American Kestrels (Falco sparverius) show territoriality on both the breeding grounds and wintering grounds. Kestrels also utilize a variety of different structures for roosting on their winter territories. In previous work, Mills hypothesized that the availability of a roost is an important territorial requirement for wintering kestrels (Mills 1975, Wilson Bulletin 87:241–247). Here I present spatial data for roost site use in relation to diurnal foraging territories for 20 color-marked wintering kestrels in South Texas agricultural areas. The average diurnal territory size for these 20 kestrels was 522.9 ± 60.3 m (maximum linear distance among all observations). I observed 14 of 20 kestrels roosted ≤142 m from their territory, but 6 of 20 kestrels roosted ≥275 m from their territory. Three of the 6 kestrels traveled at least 1000 m from the nearest recorded diurnal location. Although the availability of a roost does appear to be an important territorial requirement for kestrels, my results suggest that roost sites are not always within or adjacent to the diurnal territories. Having the ability to roost away from the diurnal territories likely allows kestrels to utilize foraging habitat that is devoid of suitable roosts, but kestrels also may experience a fitness tradeoff between traveling away from the diurnal territory to roost and staying on the diurnal territory in a less-safe roost site.

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