Species and Sex Composition of Ptarmigan Consumed by Nesting Gyrfalcons on the Seward Peninsula, Alaska
Conservation of predators requires a comprehensive understanding of their life history and ecology, including the delineation of temporal and spatial dietary habits. Gyrfalcons (Falco rusticolus), Willow Ptarmigan (Lagopus lagopus), and Rock Ptarmigan (L. muta) are Arctic specialists, strongly linked in a dynamic predator-prey relationship, and facing similar conservation threats. We studied Gyrfalcon predation of ptarmigan on Alaska's Seward Peninsula and investigated whether species-specific contributions to diet reflected preferential selection or exploitation of the more abundant prey species. Additionally, we examined how the sex ratios of ptarmigan in Gyrfalcon diet varied throughout the breeding season. We collected ptarmigan prey remains from in and around occupied Gyrfalcon nesting sites in 2017, and identified species and sex by molecular techniques. We compared proportions of the two ptarmigan species in Gyrfalcon diet to proportions of the Rock and Willow Ptarmigans' preferred habitat around Gyrfalcon nesting sites (n = 205 skeletal remains from 12 nests), and compared sex ratios of ptarmigan prey remains temporally (n = 252 skeletal remains from 12 nests). We found that prey remains were more likely to be Rock Ptarmigan when areas around Gyrfalcon sites had greater slope and higher elevation (i.e., they better matched the habitat preferences of Rock Ptarmigan), which may suggest Gyrfalcon diet tracked ptarmigan availability without preference for species. Ptarmigan remains were biased toward male birds during June, when most female ptarmigan are incubating on concealed nests. Although we recommend additional analysis of these topics, our findings further our understanding of Gyrfalcon and ptarmigan ecology as the Arctic faces rapid changes to its climate and landscape. Especies y Composición de Sexos de Ejemplares de Lagopus Consumidos Por Individuos de Falco rusticolus Que Anidan en la Península Seward, Alaska La conservación de los depredadores requiere un entendimiento integral de sus historias de vida y de su ecología, incluyendo la descripción de los hábitos temporales y espaciales de la dieta. Falco rusticolus, Lagopus lagopus y L. muta son especialistas del Ártico que están fuertemente vinculados en una relación depredador-presa dinámica y que enfrentan amenazas de conservación similares. Estudiamos la depredación de F. rusticolus sobre Lagopus en la península Seward de Alaska e investigamos si las contribuciones específicas de cada especie a la dieta reflejan selección preferencial o explotación de la especie presa más abundante. Adicionalmente, examinamos cómo la proporción de sexos de Lagopus en la dieta de F. rusticolus varió a lo largo de la estación reproductora. Recogimos restos de presas de Lagopus dentro y alrededor de los lugares de cría ocupados por F. rusticolus en 2017, e identificamos la especie y el sexo mediante técnicas moleculares. Comparamos las proporciones de las dos especies de Lagopus en la dieta de F. rusticolus con las proporciones en los hábitats preferidos de L. muta y L. lagopus alrededor de los lugares de cría de F. rusticolus (n = 205 restos de esqueletos provenientes de 12 nidos), y comparamos el cociente de sexos de los restos de presas de Lagopus por estación (n = 252 restos de esqueletos provenientes de 12 nidos). Encontramos que los restos de presas tuvieron mayor probabilidad de pertenecer a L. muta cuando las áreas alrededor de los lugares de cría de F. rusticolus tuvieron mayor pendiente y altitud (i.e., correspondiendo mejor con las preferencias de hábitat de L. muta), lo que sugiere que la dieta de F. rusticolus sigue la disponibilidad de individuos Lagopus, sin una preferencia por la especie. Los restos de Lagopus estuvieron sesgados hacia los machos durante junio, cuando la mayoría de las hembras de Lagopus está incubando en nidos ocultos. Aunque recomendamos realizar análisis adicionales sobre estos temas, nuestros resultados amplían nuestra comprensión de la ecología de F. rusticolus y de Lagopus, en un contexto en el que el Ártico se enfrenta a rápidos cambios climáticos y de paisaje. [Traducción del equipo editorial]ABSTRACT
RESUMEN
Gyrfalcon and ptarmigan breeding phenology estimates for Alaska's Seward Peninsula. Gyrfalcon phenology dates from Henderson (2019) and unpublished Peregrine Fund data. Mean clutch initiation is an estimate of the date of first laid egg and was calculated following Anderson et al. (2017). Ptarmigan breeding phenology from Kessel (1989) for both species. Ptarmigan breeding phenology is strongly influenced by percent snow cover, thus phenology is subject to interannual variation (Cotter 1999).
Effect of abiotic characteristics on species-specific ptarmigan prey remains in and around Gyrfalcon nesting sites, Seward Peninsula, Alaska, 2017. Circle size represents number of ptarmigan samples at nests and gray shading represents the 95% confidence interval. (A) Ptarmigan remains were more likely to be Rock Ptarmigan, relative to Willow Ptarmigan, when the area around nesting sites had a greater mean slope and (B) higher elevation (n ¼ 12 nests and 205 individual ptarmigan remains). Statistics delineated in Table 3.
Probability of female ptarmigan remains in and around Gyrfalcon nesting sites during the breeding season, Seward Peninsula, Alaska, 2017. The sex ratio of Rock Ptarmigan remains favored females in May–early June and no other time period differed from 1:1. The sex ratio of Willow Ptarmigan was similar to 1:1 in May–early June and favored males in mid-late June and females in July. Categories that differed statistically from 0.50 are indicated by an asterisk. In mid-late June, remains of both species were less likely to be female compared to other time periods (n¼252 samples from 12 nests. Statistics delineated in Table 3.)
Contributor Notes
1 Email address: Henderson.michael@peregrinefund.org
Associate Editor: Steven J. Slater