Abundance of owls and Bramblings Fringilla montifringilla in relation to mast seeding in south-eastern Sweden

Abstract

We investigated the relationship between mast seeding in beech Fagus sylvaticus and oak Quercus spp., and the occurrence of the rodent-dependent predators Tengmalm’s Owl Aegolius funereus, Long-eared Owl Asio otus and Tawny Owl Strix aluco in south-eastern Sweden (Blekinge) 1992—2000. We also compared available data on seed production with data on winter flocks of Brambling Fringilla montifringilla, mainly depending on beech seeds as winter food. Years of good beech mast production were followed by years of high frequency of Tengmalm’s Owl and Long-eared Owl, indicating a rapid numerical response to improved food conditions. The Tawny Owl did not respond in the same way, but tended to reach high densities two years after mast seeding. These results suggest that mast seed production may influence the populations of rodent-feeding predators, and that the numerical response may be very rapid in potentially nomadic specialist predators. This was indicated particularly for the Tengmalm´s Owls arriving in Blekinge rather late in spring of 1999. Winters following good beech mast production normally held large numbers of Bramblings. However Bramblings were absent in Blekinge during the winter of 1992/93 despite the very good year for beechnut production in 1992. Thus, low number of Bramblings was no reliable indication for low numbers of owls the following spring.

Widespread breakdown in masting in European beech due to rising summer temperatures

“…Peak seed crops represent a resource pulse to a wide variety of seed consumers and their predators. For instance, beech mast has been associated with migration of seed consuming birds like bramblings that depend on mast as a winter food (Jenni, 1987;Lithner & Jönsson, 2002), increased rodent densities (Bregnard et al, 2021;Reil et al, 2016), as well as higher densities of secondary consumers such as owls and pine martens (Lithner & Jönsson, 2002;Maag et al, 2024). Birds may also suffer from increased densities of rodents, for instance through higher nest predation rates (Elliott & Kemp, 2016;Szymkowiak & Thomson, 2019).…”

MASTREE+: Time‐series of plant reproductive effort from six continents

“…The pulses of resources associated with large reproductive events are key drivers of the population dynamics of seed‐eating insects, mammals and birds, with cascading impacts through ecosystems (Bouchard et al, 2018; Kanamori et al, 2017; Selonen et al, 2016). Time‐series in MASTREE+ can be combined with existing long time‐series of animal populations and behaviour to identify the drivers of population dynamics, both in seed‐dependent species and further down the trophic cascade (Kleef & Wijsman, 2015; Lithner & Jönsson, 2002). Where species are well replicated in MASTREE+, the spatial synchrony of masting can also be quantified, allowing researchers to determine where regional estimates of masting can be appropriately used as indicators of local variability in seed or fruit availability.…”