Experimentally disentangling intrinsic and extrinsic drivers of natal dispersal in a nocturnal raptor.

Equivocal knowledge of the phase-specific drivers of natal dispersal remains a major deficit in understanding causes and consequences of dispersal and thus, spatial dynamics within and between populations. We performed a field experiment combining partial cross-fostering of nestlings and nestling food supplementation in little owls (Athene noctua). This approach disentangled the effect of nestling origin from the effect of the rearing environment on dispersal behaviour, while simultaneously investigating the effect of food availability in the rearing environment. We radio-tracked fledglings to quantify the timing of pre-emigration forays and emigration, foray and transfer duration, and the dispersal distances. Dispersal characteristics of the pre-emigration phase were affected by the rearing environment rather than by the origin of nestlings. In food-poor habitats, supplemented individuals emigrated later than unsupplemented individuals. By contrast, transfer duration and distance were influenced by the birds' origin rather than by their rearing environment. We found no correlation between timing of emigration and transfer duration or distance. We conclude that food supply to the nestlings and other characteristics of the rearing environment modulate the timing of emigration, while innate traits associated with the nestling origin affect the transfer phases after emigration. The dispersal behaviours of juveniles prior and after emigration, therefore, were related to different determinants, and are suggested to form different life-history traits.

Juvenile survival of little owls decreases with snow cover.

Global environmental changes are associated with warmer average temperatures and more extreme weather events, potentially affecting wildlife population dynamics by altering demographic processes. Extreme weather events can reduce food resources and survival in all seasons of the year. Estimates of season-specific survival probabilities are therefore crucial to understand the moderating effect of extreme events on annual mortality. Here, we analysed survival probabilities of 307 radio-tracked juvenile little owls (Athene noctua) over two-week periods from fledging to their first breeding attempt in the following spring to assess the contribution of extreme weather events. Survival probabilities were typically lowest during the first weeks after fledging in summer but were moderated by seasonal extremes in winter. The duration of snow cover in winter had a strong negative effect on survival probability, while being food supplemented during the nestling stage increased survival during the first weeks after fledging in summer and ultimately led to a larger proportion of birds surviving the first year. Overall annual survival probability over the first year varied by 34.3% between 0.117 (95% credible interval 0.052-0.223) and 0.178 (0.097-0.293) depending on the severity of the winter, and was as high as 0.233 (0.127-0.373) for food-supplemented fledglings. In years with mild winters, the season with the lowest survival was the summer post-fledging period (0.508; 0.428-0.594), but in years with extensive snow cover the winter was the season with the lowest survival (0.481; 0.337-0.626). We therefore show that extreme weather events occurring in a particular season reduced the proportion of first-year survivors. Increasing extreme weather events can moderate seasonal survival probability through altering food supply of juvenile little owls either during the nestling period or in winter, with similarly large effects on annual survival and the viability of populations.

Periophthalmus variabilis (Teleostei: Gobiidae: Oxudercinae), a valid species of mudskipper, and a re-diagnosis of Periophthalmus novemradiatus.

The current concept of Periophthalmus novemradiatus (Hamilton, 1822) includes two species. Procurement of fresh material and examination of extant type specimens revealed that P. novemradiatus differs from P. variabilis Eggert, 1935, primarily in the number of anal fin rays (I, 12 or 13 vs. I, 10-12); the lateral scale series (62-67 vs. 48-60); the extent of fusion of the pelvic fins (56.0-90.0% vs. 32.0-48.8%); and the patterns of the first and second dorsal fins.