Individual study: The effect of supplemental feeding on growth and mortality of male vs. female Tengmalm's owl Aegolius funereus nestlings, Västerbotten, northern Sweden
Hipkiss T., Hörnfeldt B., Eklund U. & Berlin S. (2002) Year-dependent sex-biased mortality in supplementary-fed Tengmalm's owl nestlings. Journal of Animal Ecology, 71, 693-699
Tengmalm's owl Aegolius funereus is a widespread species, primarily occurring in boreal forests of Northern Europe, Northern Asia and North America. Females are slightly larger than males. There are good and poor breeding years, dependant on vole (main prey) abundance. In coniferous forest in Västerbotten province, northern Sweden (64ºN, 20ºE), sex-specific growth and mortality in broods provided with supplemental food was compared with that in non-supplemented nests (controls).
The experiment was carried out in the breeding seasons of 1998 and 1999; nearly 300 nest boxes (20 x 20 cm base; 8.5 cm diameter entrance hole) were erected in trees close to roadsides at approx. 1-km intervals. Each was inspected every 3-4 weeks (March to June) and laying date of each clutch determined. Data were subsequently collected on clutch size, brood size, hatching order and mortality. Nestlings were sexed using molecular techniques and ringed for identification. In 1998, nestling weight and wing length was measured about every 2 days.
Every second brood was provided with additional food (dead mice every other day, averaging 100 g mice/day) during the nestling period (11 fed and 13 controls in 1998; 17 fed and 26 controls in 1999).