We recorded parental provisioning rates of Varied Tits (Poecile various) at different altitudes (n = 17, 7, and 11 nest boxes at 300, 900, and 1,400 m, respectively) to examine if males and females cooperate in response to increased provisioning pressure due to nutritional demands of nestlings. Females provisioned nestlings more than males irrespective of altitude. Provisioning rates of males and females tended to increase with elevation, but the increase was greater for males. Provisioning was low early in the nestling period, and gradually increased reaching a peak between 9 and 10 days after hatching. The provisioning rate of females at the peak provisioning period (8–10 days) did not increase markedly at any altitude. Provisioning by males during the peak period increased and they contributed more during this period than at other times. The provisioning rate of males increased linearly with elevation. The provisioning rate of female Varied Tits also increased with elevation, but the increment ratio was lower than that of males. Changes in provisioning rates with elevation may be due to the need to invest more in parental care under unfavorable environmental conditions. Parents at high altitudes experience more difficulty provisioning, not only because of nestling growth, but also because provisioning is required more often. Thus, increased provisioning by males, which have a lower provisioning frequency relative to females, may be an investment to reduce foraging pressure on females and to ensure survival of nestlings.
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Vol. 123 • No. 2