Owl Predation Research Articles

Ferruginous Pygmy-owl (Glaucidium brasilianum) predation on a mobbing Fork-tailed Flycatcher (Tyrannus savana) in south-east Brazil

I observed, and documented in detail, Ferruginous Pygmy Owl (Glaucidium brasilianum) predation on a mobbing Fork-tailed Flycatcher (Tyrannus savanna) in south-east Brazil. Events described in this account are supported with original photos. In addition to the predation event, I list 12 mobbing bird species that were alarmed by the presence of this raptor in the study area, including Trochilidae (Eupetomena macroura), Formicariidae (Formicivora rufa), Tyrannidae (Camptostoma obsoletum, Elaenia cristata, E. flavogaster, E. chiriquensis, T. savana, Myiarchus tyrannulus), Turdidae (Turdus leucomelas, T. amaurochalinus, T. rufiventris) and Emberizidae (Coryphospingus cucullatus). The Fork-tailed Flycatcher may represent 35-76.8% of the pygmy-owl body mass (41-80 g), which supports early reports about the predation on relatively large prey by this owl species. Although most authors have suggested that mobbing birds are subject to a low predation risk, this report and others confirmed that these events are not negligible and can be deadly dangerous to the mobbers, and conversely profitable to the raptor.

Use of temperature sensors to monitor patterns of nocturnal desertion by incubating Common Terns

ABSTRACT Adult Common Terns (Sterna hirundo) reportedly respond to nocturnal predation by deserting colonies from the time of disturbance until dawn the following morning. However, direct evidence for this behavioral response is limited because the nocturnal behavior of individual terns is difficult to monitor due to low visibility and the vulnerability of terns to disturbance at night. We monitored the nocturnal incubation behavior of 10 pairs of nesting Common Terns using continuously recording temperature sensors disguised in dummy eggs and placed in the nests. Changes in egg temperature during the night suggested that a range of desertion behaviors occurred in response to visits by a Great Horned Owl (Bubo virginianus) that preyed on chicks and adult terns. As many as three desertion events (desertion at one or more nests in a single 12-min period) were detected on a single night and nests were unattended for an average of 54 min (range 8–352 min) per desertion event. Nest temperature sensors allowed us to detect individual responses to nocturnal predators and to determine that desertion behavior was more varied than previously assumed. Desertion events were most frequent when physical evidence indicated the presence of an owl in the colony and during nights after owl predation, suggesting that desertion is a response to the actual risk of predation as well as to the possibility of predation.

Behavioural Response of Bats to Perceived Predation Risk While Foraging

AbstractThe ability to detect and respond to predation risk while foraging may have important fitness consequences for prey organisms. Anti‐predator behaviours may reduce the probability of mortality because of predation, but they may also be associated with reduced foraging efficiency. Several behaviours of bats have been suggested to serve as anti‐predator responses, and there is evidence that predation, particularly by avian predators such as owls, may be an important cause of bat mortality. Previous studies have attempted to determine whether predator presence affects bat behaviour when emerging from roost sites, but few have examined effects of predator presence on bat behaviour while foraging. In this study, we investigated whether foraging bats respond to predator cues by presenting bats with an acoustic cue simulating the presence of an owl. Within matched trials, which were conducted at different locations each of 18 nights, significantly fewer bat detections were recorded at owl playback stations than at control stations (no auditory cue), suggesting an avoidance response by bats. An acoustic control (i.e. station playing woodpecker calls), however, did not have significantly more detections than the stations playing the owl calls, suggesting that bats may simply be avoiding noise and more detailed investigation is warranted. Although evidence for owl predation on bats is minimal in North America, results of this study may indicate that the perceived presence of owls may represent a factor influencing the behaviour of bats while foraging.

OWL SELECTION ON SIZE AND SEX CLASSES OF RODENTS: ACTIVITY AND MICROHABITAT USE OF PREY

Abstract Our aim was to study selective owl predation on size and sex classes of rodents in northwestern Argentine Patagonia and to determine which behavioral traits may be involved in differential prey vulnerability. Diet of Magellanic horned owls (Bubo magellanicus) was studied for 2 years. Jaw and pelvic measurements were used to estimate size and sex of rodents eaten by owls. Livetrapping of rodents was conducted simultaneously with diet analysis to estimate proportion of size and sex classes of rodents available and to examine movement and microhabitat use. Owls generally selected individuals of smaller size and avoided heavier individuals. Sex classes of prey were consumed according to availability, except that the female silky desert mouse (Eligmodontia morgani) was selected. Smaller individuals of E. morgani, the long-haired mouse (Akodon longipilis), and the yellow-nosed mouse (A. xanthorhinus) tended to use open microhabitat where risk of predation was higher. Vulnerability to predation seemed t...

Common blossom bats ( Syconycteris australis) as pollinators in fragmented Australian tropical rainforest

The effectiveness of the common blossom bat ( Syconycteris australis) as a pollinator of the rainforest tree Syzygium cormiflorum was assessed by comparing movements, pollen loads, and visitation frequency, with those of birds. The study was carried out in a fragmented rainforest landscape in north Queensland and movements were determined by radio-telemetry. Bats carried 6 times more pollen than birds, but were less frequent visitors to S. cormiflorum, suggesting that the quantity of pollen distributed by the two groups may not differ greatly. However, pollen quality (the geographic/genetic distance moved) may be a more important factor in determining pollinator effectiveness. Bats were very mobile in the fragmented landscape, with home ranges varying from 12 to 1796 ha and often encompassing more than one forest fragment. Bats usually moved along strips of vegetation, but were found to fly up to 5.8 km (mean=1 km) across cleared land. This mobility led to similar numbers of S. australis being caught in small (<50 ha) forest fragments and continuous forest. At the scale of individual trees, bats fed for an average of 1.2 min per tree, compared to 5.0 min for birds. Short foraging times per tree and frequent movements of >200 m should increase the opportunity for movement of pollen between individual trees. Collectively, these observations suggest that S. australis carry pollen of high quality. Although they fly across cleared land, fragmentation may impact bat populations by allowing increased owl predation in open country and by increasing the reliance of bats on flower resources in the agricultural matrix. On the Atherton Tablelands, the latter is provided mostly by cultivated bananas ( Musa sp.), which produce nectar but not pollen.

Does Disturbance by Nocturnal Predators Affect Body Mass of Adult Common Terns?

-To investigate the effects of nocturnal disturbance of predators on body condition of adult Common Terns (Sterna hirundo) we present data on body mass, mass change from evening to morning as well as data on flight activities and reproduction during nocturnal predation by a Long-eared Owl (Asio otus) and compare the periods before, during, and after predation. Birds were individually marked with transponders, allowing continuous recording of activity on resting sites and body mass. Predation caused prolonged incubation periods and lowered hatching and breeding success. Terns showed high flight activities during evenings (panic flights) and adults left the colony one hour after sunset, performing flights through the entire night and returned to resting sites in the morning at the time when normal flight activity started in the periods without predation. In 24% of the nests, mostly outside of the predation center, the incubation period was not prolonged, indicating a return of these adults in the night to their nests. During the predation period, body masses were not reduced and mass loss during night (difference in mass between evening and morning) did not differ compared to non-predation periods. The only difference between periods was the lack of correlation between evening mass and mass change during the night and a positive correlation between morning mass and mass change during night in the period when owl predation occurred. Terns appeared to forage very early in the morning before returning to the colony site, compensating for increased energy expenditure after extensive flight activity. Adults chose to leave the colony site to escape predation and to forage before returning, a strategy that appeared to favor self-survival over survival of offspring. Received 20 April 1999, accepted 27June 1999.

An estimate of the impact of predators on the British Field Vole Microtus agrestis population

AbstractThe productivity of the British Field Vole population is estimated to be between 677 000 and 982 000 voles per year, depending on the precise basis of the calculation. The total consumption of Field Voles by all the major predators, mammalian and avian, is estimated to be around 980 000, if the very uncertain consumption by Feral Cats is included. Two specialist vole predators (Weasel, Kestrel) and two generalist predators (Red Fox, Feral Cat) consume 85% of the total between them, the other 10 species share the rest between them. However important voles might be for owls, owl predation is unimportant to voles. The slight evidence for cyclicity in Field Vole populations in Britain might be explained by the overall take of voles by the predators, and by the rather generalized diet of even the vole specialists.

Effects of avian mobbing on roost use and diet of powerful owls,Ninox strenua

We observed the species and numbers of mobbing birds and their effects on a large, nocturnal, bird-eating predator, the powerful owl, together with the pattern of owl predation on mobbing and non-mobbing species. Owls were mobbed on 35 occasions by seven of 44 species of forest birds at a site composed of open forest (88% by area) and rainforest (12%). The majority of bouts involved individuals of a single species, although mixed groups were observed on nine occasions. Regular mobbers were between 4 and 26% of the owls’ body weight. Owls abandoned their daytime roosts during 20% of bouts and responded by calling or actively monitoring mobbers during 54% of bouts. Mobbing appeared to explain why owls roosted in rainforest significantly more often than expected by its availability, mobbing being significantly less frequent in rainforest than in open forest. Only one mobbing species regularly occupied rainforest and the canopy of roosts in rainforest was denser than that in open forest, thus reducing the chances of an owl being detected by potential mobbers. Twelve species of forest birds were within the range of prey size of the powerful owl (75–800g): six were mobbers and six non-mobbers. The frequency of owl predation on non-mobbers was 8.75 times that on mobbers. The species in this study took a high risk by mobbing a very large predator, but benefited by greatly reducing their chances of predation.

Refuge sites of voles under owl predation risk: priority of dominant individuals?

In an aviary experiment, we studied whether body size or habitat familiarity of field voles (Microtus agrestis) affected predation risk by Tengmalm's owls (Aegolius funnrus). In the field, we compared die body size of field voles snap-trapped in good (covered) and poor (open) habitats in 1992 and 1994 to determine whether there were habitat-related differences in the body size of voles. In the aviary, large individuals occupied the good habitat significantly more than small individuals both in the control (owl not present) and experimental treatments (owl present). Furthermore, habitat-familiar voles inhabited die good habitat more than habitat-unfamiliar voles did when an owl was present. Our field data were consistent with our aviary data: larger field voles were more frequently found in good habitats than in poor habitats. In the aviary, Tengmalm's owl predation risk was higher for small and habitat-unfamiliar voles. This suggests that large field voles may have priority to sheltered habitats. Furthermore, habitat familiarity may play a central role in avoiding risky habitats.

Assessing the impact of owl predation on the growth rate of a rodent prey population

Data from a comparative survey of rodent population dynamics and predation by owls were used to estimate the impact of owl predation on the demography of the sigmodontine rodent Akodon azarae. A stage-structured matrix model based on mass classes was used to estimate rodent demographic parameters through the analysis of field estimated transition matrices. Owl pellet analysis allowed estimation of predation rate upon different mass classes of A. azarae and computation of estimated predation-free transition matrices. Owl predation affected exclusively small individuals of initial body mass stages. Although population growth rate in A. azarae was most sensitive to transitions involving initial stages, exclusive consumption of small individuals by owls did not have a significant impact on population growth rate. Owls preyed upon individuals of A. azarae only slightly, possibly on account of specific features of its habitat use and escape response, which may result in predators not affecting prey abundance or demography. The proposed methods can be of special convenience for endangered or rare vertebrate species in which large-scale experimental manipulations might be questionable.

Two gerbils of the Negev: A long-term investigation of optimal habitat selection and its consequences

Optimal foraging theory has entered a new phase. It is not so much tested as used. It helps behavioural ecologists discover the nature of the information in an animal's brain. It helps population ecologists reveal coefficients of interaction and their patterns of density-dependent variation. And it helps community ecologists examine niche relationships. In our studies on two species of Negev desert gerbil, we have taken advantage of the second and third of these functions. Both these gerbils prefer semi-stabilized dune habitat, and both altered their selective use of this habitat and stabilized sand according to experimental changes we made in their populations. Their changes in selectivity agree with a type of optimal foraging theory called ‘isoleg theory’. Isoleg theories provide examples of dipswitch theories – bundles of articulated qualitative predictions – that are easier to falsify than single qualitative predictions. By linking behaviour to population dynamics through isoleg theory, we were able to use the behaviour of the gerbils to reveal the shapes of their competitive isoclines. These have the peculiar non-linear shapes predicted by optimal foraging theory. Finally, when owl predation threatens, the behaviour of Gerbillus allenbyi reveals the shape of their victim isocline. As has long been predicted by predation theory and laboratory experiments, it is unimodal.

Owl predation on snowshoe hares: consequences of antipredator behaviour.

We show evidence of differential predation on snowshoe hares (Lepus americanus) by great horned owls (Bubo virginianus) and ask whether predation mortality is related to antipredator behaviour in prey. We predicted higher predation on (1) young and inexperienced hares, (2) hares in open habitats lacking cover to protect from owl predation, and (3) hares in above average condition assuming that rich food patches are under highest risk of predation. Information on killed hares was obtained at nest sites of owls and by monitoring hares using radio-telemetry. The availability of age classes within the hare population was established from live-trapping and field data on reproduction and survival. Great horned owls preferred juvenile over adult hares. Juveniles were more vulnerable to owl predation before rather than after dispersal, suggesting that displacement or increased mobility were not causes for this increased mortality. Owls killed ratio-collared hares more often in open than in closed forest types, and they avoided or had less hunting success in habitats with dense shrub cover. Also, owls took hares in above average condition, although it is unclear whether samples from early spring are representative for other seasons. In conclusion, these results are consistent with the hypothesis that variation in antipredator behaviours of snowshoe hares leads to differential predation by great horned owls.

Palaeontology and Zooarchaeology of Mezmaiskaya Cave (Northwestern Caucasus, Russia)

Over 6000 large mammal and numerous small vertebrate remains have been recovered from preliminary excavations at Mezmaiskaya Cave, situated at 1300–1350 m above sea level in the northwestern foothills of the Caucasus Mountains. Most remains were recovered from layers containing Mousterian artefacts that date to the late Middle Pleniglacial (35,000 bpand older). The faunal assemblage reflects a very low degree of weathering, and many bones exhibit traces of stone tool cuts and carnivore modification. Steppe bison ( Bison priscus), Caucasian goat ( Capra caucasica), and Asiatic mouflon ( Ovis orientalis) are the most common large mammals. The remains of reindeer ( Rangifer tarandus) were encountered for the first time in the Caucasus. Although most of the smaller vertebrate remains appear to have been accumulated by nonhuman biotic processes (e.g. owl predation), the majority of the ungulate remains probably represent animals hunted by the Mousterian occupants of the cave.

Snowy Owl Predation on a Northern Pocket Gopher: Evidence of Nocturnal Foraging?

Snowy Owl Predation on a Northern Pocket Gopher: Evidence of Nocturnal Foraging?

Food habits of the barn owl,Tyto alba, in a Mediterranean rural area: Comparison with the diet of two sympatric carnivores

Abstract The feeding ecology of the barn owl, Tyto alba, was studied in a Mediterranean hilly area of central Italy. Two hundred and fourteen pellets, from five different sites, were collected, dried, and dissected. Data were analysed on seasonal and annual base. Barn owl predation on small mammals was compared with the food habits of two sympatric carnivores, the red fox and the stone marten. Barn owls fed mainly on mammals whereas birds, amphibians, reptiles and insects were occasionally taken. Apodemus mice were the most frequent (34.16%) prey among mammals. Shrews were eaten only by the barn owl (30.23%), but did not occur in the carnivore diets. In conclusion the results confirmed the barn owl as an ecological specialist of small mammals, on which it seems to prey unselectively, whereas the red fox and the stone marten do not appear to prey on shrews.

Predation Risk and the Organization of the Parus Guild

Many studies indicate that interspecific competition restricts the use of foraging sites in mixed-species winter flocks of tits. However, predation is a strong mortality factor for members of the tit guild and predation risk may restrict the selection of foraging sites and organisation of the mixed-species tit flock. We evaluated pygmy owl (Glaucidium passerinum) predation can North European tits and goldcrests. A literature analysis of six areas showed clearly that pygmy owls feed more heavily on those species that forage on exterior tree parts (coal tit P. ater and golderest Regulus regulus)

Impact of powerful owl predation on a population of the greater glider: A response to Kavanagh (1988)

Abstract Kavanagh (1988) concluded that 46 months of predation by a pair of powerful owls caused a decline of over 90% in a greater glider population over an area of 100 ha in southeastern New South Wales. He suggested the owls concentrated their foraging activities by taking gliders in one pocket of their home range before moving to the next pocket once the glider population fell to levels at which they were difficult to catch. The assumptions on which Kavanagh's work is based and the methodology of the study are assessed here, and it is argued that the data presented are not sufficient to support the interpretations of powerful owl ecology made in the paper. In particular, alternative explanations of the glider population decline were not considered and home range use by the owls was not examined in enough detail to discern any spatial movement patterns.

Behavioral resource depression and decaying perceived risk of predation in two species of coexisting gerbils

Behavioral resource depression occurs when the behavior of prey individuals changes in response to the presence of a predator, resulting in a reduction of the encounter rate of the predator with its prey. Here I present experimental evidence on the response of two species of gerbils (Gerbillus allenbyi and G. pyramidum) to the presence of barn owls. I conducted the experiments in a large aviary. Both gerbils responded to the presence of barn owl predators by foraging in fewer resource patches (seed trays) and by quitting foraged resource patches at a higher resource harvest rate (giving-up density of resource; GUD). This reduced the amount of time gerbils were exposed to owl predation, and hence the encounter rate of owls with gerbils, i.e., behavioral resource depression. Thus, the presence of owls imposes a foraging cost on gerbils due to risk of predation, and also on the owls themselves due to resource depression. I then examined how resource depression relaxed over time following exposure to owls. In the days following an encounter with the predator, the reduction in foraging activity for both gerbil species eased. Increasing numbers of trays were foraged each day, and GUDs in seed trays declined. The two gerbils differed in their rate of recovery, with G. pyramidum returning to prepredator levels of foraging after 1 or 2 nights and G. allenbyi taking 5 nights or longer. Interspecific differences in recovery rates may be based on differences between the species in vulnerability to predation and/or ability to detect the presence of predators. The differences in recovery rates may be due to optimal memory windows or decay rates, where differences between species are based on risk of predation or on how perceived risk changes with time since a predator was last encountered. Finally, differences between or among competitors in recovery from resource depression may provide foraging opportunities in time for the species which recover most quickly and may have implications for species coexistence.

Direct Observations of Owls and Heteromyid Rodents: Can Predation Risk Explain Microhabitat Use?

Coexisting heteromyid rodent species of North American deserts differ in habitat use and in locomotory morphology. Quadrupedal species forage primarily in structurally complex microhabitats, such as under bush canopies, while bipedal species forage in open spaces. A common explanation for this morphology—microhabitat association is that species differing in morphology also differ in vulnerability to predators, that microhabitat structure affects predation risk, and that animals preferentially forage in the safest microhabitats. We tested this for two bipedal and two quadrupedal heteromyid species (matched by body size), and one cricetid species, by quantifying effects of habitat and illumination on activity and on risk of predation by Great Horned Owls. Capture frequencies were lower for all heteromyid species than for the cricetid species, Peromyscus maniculatus. Heteromyid activity was lower in open habitat and under bright illumination. Illumination had no significant effect on risk, perhaps because rodents changed activity patterns under full moon to compensate for a potential increment in risk. Habitat, however, did affect risk: all species were attacked and captured more frequently in the open. Bipedal species were attacked relatively more in the open than were quadrupeds. If these results apply to all predators, they indicate that predation alone cannot account for the divergent microhabitat associations of bipedal and quadrupedal species. Bipedal heteromyids, however, escaped owl attacks more frequently than did quadrupeds of equivalent size. It is therefore conceivable that they experience lower overall risk in nature, where owls may preferentially attack more easily captured prey species when given a choice. Under these circumstances, owl predation could reinforce divergent microhabitat specializations based on some other factor, such as foraging economics, by restricting quadrupeds more strongly than bipeds to the safety of bushes.

Factors Affecting Gerbil Foraging Behavior and Rates of Owl Predation

We experimented on how illumination, habitat structure, and three different species of owls affected the foraging behavior of Gerbillus allenbyi and G. pyramidum, two gerbil species that coexist on sand dune habitats in the Negev Desert, Israel. We also tested how illumination and habitat structure affected rates of predation by owls on the two gerbil species. In a large aviary, we manipulated presence and absence of owls, owl species, presence and absence of illumination, and shrub cover. In response to the presence of owls or to increased illumination, gerbils foraged less, shifted foraging activity to the bush microhabitat, and quit patches at a higher giving—up density of resources. In accord with moonlight avoidance, both gerbil species suffered higher predation rates under illumination than in the absence of illumination. In addition, G. pyramidum distinguished among owl species, as indicated by changes in patch use and habitat selection. Habitat structure also affected foraging behavior and rates of predation. Gerbils foraged less in the open than in the bush microhabitat, foraged less when there was no cover present, and foraged less in the bush microhabitat when patches were encumbered by entangling branches. In accord with avoidance of open areas, both gerbil species suffered higher rates of predation when shrub cover was 0% than when shrub cover was 10%. With 0% cover, G. allenbyi suffered higher predation rates than G. pyramidum, but with 10% cover, rates of owl predation did not differ between gerbil species. Rates of owl predation on the two species corresponded to their natural patterns of macro— and microhabitat partitioning; relative to G. allenbyi, G. pyramidum predominates on open sand dunes and biases its behavior toward the open microhabitat. The results suggest that predation interacts with resource competition to determine the distribution and habitat separation of G. allenbyi and G. pyramidum.