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

<div><p>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.</p> </div

Change in hunting times for males throughout the breeding season in relation to the sun angle, based on prey delivery data in the Czech (filled squares) and Finnish population (open squares) (box: 5–95%, whiskers: non-outlier range, point: median).

<p>Change in hunting times for males throughout the breeding season in relation to the sun angle, based on prey delivery data in the Czech (filled squares) and Finnish population (open squares) (box: 5–95%, whiskers: non-outlier range, point: median).</p

Mean prey mass (weight (g) ±SE) delivered during 30 min periods in the Czech (filled squares) and Finnish population (open squares) in relation to the nestling requirement (nestling number×nestling age).

<p>The reduction of the feeding rate with older nestlings is a consequence of that broods fledge asynchronously.</p

Statistical analyses (Generalized Linear Mixed Models: GLIMMIX module in SAS 9.3) of factors affecting the (a) number of prey items (Poisson distribution, log-link), and (b) the average prey mass delivered to nest within half-hour intervals (Lognormal distribution, identity-link) in the Czech population (Ore Mountains; 12 nest boxes; number of days observed (mean ± SE): 25.8±2.2) and in the Finnish population (Kauhava region; 9 nest boxes; number of days observed: 22.9±2.4).

<p>Box identity nested within country was added as a random factor to the model to control for the repeated measurement at the same nest boxes.</p><p>1: sun angle (in degrees) in relation to the horizon. 0° = sunrise/sunset.</p><p>2: calculated as the product of number of nestling×age of oldest nestling.</p><p>3: duration of the night: minutes between sunset and sunrise.</p

Night length (time between sunset and sunrise) at the two sites during the breeding season and basic breeding data (mean ± SE) of the Finnish and Czech populations of Tengmalm's owls.

<p>The statistical comparisons are done with General Linear Mixed Models (Normal distribution, identity-link) (date egg laying, fledging date), or with a Generalised Linear Mixed Model (Poisson distribution, log-link) (clutch size, number of nestlings, number of fledglings, brood reduction). All models included year as random factor. Brood reduction was calculated as the number of nestlings which disappeared from a nest box before fledgling date (ring or dead nestling found in nest box).</p

Mean (±SE) number of prey items delivered during 30 min periods in the Czech (filled squares) and Finnish population (open squares) in relation to the nestling requirement (nestling number×nestling age).

<p>The reduction of the feeding rate with older nestlings is a consequence of that broods fledge asynchronously.</p

Mean (±SE) number of prey items delivered during 30 min periods in the Czech (filled squares) and the Finnish population (open squares) against the sun angle in relation to the horizon (in degrees, 0 = sunrise/sunset, twl. = twilight).

<p>Mean (±SE) number of prey items delivered during 30 min periods in the Czech (filled squares) and the Finnish population (open squares) against the sun angle in relation to the horizon (in degrees, 0 = sunrise/sunset, twl. = twilight).</p

Mean prey mass (weight (g) ±SE) delivered during 30 min periods in the Czech (filled squares) and the Finnish population (open squares) against the sun angle in relation to the horizon (in degrees, 0 = sunrise/sunset, twl. = twilight).

<p>Mean prey mass (weight (g) ±SE) delivered during 30 min periods in the Czech (filled squares) and the Finnish population (open squares) against the sun angle in relation to the horizon (in degrees, 0 = sunrise/sunset, twl. = twilight).</p

Candidate model set of factors affecting the (a) number of prey items (Poisson distribution, log-link), and (b) the average prey mass delivered to nest within half-hour intervals (Lognormal distribution, identity-link).

<p>s (sun angle), r (nestling requirements), n (night length), c (country), p (prey availability).</p><p>ΔAICc = difference in AICc relative to the best model; <i>wi</i> = ΔAICc weight of the model. All models with AICc weight (<i>wi</i>)≥0.001 are shown.</p

Nest box occupancy data for red squirrels

Data describes nest box occupancy of red squirrels. 1= presence, 0=absence. BCODE= name of the box. ID= unique combination of box name and census year. Goshawk and Ural owl columns describe modelled predation risk from these predators at the nest box site, see methods in research article. h1...h27 refer to land use classes as in Table 1 of the article. _300 and _2500 m refer to the area of each land use class within 300 meters and 2.5 kilometers from the nest box in square meters. Ending 's' (h8_300ms) refers to the standardized area. TimePeriod describes the slightly different detection probability before and since 2006