Dark or Short Nights: Differential Latitudinal Constraints in Nestling Provisioning Patterns of a Nocturnally Hunting Bird Species

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.

Effect of male provisioning on the parental behavior of female Boreal Owls Aegolius funereus

“…When this study was conducted, 2005 was an increasing year in the vole cycle with intermediate small-mammal abundances during the breeding season; the number of small mammals trapped was 13.3 ± 3.0 individuals/100 trap nights (bank voles Myodes glareolus 48.2%, sibling voles Microtus rossiaemeridionalis 16.9%, and common shrews Sorex araneus 16.3% were the most commonly trapped species) (Zárybnická et al 2012).…”

“…To avoid pseudoreplication resulting from repeated measurements at each nest, we used the identity of the nest as a variable with a random effect. Because the prey delivery rate increases with nestling age (Zárybnická et al 2012), we used the age of the oldest young as a covariate in both models. Furthermore, due to the negative correlation between feeding rates of males and females (Spearman's rank correlation: r s = −0.3, p < 0.001), we compared female feeding rates at successful and unsuccessful nests after controlling for the male feeding rate and vice versa, placing the other factor in the model as the covariate.…”

Non-native spruce plantations represent a suitable habitat for Tengmalm’s Owl (Aegolius funereus) in the Czech Republic, Central Europe

“…Basic breeding data were collected by inspecting all nest boxes during the onset of the breeding season (Zárybnická et al 2012). From 1999 to 2012, a total of 234 breeding attempts by Tengmalm's Owls were recorded, of which 119 were successful (i.e., at least one fledgling was produced), and the number of fledglings in each of 110 nest boxes was determined (Table 2).…”

“…(1) Food availability of small mammals in Central Europe is relatively stable from year to year, and there is simultaneously a higher diversity in the prey community (both mice and voles) compared to northern areas (Korpimäki and Hakkarainen 2012;Zárybnická et al 2012Zárybnická et al , 2013. (2) There is low competition with other vole-specialized predators (the abundances of other birds of prey and owls in the study area are very low: only Tawny Owl and Eurasian Pygmy Owl Glaucidium passerinum breed in densities of 1-2 pairs/100 km 2 within old beech and spruce forest remnants; Š t'astný et al …”

Influence of human footprint and sensory disturbances on night-time space use of an owl

“…Nocturnal predators, such as owls, rely on both sight and sound when hunting (Dice 1945, Kaufman 1974, requiring a minimum level of light and acoustic cues to detect prey. Accordingly, most owls are more active (Penteriani et al 2011, Frye & Jageman 2012 and bring more prey back to their young (Poulin & Todd 2006, Zárybnická et al 2012, during crepuscular periods. Therefore, artificial light could actually benefit owls if it extends the period of time that they can see prey at night.…”