MINIMUM ESTIMATES OF SURVIVAL AND POPULATION GROWTH FOR CERULEAN WARBLERS (DENDROICA CERULEA) BREEDING IN ONTARIO, CANADA

Auk, The, Jan 2004 by Jones, Jason, Barg, Jennifer J, Sillett, T Scott, Veit, M Lisa, Robertson, Raleigh J

ABSTRACT.-The Cerulean Warbler (Dendroica cerulea) tops many lists of species of conservation concern because of severe population declines and habitat loss. Here we present the first robust estimates of annual survival and population growth rates for this species. We used capture-mark-recapture models to estimate survival of adult male Cerulean Warblers in an eastern Ontario population that has been studied since 1994. Adult male survival probability ([phi]) was constant over time in our best-supported model. Our second-best-supported model indicated a negative effect of a 1998 ice storm on survival. The third-best-supported model indicated a significant year effect on survival. On the basis of those results and previously published estimates of annual fecundity, we calculated a population growth rate using a two-stage Leslie matrix. Population growth rate ([lambda]) was 0.73, using the estimate for constant survival. Model elasticities imply that adult mortality had a stronger effect on A than did seasonal fecundity. Oversummer survival estimates suggest that events during migration or on wintering grounds are responsible for most adult male mortality. It appears that our study population, thought to be one of the healthiest known for this species, may not be currently reproducing at a high enough rate to accommodate adult mortality. However, caution must be used when interpreting those results, given the possibility of underestimating survival and fecundity of this species. Received 8 june 2002, accepted 28 August 2003.

RESUMEN. - Dendroica cerulea encabeza muchas listas de especies cviyo estado de conservacion es preocupante debido a sus dddisminuciones poblacionales severas y a la perdida de habitat. Aqui presentames la primera estimacion robusta de la supervivencia anual y de las tasas de crecimiento poblacional para esta especie. Empleamos modelos de captura, marcado y recaptura para estimar la supervivencia de machos adultos de D. cerulea en una poblacion del este de Ontario que ha sido estudiada desde 1994. La probabilidad de supervivencia de los machos adultos ([phi]) fue constante a Io largo del tiempo (0.49 � 0.05) en el modelo con mayor respaldo. El modelo en segundo orden de respaldo indico un efecto negativo en la supervivencia de una tormenta de hielo ocurrida en 1998: sin tormenta de hielo [phi] = 0.54 � 0.06; tormenta de hielo [phi] = 0.40 � 0.08. El modelo en tercer orden de respaldo indico un efecto significativo de ano en la supervivencia. Basados en estos resultados y en estimaciones anuales de fecundidad previamente publicadas, calculames una tasa de crecimiento poblacional usando una matriz de Leslie de dos etapas. La tasa de crecimiento poblacional (A) fue de 0.73, usando el parametro estimado para supervivencia constante. La elasticidad de los modelos implica que la mortalidad de los adultos tuvo un efecto mayor sobre A que la fecundidad estacional. La supervivencia a fines del verano fue estimada en 0.98 � 0.05, sugiriendo que eventos durante la migracion o en las areas de invernada son responsables de la mayor parte de la mortalidad de los adultos. Aparentemente, nuestra poblacion de estudio, que se supone es una de las mas saludables que se conoce para esta especie, podria no estar reproduciendose actualmente a una tasa suficientemente alta para balancear la mortalidad de los adultos. Sin embargo, debe actuarse con cautela al interpretar estos resultados, dada la posibilidad de subestimar la supervivencia y fecundidad de esta especie.

DECLINE OF SPECIES that migrate between Nearctic breeding grounds and Neotropical wintering grounds is one of the most pressing North American bird conservation issues of the past two decades (Robbins et al. 1989, Terborgh 1989, Askins et al. 1990). Although concern for Neotropical migrants as a group may have waned in recent years (Faaborg 2002), certain species remain in the spotlight. The Cerulean Warbler (Dendroica cerulea) is of particular concern, with documented declines of up to 3% per year since 1966 (Robbins et al. 1992, Link and Sauer 2002), probably resulting from habitat loss in both North and South America. The Cerulean Warbler has been variously designated as threatened, rare, or of special concern in the United States and as a species of special concern in Canada (Robbins et al. 1992, Committee of the Status of Endangered Species in Canada 2000, Hamel 2000). Most of what is known about this species involves general habitat associations and reproductive ecology (Oliarnyk and Robertson 1996, Hamel 2000, Jones and Robertson 2001, Jones et al. 2001), and there is no published information on adult survivorship.

Here we present annual and seasonal survival estimates and estimates of population growth rates for Cerulean Warblers breeding in eastern Ontario. That population is thought to have historically enjoyed high reproductive success (Oliarnyk and Robertson 1996). However, a major ice storm that swept through the region in January 1998, causing widespread damage to forest structure (Kerry et al. 1999), negatively influenced short-term fecundity (Jones et al. 2001). To estimate annual and seasonal survival probabilities and to assess the ice storm's effect on survival, we used capture-mark-recapture models.