A breeding population of Aplomado Falcons (Falco femoralis) remains in the desert grasslands of Chihuahua, Mexico, despite their extirpation in the nearby southwestern United States. We monitored falcon reproduction at 35 territories during 1996-2002, a period of severe drought in the region. To test the influence of prey abundance and vegetation on falcon reproduction, we surveyed avian prey abundance through plot counts in 1998-2002 and characterized vegetation at each plot. Aplomado Falcon productivity declined from 1.57 fledglings per occupied territory in 1997 to 0.63 in 2002 at the larger of two study areas, a trend consistent with cumulative effects of consecutive years of low rainfall. Reproduction in the smaller area remained low throughout the study. Both productivity and incubation start date were significantly associated with prey bird abundance. Summer rain most likely influenced falcon reproduction by affecting seed abundance and therefore abundance of granivorous prey birds the following winter and spring. Falcons nested in open grasslands with sparse woody vegetation, an adaptation likely related to higher prey vulnerability and fewer predators. However, important prey birds were positively correlated with woody-plant density, which suggests that proximity of shrublands increased nest-site suitability.
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Vol. 121 • No. 4