Northern Harriers Have a Geographically Broad Four-year Migration Cycle
We used observation data from major hawk-watch sites in northeastern North America to assess multiyear cycles in breeding productivity of the Northern Harrier (Circus hudsonius). We found that autumn totals of migrating harriers for 1983–2017 autocorrelated positively at the 4-yr lag for interior, but not Atlantic coastal, counts. The autocorrelation was especially high for harriers counted in October. The autumn counts of south-bound harriers were not correlated with counts of north-bound harriers in the previous spring. Interannual differences in atmospheric circulation during migration in 1983–2017, as expressed by the autumn North Atlantic Oscillation Index, correlated with the autumn harrier counts but did not have a 4-yr cycle. The temporal pattern revealed in this study should be considered when using counts of autumn migrants to infer regional population trends. Future research could focus on the cause of the cyclic pattern in harrier numbers. One obvious question concerns the relationship between spring populations of meadow voles (Microtus pennsylvanicus), which are important prey for harriers during breeding, and harrier numbers reported during the subsequent autumn migration.
Circus hudsonius Tiene un Ciclo Migratorio de Cuatro Años Geográficamente Amplio Usamos datos de observaciones provenientes de los principales sitios de observación de aves rapaces ubicados en el noreste de América del Norte para evaluar ciclos multianuales en la productividad de Circus hudsonius. Encontramos que los números totales de individuos migratorios en otoño de C. hudsonius para el período 1983–2017 se autocorrelacionan positivamente con un desfase de 4 años para los conteos del interior, pero no para los conteos de la costa atlántica. La autocorrelación fue especialmente alta para los individuos contados en octubre. Los conteos otoñales de individuos migrando hacia el sur no se correlacionaron con los conteos de individuos migrando hacia el norte en la primavera previa. Las diferencias interanuales en la circulación atmosférica durante la migración en el período 1983–2017, según lo expresado por el Índice de Oscilación del Atlántico Norte de otoño, se correlacionaron con los conteos otoñales de C. hudsonius, pero no tuvieron un ciclo de 4 años. El patrón temporal mostrado en este estudio debería ser considerado cuando se usen los conteos de migrantes otoñales para inferir tendencias poblacionales regionales. Las investigaciones futuras podrían enfocarse en las causas de este patrón cíclico en los números de C. hudsonius. Una línea de investigación obvia sería la relación entre las poblaciones de primavera del roedor Microtus pennsylvanicus, una presa importante para C. hudsonius durante la reproducción, y el número de individuos de C. hudsonius hallados durante la migración otoñal subsecuente. [Traducción del equipo editorial]Abstract
Resumen
Location of five raptor count sites for the Northern Harrier migration cycle study, 1983–2017. Sites denoted by a circle had counts made during autumn migration, and the site denoted by a triangle had counts made during spring migration.
Temporal autocorrelograms for residuals of log10 autumn-migrating Northern Harriers (Interior Sum, IS; Cape May, CM), residuals of the autumn North Atlantic Oscillation Index (NAOI), and partial autocorrelogram for log10 Interior Sum (IS Partial), for years 1983–2017. Markers filled black represent rk or partial-rk different from 0 at P < 0.05, where k is the lag (yr). IS and IS Partial had identical rk values of –0.480 at the k = 2-yr lag, with the IS marker left unfilled so that both can be seen.
Detrended raw counts of migrating Northern Harriers for years 1983–2017: Interior Sum (IS), Cape May (CM), and Derby Hill (DH).
Temporal autocorrelograms for residuals of log10 monthly Interior Sum counts of migrating Northern Harriers for years 1983–2017: August (A), September (S), October (O), November (N). Solid black markers represent rk different from 0 at P < 0.05, where k is the lag (yr).
Residuals of log10 autumn-migrating Northern Harriers for Cape May (CM) and Interior Sum (IS) plotted against the autumn North Atlantic Oscillation Index for the same year, 1983–2017.
Contributor Notes
Associate Editor: Christopher J. Farmer