Recent wildlife population declines are usually attributed to multiple sources such as global climate
change and habitat loss and degradation inducing decreased food supply. However, interactive
efects of fuctuations in abundance of main foods and weather conditions on population densities
and reproductive success have been studied rarely. We analysed long-term (1973–2018) data on
Tengmalm’s owl (Aegolius funereus) and the infuence of prey abundance and weather on breeding
densities and reproductive success in western Finland. We found that fedgling production per
breeding attempt declined and laying date of the owl population delayed during the period between
1973 and 2018. The breeding density of the owl population decreased with increasing temperature in
winter (October–March), fedgling production increased with increasing temperature and precipitation
in spring (April–June), whereas the initiation of egg-laying was delayed with increasing depth of snow
cover in late winter (January–March). The decreasing trend of fedgling production, which was mainly
due to starvation of ofspring, was an important factor contributing to the long-term decline of the
Tengmalm’s owl study population. Milder and more humid spring and early summer temperatures due
to global warming were not able to compensate for lowered ofspring production of owls. The main
reason for low productivity is probably loss and degradation of mature and old-growth forests due to
clear-felling which results in loss of coverage of prime habitat for main (bank voles) and alternative
foods (small birds) of owls inducing lack of food, and refuges against predators of Tengmalm’s owls.
This interpretation was also supported by the delayed start of egg-laying during the study period
although ambient temperatures increased prior to and during the egg-laying period.