Predation is a major selective force in the evolution of both morphological and behavioural characters in animals. The flexibility in behavioural traits according to perceived predation risk has received much attention in recent years. Since resources often are limited, prey animals face a trade-off between the risk of predation and other energy demanding activities in life. This thesis investigates the effect of predation risk on some behavioural traits in birds. The two main topics under study are predation risk and niche use in the genus Parus, and mass-dependent predation risk in birds.
In a field study investigating hunting behaviour in relation to prey choice in wild pygmy owls, Glaucidium passerinum, I found that when hunting for birds, owls used a hunting strategy attacking the birds with a height advantage from ambush. This observation suggests that birds foraging high in the tree and close to the trunk, sheltered by branches, face a lower predation risk than birds foraging further out and lower down in the tree. In a laboratory experiment with wild caught willow tits, Parus montanus, I investigated the use of foraging sites in the tree according to perceived predation risk. The tits adjusted foraging behaviour to the present predation risk by choosing more sheltered foraging sites after the presentation of a model raptor. Since the hunting strategy of pygmy owls lead to selective predation of birds foraging in the outer parts of branches, the pygmy owl might acts as a keystone predator enabling coexistence of tit species that are separated in their foraging niches. The coexistence of willow tits, crested tits, Parus cristatus, and coal tits, P. ater, could be a result of a two-way asymmetric interaction between species competing for food and predator safe foraging sites.
Laboratory experiments investigating take-off ability in relation to diurnal body mass increase ((8%) in wintering willow tits and great tits, Parus major, showed no measurable effect on flight ability, indicating that the small energy reserves accumulated during a day in wintering tits do not increase predation risk as a consequence of reduced take-off ability. However in a similar experiment studying migratory fuel load (up to 59% of lean body mass) in blackcaps, Sylvia atricapilla, a reduction in both velocity and angle of ascent was found suggesting that the large fuel loads needed for migration place the birds at an increased risk of predation. The study of alarmed take-off flights in great tits further suggests that great tits adjust take-off strategy to the attack trajectory of an attacking raptor, indicating that the trade-off between velocity and angle of ascent during take-off is affected by the nature of the predator attack.
Stockholm: Department of Zoology, Stockholm University , 1998. , 35 p.