Pine marten predation of common goldeneye nests: Effects of cavity age and habitat override any effect of microtine rodent abundance.

According to the alternative prey hypothesis (APH), the temporal synchrony in population fluctuations of microtine rodents and other small herbivores in boreal areas is caused by generalist predators with numerical and functional response to microtines, leading to an increased predation of prey alternative to microtines in the low phase of the microtine population fluctuations. The tree-climbing pine marten (Martes martes) is a food generalist that includes bird eggs among its alternative prey, also eggs of the cavity-nesting common goldeneye (Bucephala clangula). We used long-term data to test whether pine marten predation of goldeneye eggs in nest boxes varied as predicted by the APH. As a measure of microtine abundance at the time of nesting, we applied two measures. First, for goldeneye nests located <40 km from our microtine trapping site, we applied the trapping index of microtine rodents. Second, to also use data from nests located >40 km from our microtine trapping site, and from nests in years when trapping was not conducted, we used two proxies for the microtine abundance: whether boreal owls (Aegolius funereus) nested in any of our boxes <40 km from each goldeneye nest and the average clutch size of these boreal owls. The probability of predation of a goldeneye nest was independent of the microtine trapping index and independent of the proxies for microtine abundance. However, it increased with cavity age, taken as the number of nesting seasons elapsed since the actual nest box was installed, and declined with distance from habitat with forest canopy. The effect of cavity age confirms that the long-term spatial memory of pine marten is an important factor in the pattern of its predation on nests in tree cavities.

Hawk owl irruptions: spatial and temporal variation in rodent abundance drive push and pull dynamics.

Bird irruptions are thought to be triggered by low food availability in breeding areas, thereby causing emigration (push factor). However, few studies have tested whether emigrating individuals are drawn towards areas of high food availability (pull factor). The Northern hawk owl (Surnia ulula), a rodent specialist, occurs irruptively to southern parts of Fennoscandia. We analysed whether irruption size during 1980-2020 in southeastern Norway was related to rodent abundance at four sites 450-990 km to the north-northeast (potential source areas) and at two sites in southeastern Norway to test push and pull dynamics of irruptions. Irruptions occurred when rodent abundance in potential source areas were low, supporting the push hypothesis. High rodent abundance in potential source areas 1-2 years before irruptions suggested that irruptions were preceded by high reproduction. Upon arrival to southeastern Norway, hawk owls did not encounter high rodent abundance in their main habitat (boreal forest). However, hawk owls stayed in boreal forest in hills in years with higher microtine rodent abundance, but occurred in farmland areas in the lowlands when microtine rodents were less abundant. Use of lowlands coincided with higher than median numbers of wood mouse (Apodemus sylvaticus) for 87% of the hawk owls settling in the lowlands, thus suggesting support for the pull hypothesis. In conclusion, hawk owl irruptions to southern Fennoscandia were triggered by low food availability in northern areas (push factor), and appeared to be drawn by high food availability in southeastern Norway to some degree (pull factor).

Predation of boreal owl nests by pine martens in the boreal forest does not vary as predicted by the alternative prey hypothesis.

The alternative prey hypothesis (APH) states that temporally synchronous population fluctuations of microtine rodents and other small herbivores are caused by generalist predators that show functional and numerical responses to the abundance of microtines. This would lead to an increased predation of alternative prey in the low phase of the microtine population fluctuations. One candidate for such a predator is the tree-climbing pine marten (Martes martes), which includes bird eggs in its diet, among them eggs of the cavity-nesting boreal owl (Aegolius funereus). I used long-term data to test whether pine marten predation of boreal owl eggs in nest boxes varied as predicted by the APH. The probability of predation of owl nests situated < 45 km from a site where microtines were trapped in spring during four decades increased with microtine trapping index, which is opposite to the prediction from the APH. As the data set was limited to one nest per box, I extended it spatially and temporally using the clutch size of each boreal owl nest as a proxy for the actual microtine abundance at the site. The probability of nest predation increased with clutch size. However, the effects of microtine index and owl clutch size became non-significant when I controlled for habitat, and in particular cavity age, which had an overriding effect. The increase in predation probability with cavity age suggests that the long-term spatial memory of pine marten is an important factor in the pattern of its nest predation in tree cavities.

Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors.

In birds with bi-parental care, the provisioning link between prey capture and delivery to dependent offspring is regarded as often symmetric between the mates. However, in raptors, the larger female usually broods and feeds the nestlings, while the smaller male provides food for the family, assisted by the female in the latter part of the nestling period, if at all. Prey items are relatively large and often impossible for nestlings to handle without extended maternal assistance. We video-recorded prey delivery and handling in nests of a raptor with a wide diet, the Eurasian kestrel Falco tinnunculus, and simultaneously observed prey transfer from male to female outside the nest. The male selectively allocated larger items, in particular birds and larger mammals, to the female for further processing and feeding of nestlings, and smaller items, in particular lizards and smaller mammals, directly to the nestlings for unassisted feeding. Hence, from the video, the female appeared to have captured larger prey than the male, while in reality no difference existed. The female's size-biased interception of the male's prey provisioning line would maximize the male's foraging time, and maximize the female's control of the allocation of food between her own need and that of the offspring. The male would maximize his control of food allocation by capturing smaller prey. This conflict would select for larger dominant females and smaller energy-efficient males, and induce stronger selection the longer the female depends on the male for self-feeding, as a proportion of the offspring dependence period.

Female offspring desertion and male-only care increase with natural and experimental increase in food abundance.

In species with biparental care, one parent may escape the costs of parental care by deserting and leaving the partner to care for the offspring alone. A number of theoretical papers have suggested a link between uniparental offspring desertion and ecological factors, but empirical evidence is scarce. We investigated the relationship between uniparental desertion and food abundance in a natural population of Tengmalm's owl Aegolius funereus, both by means of a 5-year observational study and a 1-year experimental study. Parents and offspring were fitted with radio-transmitters in order to reveal the parental care strategy (i.e. care or desert) of individual parents, and to keep track of the broods post-fledging. We found that 70 per cent of the females from non-experimental nests deserted, while their partner continued to care for their joint offspring alone. Desertion rate was positively related to natural prey population densities and body reserves of the male partner. In response to food supplementation, a larger proportion of the females deserted, and females deserted the offspring at an earlier age. Offspring survival during the post-fledging period tended to be lower in deserted than in non-deserted broods. We argue that the most important benefit of deserting may be remating (sequential polyandry).

UV-Absorbing compounds in the aqueous humor from aquatic mammals and various non-mammalian vertebrates.

OBJECTIVE: To evaluate the absorbance of ultraviolet radiation (UVR) in the aqueous humor of various animal species in relation to the ambient radiation of their respective habitats, and to identify substances responsible for this absorbance. Representatives of all five classes (fish, amphibian, reptile, bird, and mammal) have been tested. METHODS: Absorbance was recorded using a spectrophotometer. The ascorbic and uric acid concentrations were determined by HPLC, and the amino acid profiles with an automatic analyzer. Screening for potential UV-absorbing substances was performed by HPLC and a total of 12 species were examined, 7 of them birds. RESULTS: UV-absorbing substances in the aqueous humor were proteins, tryptophan, tyrosine and ascorbic and uric acid. In addition, an unknown UV-absorbing component present in bird aqueous humor caused a high, red-shifted UV-absorbance spectrum, particularly in tentatively heavily exposed species such as goose when migrating at 10,000 m altitude. By comparison, the UV absorbance above the 288-nm wavelength was low in the aqueous humor of fish, frogs, aquatic mammals and two ground-living birds. The crocodile, whose aqueous humor contained significant amounts of both ascorbic and uric acid, revealed a concentration mechanism for ascorbic acid. CONCLUSIONS: The UV absorbance of aqueous humor varies considerably from one species to the next, and independent of class. It is noteworthy that the species being at highest risk for high-dose UV exposure, the migrating goose, showed the most red-shifted spectrum.

What causes extended lows in microtine cycles? : Analysis of fluctuations in sympatric shrew and microtine populations in Fennoscandia.

Synchronous declines of sympatric shrew and microtine populations in boreal regions of Fennoscandia have been claimed as evidence that predation deepens and extends the low phase of the microtine population cycles. Assuming that shrews and microtines have the same maximum intrinsic rate of increase, and that the predators kill the shrews and microtines that they encounter unselectively, this hypothesis predicts that sympatric shrew and microtine populations will decline and increase simultaneously. In this study conducted at 60°56'N in southeast Norway, as well as in 13 of 15 other studies at sites in Fennoscandia, the shrew populations declined simultaneously with the sympatric microtine populations, but recovered sooner. The shrew-microtine ratio among animals trapped peaked about 2 years after the peak in the microtine population. The same pattern was found in the diet of raptors in Fennoscandia. Since the maximum intrinsic rate of increase does not seem to be higher in shrews than in microtines, the pattern suggests that either predation is not causing the extended lows in the microtine populations, or that the proportion of predators that selectively kill microtines fluctuates during the microtine cycle. The most likely candidates for selective killers are weasels.