Geir A. Sonerud

The effect of a natural reduction of red fox Vulpes vulpes on small game hunting bags in Norway

An epizootic of sarcoptic mange among red foxes Vulpes vulpes reached central Norway in 1976, and by 1986 it had spread to the whole country, resulting in a severe decline in the red fox population. We analyse the change in the hunting bags of the predator species red fox and pine marten Martes martes, and the prey species capercaillie Tetrao urogallus, black grouse Tetrao tetrix, mountain hare Lepus timidus and willow grouse Lagopus I. lagopus from the period prior to and during the mange epizootic. The data are gathered from publications by Statistics Norway and are based on answers from more than 5,000 hunters yearly. On the national level, the hunting bags of capercaillie, black grouse, mountain hare and pine marten were significantly negatively correlated with that of the red fox. When the time series at the national level were detrended, there were positive correlations between the hunting bags of all species without time lag, except that of pine marten, which lagged one year behind the other species. At the local level there were negative correlations between the hunting bag of red fox and those of the small game species except for willow grouse. The study confirms that the red fox is a keystone predator in Scandinavia.

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 Tengmalms 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).

Experimental increase in food supply influences the outcome of within-family conflicts in Tengmalm’s owl

Within a family there are conflicts of interest between parents and offspring, and between male and female parents, over the supply of parental care. The observed pattern of parental care is the outcome of negotiations within the family, and may be influenced by environmental factors such as food abundance. We experimentally increased food supply to ten Tengmalm’s owl (Aegolius funereus) nests from hatching to fledging, mimicking natural cached prey. Ten un-supplemented nests served as controls. Parents and offspring were fitted with radio-tags. Food provisioning by parents was measured both in the (1) mid- and (2) late nestling stage and in the (3) early and (4) late post-fledging stage. In response to food supplementation, both males and females reduced food provisioning, but the effect was more pronounced in females. Females generally contributed much less to food provisioning than males, and food supplementation increased the difference between the sexes. Mass loss during the brooding stage was substantially lower for supplemented than for control females. Food supplementation did not improve offspring survival, and had no effect on body measurements of nestlings. In conclusion, parents of both sexes used the increased food supply to reduce the costs of caring for their current offspring, but females responded more strongly than males.

Sex roles during post-fledging care in birds: female Tengmalm's Owls contribute little to food provisioning

AbstractPost-fledging care constitutes a large proportion of the total costs of parental care in many bird species. Despite being recognized as of critical importance to the survival of the offspring and their recruitment into the breeding population, post-fledging care, including the relative contribution by male and female parents, is under-studied. In this study, we quantified food provisioning (prey deliveries) by male and female Tengmalm’s Owl (Aegolius funereus) parents to their offspring both in the nestling and the post-fledging stages, in years of differing natural prey abundance. Parents and at least one offspring in 26 families were fitted with radio-transmitters. Male parents exhibited higher delivery rates than did females throughout the late nestling and post-fledging stages, but the intersexual difference was smaller in broods that were not deserted by the female at any stage. The female deserted her mate and offspring at some stage in 63% of the broods. Overall, deserted males delivered more prey to their offspring than did non-deserted males. Delivery rates were generally higher post-fledging. Prey delivery rates differed among years, and were highest in low vole years (probably because of smaller prey items), intermediate in peak vole years, and lowest in years of vole increase. Prey delivery rates increased with increasing brood size for both sexes, but the response was stronger in females. We suggest that female Tengmalm’s Owls contribute less than males because they are in a position to decide on their level of provisioning effort first, and because of the potential for re-mating after deserting the first brood.ZusammenfassungGeschlechterrollen bei der Fürsorge für Flügglinge: Weibliche Raufußkäuze tragen wenig zur Futterversorgung bei Die Fürsorge für Flügglinge macht bei vielen Vogelarten einen großen Anteil der Gesamtkosten der Brutpflege aus. Obwohl bekannt ist, dass die Fürsorge für Flügglinge von entscheidender Bedeutung für das Überleben der Nachkommen und ihre Rekrutierung in die Brutpopulation ist, ist dieser Aspekt der Brutpflege, einschließlich des relativen Beitrags von Männchen und Weibchen, nicht gut untersucht. In dieser Studie haben wir in Jahren mit unterschiedlicher natürlicher Beuteabundanz quantifiziert, wie männliche und weibliche Raufußkäuze (Aegolius funereus) ihre Nachkommen mit Futter versorgten, sowohl im Nestlingsstadium als auch nach dem Ausfliegen. Die Elternvögel und mindestens ein Jungvogel aus 26 Familien wurden mit Radiosendern ausgestattet. Männchen wiesen während des späten Nestlingsstadiums und nach dem Ausfliegen höhere Fütterraten auf als Weibchen, aber der Unterschied zwischen den Geschlechtern war geringer in Bruten, die nicht zu irgendeinem Zeitpunkt vom Weibchen verlassen wurden. Das Weibchen verließ irgendwann seinen Partner und seine Nachkommen in 63% der Bruten. Insgesamt brachten verlassene Männchen ihren Nachkommen mehr Beute als nicht verlassene. Die Fütterraten waren nach dem Ausfliegen generell höher. Sie unterschieden sich zwischen den Jahren und waren am höchsten in Jahren mit geringer Wühlmausabundanz (wahrscheinlich aufgrund kleinerer Beutestücke), mittel in Jahren mit höchster Wühlmausabundanz und niedrig in Jahren mit zunehmender Wühlmausabundanz. Die Fütterraten nahmen mit ansteigender Brutgröße für beide Geschlechter zu, doch die Antwort war bei Weibchen stärker ausgeprägt. Wir schlagen vor, dass weibliche Raufußkäuze einen geringeren Beitrag leisten als Männchen, da sie in der Lage sind, über das Ausmaß ihres Fütterungsaufwands zuerst zu entscheiden, und für sie die Möglichkeit besteht, sich nach Verlassen der ersten Brut wieder zu verpaaren.

Prey Delivery Rates as Estimates of Prey Consumption by Eurasian Kestrel Falco tinnunculus Nestlings

Steen R., Løw L.M., Sonerud G.A., Selås V. & Slagsvold T. 2011. Prey delivery rates as estimates of prey consumption by Eurasian Kestrel Falco tinnunculus nestlings. Ardea 99: 1–8. In altricial birds the type of prey selected by parents for their nestlings may affect the allocation of time and energy spent on hunting, preparing prey and feeding the nestlings, which in turn may affect the rate of provisioning. Raptors take relatively large prey items, which facilitates the quantification of rates of prey items and prey mass delivered to nestlings. Estimates of rates of prey delivery in raptors are nevertheless few and have been based on direct observations from a hide in combination with analyses of prey remnants and regurgitated pellets. To obtain better estimates we video monitored prey deliveries at 55 nests of Eurasian Kestrels Falco tinnunculus. Of the 2282 prey items recorded, voles were most abundant by number, followed by birds, shrews and lizards, while insects and frogs were rare. An average brood of 4.3 nestlings was estimated to consume 18.3 g/h, hence a nestling consumed on average 4.2 g/h. This is equivalent to 67.8 g/d, given an average daily activity period of 16.1 h. The estimated delivery rate of prey items required to feed an average brood in our study was 91 per h if the kestrels had provided only insects, and 3.4, 1.9, 0.83 and 0.52 if they had provided only lizards, shrews, voles or birds, - respectively. This corresponds to one prey delivery per 40 s if feeding solely on insects and one per 18, 32, 75 and 120 min if feeding solely on lizards, shrews, voles or birds, respectively. We argue that kestrels in the boreal forest would be unable to raise an average brood solely on insects or lizards, unlikely to do so solely on shrews, but able to do so solely on voles in a vole peak year.

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.

The feeding constraint hypothesis: prey preparation as a function of nestling age and prey mass in the Eurasian kestrel

The feeding constraint hypothesis states that an inability of young nestlings to ingest prey included in the diet of older nestlings and adult birds affects the evolution of parental behaviour, and predicts that the extent of prey preparation would increase with prey size and decrease with nestling age. In the Eurasian kestrel, Falco tinnunculus, a small raptor with a wide diet, parents often prepare prey prior to delivery at the nest, most notably by decapitation. We studied this behaviour by video monitoring prey deliveries at 29 nests for a total of about 200 days over 3 years. The probability of a prey item being decapitated prior to delivery differed between prey types and prey sizes; none of the insects or common lizards, Zootoca vivipara, and almost none of the shrews (Soricidae) were decapitated, whereas voles (Microtinae) and birds were commonly decapitated. For voles the probability of being decapitated decreased with nestling age, which supports the feeding constraint hypothesis because the nestlings’ gape size limit and swallowing capacity would increase with age. For both voles and birds the probability of being decapitated increased with prey body mass, suggesting that kestrel nestlings may be unable to swallow, digest or egest skulls from larger prey. We suggest that the extent to which kestrel parents decapitate prey prior to delivery is an effect of their nestlings’ age-dependent swallowing capacity and that the age of the nestlings therefore imposes constraints on the kestrel parents’ foraging behaviour.

Moose recruitment in relation to bilberry production and bank vole numbers along a summer temperature gradient in Norway

The plant stress hypothesis states that plant stress factors other than herbivory improve herbivore performance due to changes in the content of nutritive or defensive compounds in the plants. In Norway, the bilberry (Vaccinium myrtillus) is important forage for the bank vole (Myodes glareolus) in winter and for the moose (Alces alces) in summer and autumn. The observed peaks in bank vole numbers after years with high production of bilberries are suggested to be caused by increased winter survival of bank voles due to improved forage quality. High production of bilberries should also lead to higher recruitment rates in moose in the following year. We predict, however, that there is an increasing tendency for a 1-year delay of moose indices relative to vole indices with decreasing summer temperatures, because low temperatures prolong the period needed by plants to recover in the vole peak year, and thus positively affect moose reproduction also in the succeeding year. In eight out of nine counties in south-eastern Norway, there was a positive relationship between the number of calves observed per female moose during hunting and a bilberry seed production index or an autumn bank vole population index. When dividing the study area into regions, there was a negative relationship between a moose-vole time-lag index and the mean summer temperature of the region. These patterns suggest that annual fluctuations in the production of bilberries affect forage quality, but that the effect on moose reproduction also depends on summer temperatures.

Delivery of Common Lizards (Zootoca (Lacerta) vivipara) to nests of Eurasian Kestrels (Falco tinnunculus) determined by solar height and ambient temperature

Recent development in video monitoring has allowed collecting of data on prey deliveries at raptor nests, and this offers an opportunity to relate prey selection to short-term changes in environmental factors on a scale of hours. Whereas raptors may specialize on ectothermic prey at southern latitudes, only some generalist raptors may include such prey in their diet at northern latitudes. In particular, at northern latitudes the activity pattern of ectothermic reptiles is strongly dependent on the prevailing weather conditions. To test whether this dependence affects the exposure of reptiles to raptors, we used video recording of prey deliveries at nests of the Eurasian Kestrel (Falco tinnunculus L., 1758) at 61°N in Norway, where the Common Lizard (Zootoca (Lacerta) vivipara (Jacquin, 1787)) is the only lizard available to kestrels. The probability that a prey item delivered at a kestrel nest was a lizard increased towards midday and also increased independently with increasing ambient temperature, which...

A Bank Vole (Myodes glareolus) with Complete Leucism Captured by a Eurasian Kestrel (Falco tinnunculus) in Norway

Small mammals with leucism are very rarely recorded, most likely because the probability of surviving to maturity would be very low for abnormal, white individuals. Here we report a record of a bank vole (Myodes glareolus) with complete leucism delivered to a nest of the Eurasian kestrel (Falco tinnunculus) in the boreal forest in SE Norway.