. El campo canadiense-naturalista. 2000 SUYDAM, QUAKENBUSH, DiCKSON y ObRITSCHKEWITSCH: MIGRACIÓN DE ANCIANOS 447 o o o 0) "D n "^ o d 0) 3 O I o d. 8 18 de julio agosto septiembre octubre fecha figura 2. Número de horas de observación por día y se proyectan a diario paso del rey y eiders comunes durante la suma- mer/Otoño 1996 en Point Barrow, Alaska. Pasajes proyectado incluir eiders no identificados que estaban divididos entre el Rey y eiders comunes en proporción al Rey y eiders comunes que fueron identificados. Octubre (Tabla 1). Hubo relativamente pocos grandes impulsos de eiders comunes entre el 10 de julio y
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. The Canadian field-naturalist. 2000 SUYDAM, QUAKENBUSH, DiCKSON, AND ObRITSCHKEWITSCH: MIGRATION OF ElDERS 447 o o o 0) "D n «^ o d 0) 3 O I o d. 8 18 July August September Date October Figure 2. Number of hours of observation per day and projected daily passage of King and Common eiders during sum- mer/fall 1996 at Point Barrow, Alaska. Projected passages include unidentified eiders that were divided between King and Common eiders in proportion to King and Common eiders that were identified. October (Table 1). There were relatively few large pulses of Common Eiders between 10 July and 30 August (Figure 2). Several large pulses occurred between 30 August and 5 September, between 21 and 26 September and on 7 October (Figure 4). Approximately 24% of the Common Eiders passed by 1 September and 76% had passed by 1 October. We observed 4841 Common Eiders closely enough to estimate the sex ratio. The percentage of male Common Eiders passing per day between 10 July and 19 August was highly variable (0 to 100%), but consisted mostly of males (mean = 87%, SE = 3%; Figure 4). After 19 August, the flocks changed to mostly females and juveniles until 23 September when the number of adult males again increased (Figure 4). Over the entire summer/fall migration, 67% (SE = 5%) of the Common Eiders were adult males. King Eiders comprised 90% of all the eiders we identified migrating past Point Barrow during sum- mer/fall 1996. Common Eiders made up a little less than 10% of the eider migration, and we also observed 42 Spectacled Eiders and 3 Steller's Eiders. Discussion Numbers of Eiders Our estimate of the number of King Eiders pass- ing Point Barrow during spring 1996 was similar to 1994 estimates (Suydam et al. 1997). Our summer/fall estimate, however, appears to be differ- ent even though confidence intervals overlaped. We observed a large number of eiders in September and October (113 000) whereas few were observed dur- ing the same time period in 1994 (13 000; Suydam
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