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Redistribution and Growth of the Caspian Tern Population in the Pacific Coast Region of North America, 1981-2000

Robert M. Suryan, David P. Craig, Daniel D. Roby, Nathan D. Chelgren, Ken Collis, W. David Shuford and Donald E. Lyons
The Condor
Vol. 106, No. 4 (Nov., 2004), pp. 777-790
Stable URL: http://www.jstor.org/stable/3247783
Page Count: 14
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Redistribution and Growth of the Caspian Tern Population in the Pacific Coast Region of North America, 1981-2000
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Abstract

We examined nesting distribution and demography of the Pacific Coast population of Caspian Terns (Sterna caspia) using breeding records and band recoveries spanning two decades since the first population assessment. Since 1980, population size has more than doubled to about 12 900 pairs, yet the proportion of the population nesting at inland (18%) versus coastal sites (82%) has remained constant. Although the breeding range of the Pacific Coast population has expanded northward into Alaska and farther south in Mexico, there was no net latitudinal shift in the distribution of breeding pairs or new colonies. The distribution of breeding birds among areas changed dramatically, however, with 69% of breeding terns now nesting in Oregon (primarily in the Columbia River estuary) versus 4% during the late 1970s. During the past 20 years, there has continued to be a greater proportion of Caspian Terns breeding at anthropogenic sites compared to natural sites. Estimated annual survival rates for hatch-year and after-third-year birds during 1981-1998 were greater than during 1955-1980, consistent with the higher rate of population increase in recent decades. Fecundity required to maintain a stable population (λ = 1) was estimated at 0.32-0.74 fledglings pair-1, depending on band recovery probabilities for subadults. Caspian Terns readily moved among breeding sites and rapidly colonized new areas; however, a greater concentration of breeding Caspian Terns among fewer colonies in response to anthropogenic factors is an important conservation concern for this species. /// Se examinó la distribución de anidamientos y la demografía de la población de Sterna caspia en la costa del Pacífico usando registros reproductivos y anillos recobrados de aves marcadas durante dos décadas desde la primera evaluación poblacional. Desde 1980 el tamaño poblacional se duplicó a 12 900 parejas, aunque la proporción de la población anidando en la zona interior (18%) versus la zona costera (82%) permaneció constante. A pesar de que el rango reproductive de la población costera del Pacífico se expandió hacia el norte llegando a Alaska y hacia el sur hasta México, no hubo un cambio latitudinal neto en la distribución de parejas reproductivas o de nuevas colonias. Sin embargo, la distribución de aves reproductivas entre las áreas cambió dramáticamente, con un 69% de los individuos reproductivos de Sterna caspia anidando ahora en Oregón (principalmente en el estuario del Río Columbia) comparado con un 4% a fines de la década del 70. Durante los últimos 20 años continuó habiendo una mayor proporción de individuos de Sterna caspia anidando en sitios con actividad antropogénica en comparación con áreas naturales. Las tasas de supervivencia anual para aves con menos de un año y aves con más de tres años fueron mayores durante 1981-1998 que durante 1955-1980, lo que es consistente con una mayor tasa de crecimiento poblacional en décadas recientes. La fecundidad requerida para mantener estable la población (λ = 1) se estimó en 0.32-0.74 volantones producidos por pareja, dependiendo de la probabilidad de recuperación de anillos en aves subadultas. Sterna caspia es capaz de moverse entre sitios reproductivos y coloniza nuevas áreas rápidamente. No obstante, una mayor concentración de individuos reproductivos de Sterna caspia en unas pocas colonias, como respuesta a factores antropogénicos, es una importante preocupacién para la conservación de esta especie.

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