. The Canadian field-naturalist. Natural history. 478 The Canadian Field-Naturalist Vol. 115 Table 1 was from a single nest box. This box was visited daily from hatching of the first owlet on 11 May until fledging on 15 June. No fresh prey items were found in this box after 28 May, although obvi- ously, the attendant male continued to deliver food to the box. As the dietary needs of the brooding female and her five young were high, prey items were probably consumed soon after the prey was delivered by the male. Although the primary reason for daily nest visits was to gather growth data on the
. The Canadian field-naturalist. Natural history. 478 The Canadian Field-Naturalist Vol. 115 Table 1 was from a single nest box. This box was visited daily from hatching of the first owlet on 11 May until fledging on 15 June. No fresh prey items were found in this box after 28 May, although obvi- ously, the attendant male continued to deliver food to the box. As the dietary needs of the brooding female and her five young were high, prey items were probably consumed soon after the prey was delivered by the male. Although the primary reason for daily nest visits was to gather growth data on the hatchlings (Whitman, in preparation), prey occurrence was recorded. During both 1996 and 1997, diet data were collected from 14 active nest boxes. As with 1995, fresh prey found in boxes during 1996 and 1997 declined during later brood- ing periods. When frequency of occurrence of prey items was analyzed by 5-day periods during 1996, incidence of Northern Red-backed Voles (Clethrionomys rutilus) was relatively high during early incubation (late April), then declined. During the last half of May and early June, however, their incidence was again quite high (Figure 1). During the late-April through mid-May period when incidence of Northern Red- backed Voles in the diet declined, their occurrence was replaced by a higher incidence of Meadow Voles {Microtus pennsylvanicus). Other prey items made up a relatively small proportion of the diet throughout the nesting period. Discussion In North America, data from three investigations indicates that Boreal Owls rely heavily on microtines. In Canada, Bondrup-Nielsen (1978) found 76% of prey items were Clethrionomys or Microtus. In Colorado, these two genera made up 79% of the diet (Palmer 1986), and in Idaho, Hayward et al. (1993) indicated microtines made up 55% of the prey. Not unlike our observations, birds made up less than 10% of the diet in all previous North American studies. In western Finland, Korpimaki (1986, 1988) and Korpimaki
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