavera en aves que se reproducen en las montañas.

MIGRATORY BIRDS GENERALLY must adjust to environmental unpredictability when they arrive on their breeding grounds. This is particularly true for ground and low-shrub nesters breeding in extreme environments, such as high elevations and high latitudes. Winter snowpack determines when nesting and foraging habitat will become available; access can be immediate upon arrival from spring migration or delayed for weeks following arrival. In addition, spring and early summer weather is notoriously capricious in both mountain and Arctic regions. Spring storms can further reduce food availability by covering thawed areas and by hiding arthropods and seeds that accumulate on the surface of existing snow (see Edwards 1972, 1986; Edwards and Banko 1976). Those limitations on food availability, combined with energetically taxing conditions owing to subfreezing ambient temperatures and wind, may press small birds to abandon breeding areas temporarily in favor of relatively benign refugia (see Wingfield et al. 1983). For mountain birds, attractive locations may be available nearby at low elevations, where springlike conditions prevail despite stormy "weather in adjacent high country. Consequently, mountain birds can escape inclement weather on breeding grounds through short flights downslope. When those movements between high and low elevations occur seasonally, they are generally termed "altitudinal migrations" (Orr 1970, Dingle 1980; see also Rabenold and Rabenold 1985, Inouye et al. 2000). However, facultative altitudinal movements could theoretically occur whenever weather conditions deteriorate.

Evidence that mountain birds make facultative altitudinal movements prior to or during the breeding season comes primarily from observations of montane, subalpine, and alpine species at low elevations during storms (O'Neill and Parker 1978, Horvath and Sullivan 1988, Fjelds0å 1991, Morton 2002). However, documentation of the phenomenon, especially from monitoring of marked individuals, has been lacking. Consequently, we have little information regarding the nature or prevalence of conditions that induce such behavior. During a long-term study of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) breeding on the central Sierra Nevada crest of California, Morton (2002) occasionally observed color-banded birds from the Tioga Pass population at lower elevations during periods of extreme weather in the 1980s. In 1995, we began a multi-year radiotelemetry study to explore the prevalence of facultative altitudinal movements by Mountain White-crowned Sparrows of the same population. Our results demonstrate that, far from being unusual, facultative altitudinal movements following spring return from Mexico are a common feature of arrival and settlement behavior of both sexes in this population.