Abstract
In diurnal bird species, individuals breeding at high latitudes have larger broods than at lower latitudes, which has been linked to differences in the daily time available for foraging. However, it remains unclear how latitude is linked with parental investment in nocturnal species. Here, we investigate nestling provisioning rates of male Tengmalm's owls in two populations at different latitudes (Czech Republic 50°N; Finland 63°N) with the help of cameras integrated into nest boxes. Clutch sizes were smaller in the Czech population (CZ: 5.1±0.1; FIN: 6.6±0.1), but given the higher nestling mortality in the Finnish population, the number of fledglings did not differ between the two populations (CZ: 3.5±0.3; FIN: 3.9±0.2). Nestling provisioning patterns varied within days, over the reproductive season and between the two sites. Males delivered most food at dusk and dawn, having peak delivery rates at sun angles of −11° to −15° at both sites, and males increased the prey delivery rates with higher nestling requirements. Given the longer nights during summer in the Czech Republic compared to Finland, Czech males only showed a small shift in their delivery peak during the night from −17° in April to −14° in July. In contrast, Finnish males shifted their peak of prey delivery from −11° in April to −1° in July. Consequently, Czech males had a longer hunting time per night around midsummer when feeding young (360 min) than Finnish males (270 min). This suggests that nocturnal owl species in northern populations are constrained by the short nights during the breeding season, which can limit the number of young they can raise. Moreover, owls in northern populations are additionally constrained through the unpredictable changes in food availability between years, and both these factors are likely to influence the reproductive investment between populations.
Highlights
In many animals, parents provide dependent offspring with food to ensure proper development [1,2]
In the Finnish site, bank voles Myodes glareolus were the most abundant prey species (48.2%), while sibling voles Microtus rossiaemeridionalis (16.9%) and common shrews Sorex araneus (16.3%) were less abundant. Since these species differ in their body mass with shrews being much smaller than voles and mice, the average mass of prey was higher in the Czech population than in the Finnish population (19.960.5 g; GLMM: F = 15.18, p = 0.0001; including year as random factor)
Basic breeding data Basic breeding data were collected from 39 nests in the Czech population (15 in 2004, 24 in 2006), and from 75 nests in the Finnish population
Summary
Parents provide dependent offspring with food to ensure proper development [1,2]. The reproductive investment often varies between populations. Ashmole [9] hypothesized that the reproductive investment increases with latitude as a consequence of higher winter mortality, which reduces the number of competitors during the breeding season. While comparative studies confirmed this pattern [10], a field study suggested that a key factor responsible for within species differences in parental investment across latitudes is day length [11]. The energy expenditure of great tits (Parus major) indicates that parents breeding at low latitudes are time constrained, where the short daylight period limits the time available for foraging. Great tits breeding at high latitudes have more time available for foraging and can raise larger broods [11]. Individuals breeding at high latitude are likely to be more time constrained than individuals breeding at lower latitudes, affecting the reproductive decisions, but to our knowledge this hypothesis remains so far untested
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