Changes in Survey Effort Can Influence Conclusions about Migration Phenology
Changes in autumn raptor migration phenology have been documented at hawk count sites across North America. Delays in autumn migration are the most common shift reported, but these changes vary by species. We examined autumn raptor phenology and changes in count effort for 14 raptor species over 46 yr at Hawk Ridge in Duluth, Minnesota, USA, using mean passage date. Among the 14 raptors, six species showed significant shifts in migration phenology; four toward later migration and two toward earlier migration. Our results suggest fewer species are shifting toward later phenology than previously reported at Hawk Ridge and that some previously reported changes in phenology may be explained by changes in survey effort. This study highlights the importance of controlling for count effort when analyzing migration phenology, emphasizing the need for consistent effort in the future. Los Cambios en el Esfuerzo de Muestreo Pueden Influir Las Conclusiones Sobre l Fenología Migratoria En los sitios de conteo de aves rapaces en Norteamérica se han observado cambios en la fenología migratoria otoñal. El retraso en la migración otoñal es el cambio más comúnmente observado, si bien estos cambios varían según la especie. Examinamos la fenología de las aves rapaces durante el otoño y los cambios en el esfuerzo de muestreo para 14 especies de aves rapaces durante 46 años en Hawk Ridge, Duluth, Minnesota, EEUU, utilizando la fecha promedio de paso migratorio. De las 14 especies de rapaces, seis especies mostraron cambios significativos en la fenología migratoria; cuatro hacia una migración tardía y dos hacia una migración anticipada. Nuestros resultados sugieren que hay un menor número de especies retrasando su fenología migratoria comparado a lo que se informó anteriormente en esta localidad. Algunos cambios reportados previamente en la fenología migratoria podrían ser explicados por cambios en el esfuerzo de muestreo. Este estudio destaca la importancia de controlar el esfuerzo de muestreo al analizar la fenología migratoria, enfatizando la necesidad de un esfuerzo estable en el futuro. [Traducción del equipo editorial]ABSTRACT
RESUMEN
Timing of the two metrics of count effort at Hawk Ridge, Duluth, Minnesota: mean count day and ordinal date of the mean count hour. (A) Mean count days using the full data set (β = 0.37 ± 0.08, P < 0.01); (B) Mean count days using the truncated early migrant data set (β = –0.02 ± 0.02, P = 0.10); (C) Mean count days using the truncated late migrant data set (β = –0.01 ± 0.04, P = 0.47); (D) Mean count hours using the full data set (β = 0.29 ± 0.08, P < 0.01); (E) Mean count hours using the truncated early migrant data set (β = –0.02 ± 0.04, P = 0.37); and (F) Mean count hours using the truncated late migrant data set (β = –0.16 ± 0.11, P < 0.01).
Autumn migration phenology shifts (β = slope of regression of lognormal mean migration date) for 14 raptor species at Hawk Ridge. Species for which P < 0.05 and that have a confidence interval not crossing zero are considered to have a statistically significant shift in migration phenology and are marked with solid circles. Species for which P > 0.05 and that had a confidence interval that crossed zero are marked with open circles.
Autumn migration phenology for six species with significant shifts. (A) Turkey Vulture (β = 0.14 ± 0.06, P < 0.01); (B) Bald Eagle (β = –0.82 ± 0.29, P < 0.01); (C) Northern Harrier (β = 0.10 ± 0.09, P = 0.04); (D) Sharp-shinned Hawk (β = 0.18 ± 0.06, P < 0.01); and (E) American Kestrel (β = 0.17 ± 0.07, P < 0.01).
Autumn migration phenology shifts (β = slope of regression of lognormal mean migration date) for five late migrating raptor species comparing the truncated late migrant data set and the full data set. Species for which P < 0.05 and that have a confidence interval not crossing zero are considered to have a statistically significant shift in migration phenology and are marked with solid circles. Species for which P > 0.05 and that had a confidence interval that crossed zero are marked with open circles.
Contributor Notes
1 Present address: Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 USA; email address: stei1265@d.umn.edu
2 Present address: Wetland Wildlife Populations and Research Group, Minnesota Department of Natural Resources, 102 23rd Street NE, Bemidji, MN 556601 USA.
Associate Editor: Christopher J. Farmer