Hilton Jr., B. and M.W. Miller. 2003. Annual survival and recruitment in a Ruby-throated Hummingbird population, excluding the effect of transient individuals. The Condor 105:54-62.
Sobrevivencia Anual y Reclutamiento en una Población del Picaflor Archilochus colubris Excluyendo el Efecto de Individuos Ocasionales
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ABSTRACT & EXECUTIVE SUMMARY
ABSTRACT (from The Condor)
We estimated annual apparent survival, recruitment, and rate of population growth of breeding Ruby-throated Hummingbirds (Archilochus colubris), while controlling for transients, by using 18 years of capture-mark-recapture data collected during 1984-2001 at Hilton Pond Center for Piedmont Natural History near York, South Carolina. Resident males had lower apparent survival (0.30 ± 0.05 SE) than females (0.43 ± 0.04). Estimates of apparent survival did not differ by age. Point estimates suggested that newly banded males were less likely than females to be residents, but standard errors of these estimates overlapped (males: 0.60 ± 0.14 SE; female: 0.67 ± 0.09). Estimated female recruitment was 0.60 ± 0.06 SE, meaning that 60% of adult females present in any given year had entered the population during the previous year. Our estimate for rate of change indicated the population of female hummingbirds was stable during the study period (1.04 ± 0.02 SE). We suggest an annual goal of >64 adult females and >64 immature females released per banding area to enable rigorous future tests for effects of covariates on population dynamics. Development of a broader cooperating network of hummingbird banders in eastern North America would allow tests for regional or metapopulation dynamics in this species.
RESUMEN
Estimamos la sobrevivencia anual aparente, reclutamiento y tasa de crecimiento en una población reproductiva del picaflor Archilochus colubris, controlando por la presencia de individuos ocasionales. Utilizamos datos de 18 años de captura-marcaje-recaptura colectados entre 1984 y 2001 en Hilton Pond Center for Piedmont Natural History en cercanías de York, Carolina del Sur. Los machos residentes presentaron una sobrevivencia aparente menor (0.30 ± 0.05 EE) que las hembras (0.43 ± 0.04). Las estimaciones de la sobreviviencia aparente no difirieron entre edades. Estimaciones puntuales sugirieron que los machos marcados por primera vez tuvieron una menor probabilidad de ser residentes que las hembras, pero los errores estándar de estas estimaciones se sobrepusieron (machos: 0.60 ± 0.14 EE; hembras: 0.67 ± 0.09 EE). Para las hembras el reclutamiento estimado fue 0.60 ± 0.06 EE, lo que significa que el 60% de las hembras adultas presentes en un año determinado entraron a la población durante el año anterior. Nuestra estimación de la tasa de cambio indicó que la población de picaflores hembra fue estable durante el período de estudio (1.04 ± 0.02 EE). Para permitir futuras pruebas rigurosas sobre las covariables asociadas a las dinámicas poblacionales, sugerimos una meta anual de >64 hembras adultas y >64 hembras inmaduras liberadas por área de anillado. La creación de una amplia red cooperativa de anilladores de picaflores en el este de Norteamérica podría permitir estudiar y entender mejor las dinámicas regionales o metapoblacionales de esta especie.
EXECUTIVE SUMMARY
Conservation of a species requires understanding how changing environmental conditions affect the population dynamics of that species. Such an understanding is invariably difficult to achieve, but can be done for birds through the analysis of banding data. If fieldwork follows the appropriate structure, such analyses enable estimation of the four vital rates that compose population dynamics: annual survival, production, immigration, and emigration. These analyses can also relate such parameters to variation in climate, habitat, physiology, or other variables of interest.
In this paper we estimated survival and rate of population growth of breeding Ruby-throated Hummingbirds (Archilochus colubris) near York, South Carolina. We used 18 years of capture-mark-recapture data collected by the first author during 1984-2001 at Hilton Pond Center for Piedmont Natural History as part of Operation RubyThroat: The Hummingbird Project.
We employed new statistical techniques that have not, to our knowledge, been used to determine population dynamics in birds, and certainly not in hummingbirds. Our analyses showed that male Ruby-throated Hummingbirds had lower annual survival (30%) than females (43%). These estimates did not differ between adults and immature birds of the same sex. We estimated that 60% of adult females present in the study area in any given year had entered the population during the previous year. Our results indicated that the population was stable.
We did not attempt to relate annual survival to environmental variables, although we might attempt that in the future. Relating population parameters to such covariates requires that a fairly large number of birds be captured each year. We suggest an annual goal of >64 adult females and >64 immature females released per banding area to enable rigorous future tests for effects of covariates on population dynamics. We also suggest that banding efforts be concentrated in the breeding season, perhaps May 1 through August 1, to avoid including active migrants in future studies of population dynamics; presence of migrants in an analysis can skew survival estimates low. Nonetheless, banding of migrants is valuable for studies of migration behavior.
Currently, every hummingbird is marked at initial capture during each field season with a "necklace" of nontoxic green dye on its upper breast or throat. This color mark, authorized by the federal Bird Banding Laboratory, helps avoid re-trapping a banded hummingbird within a given year, thereby minimizing handling stress, as some birds enter the same trap dozens of times each day. Since this color-marking scheme is unique to Hilton Pond, it also allows for re-sightings of birds during migration away from the banding site. One possible method to separate migrants from residents would be to mark birds one way when mostly migrants are thought to be present and a second way, perhaps with a different color, when mostly breeders are thought to be present. In that manner, perhaps only breeders that arrive early or remain late in the breeding season would be subjected to several captures in the same year. Such a solution may not be easy to implement, however, as hummingbirds can be difficult to capture, band, and color mark because of their small size.
If a fairly large number of birds can be captured each year, and presence of migrants can be confidently minimized, we expect that future studies relating hummingbird population parameters to environmental conditions will be straightforward. Indeed, we hope that a cooperating network of hummingbird banding sites can be developed throughout the U.S., Canada, and Central America, where Ruby-throated Hummingbirds either breed or overwinter. Such a network would be extremely valuable for studying large-scale population dynamics of this and other hummingbird species, and would also be valuable in hummingbird conservation.
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Hilton Pond Center for Piedmont Natural History is a non-profit research & education organization in York, South Carolina USA; phone (803) 684-5852. Directed by