Year-dependent sex-biased mortality in supplementary-fed Tengmalm's owl nestlings

  • Published source details 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).


Natural food supply: Whilst spring vole abundance was similar in both study years, 1998 had a less severe decline in abundance over the previous winter and a greater increase during the summer than 1999 (inferring that natural food was more plentiful in 1998).
Weight and growth: Rate of weight gain and wing growth was similar between sexes, and there was no effect of feeding. In 1998, female nestlings attained a 5% higher weight than males. Control females (average 137.6 g) were 5% heavier than control males (130.8g). Fed nestlings attained a higher weight than controls: fed males (142.1 g) were 8% heavier than control males; fed females (155.1 g) were 11% heavier than control females.
Mortality: Overall nestling mortality and male mortality were lower (i.e. survival higher) in fed broods than controls in 1999 (when natural food supply lower) only. Female mortality was unaffected by supplemental feeding in both years. Nestling mortality in each year for each sex was:
Males: 1998 - controls 29%, fed 25%; 1999 - controls 9%, fed 11%.
Females: 1998 - controls 15%, fed 28%; 1999 - controls 26%, fed 2%.
The larger female nestlings (which require relatively more food) did not appear disadvantaged, even in 1999 when natural food supply was low, perhaps out-competing their smaller male siblings, leading to increased male mortality when food is limited.
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