Female Owls Research Articles

Breeding Dispersal In the California Spotted Owl

Abstract Spotted Owls (Strix occidentalis) are territorial, generally nonmigratory, and strongly philopatric. Nevertheless, California Spotted Owls (S. o. occidentalis) exhibited breeding dispersal during 7% of interannual observations of banded individuals (n = 54 of 743 occasions). Based on ecological theory and published literature, we made a priori predictions about the factors associated with the probability of breeding dispersal and breeding dispersal distance, and about the consequences of dispersal. Breeding dispersal probability was higher for younger owls, single owls, paired owls that lost their mates, owls at lower quality sites, and owls that failed to reproduce in the year preceding dispersal. Sex had little effect on the probability of breeding dispersal. Dispersal distance was similar for female and male owls (median = 7 km, range = 1–33 km). We found no strong relationships between dispersal distance and any of the conditions that were associated with the probability of breeding dispersal...

ENVIRONMENTAL CONDITIONS AFFECT OFFSPRING SEX-RATIO VARIATION AND ADULT SURVIVAL IN TAWNY OWLS

Abstract We studied survival of adult Tawny Owls (Strix aluco), number of breeding pairs, breeding performance, and offspring sex ratio in relation to the number of snowy days in the preceding winter in Duna-Ipoly National Park, Hungary. A new male was more likely to be present after a winter with many snowy days, although female survival was not affected by weather. Number of breeding pairs and number of fledglings declined with increasing number of snowy days. Offspring sex ratio varied according to whether snow cover was present during the egg-laying period, with broods being male biased during adverse conditions but female biased during mild conditions. Also, female nestlings were more likely to die before fledging than male nestlings. These data suggest that female Tawny Owls are able to adjust the sex ratio of their brood according to the expected differential success of nestlings under the prevailing weather conditions. This adjustment in relation to environmental conditions has important implicati...

SEXUAL AND SEASONAL DIFFERENCES IN THE DIET OF LANYU SCOPS OWLS BASED ON FECAL ANALYSIS

Sexual size dimorphism in bird species has been claimed to reduce intersexual food competition and expand the species' overall feeding niche, yet quantitative diet information is known for only a few species. Lanyu scops owl (Otus elegans), a nocturnal insectivore on Lanyu (also called Orchid Island, southeast of Taiwan), has larger females than males. Lanyu scops owls rarely produce pellets, rendering quantification of its diet particularly difficult. We studied the diet of wild Lanyu scops owls through fecal analysis and verified the reliability of our method with a captive owl. Our results showed that fecal analysis was more reliable than observation alone because observations often missed small prey items and resulted in a much higher proportion of prey being unidentified. The feces of 81 Lanyu scops owls showed that orthopterans, beetles, moths, spiders, and centipedes constituted 67% of the total occurrences and >80% of the volume in their diet. Between the breeding and nonbreeding season, owl diets differed slightly in proportion but not in food types. Intersexual diet differences existed for both the breeding and nonbreeding seasons. Adult males ate a higher proportion of aerial insects than did adult females, which is consistent with the prediction that smaller males are more agile than larger females in capturing aerial insects. Male owls ate a lower proportion of terrestrial arthropods than did both females and juveniles in the breeding season, perhaps due to differential use of forest strata when foraging by male and female owls. Fecal analysis provides more reliable information on the diet of insectivorous owls than observations alone and is a non-invasive option for studying the diet of rare or threatened owls.

WINTER HOME RANGE AND HABITAT USE OF FEMALE NORTHERN SAW-WHET OWLS ON ASSATEAGUE ISLAND, MARYLAND

We quantified home range size and habitat selection of seven female Northern Saw-whet Owls (Aegolius acadicus) on Assateague Island, Maryland, during the winters of 1996 and 1997. Home range size (95% fixed kernel) was 103.5 ha (± 50.3 SE). Home range size increased with time spent radio tracking as biweekly home ranges were smaller than those calculated for longer time periods. Home ranges often overlapped in time and space and in one instance the home range for one owl was completely within that of another owl. Northern Saw-whet Owls used primarily pine woods and shrub swamp habitats, with pine woods used more often than any other habitat type and significantly more than expected based on habitat availability.

Offspring Desertion by Double-Brooded Female Barn Owls (Tyto Alba)

Offspring Desertion by Double-Brooded Female Barn Owls (Tyto Alba)

Vocal identification of individual African Wood Owls Strix woodfordii: a technique to monitor long‐term adult turnover and residency

The hoot‐calls of Wood Owls Strix woodfordii, recorded in the Kruger National Park, South Africa, from 1986 to 1998, are sufficiently different to distinguish individuals. Thirteen hoot‐call variables, measured from recordings over this period, show that male and female Wood Owls have such temporal stability that these calls can be used reliably as a long‐term census technique. manova, based on ordinations from principal component analysis, was used to identify individuals statistically between sampling periods. A forward stepwise discriminant function analysis achieved 100% classification success of individual male (n = 3) and female (n = 4) owls from a single sampling period. For all the individuals recorded over the whole study period we achieved a classification success of 80.9% (n = 9) for male hoot calls and of 96.3% (n = 13) for female calls. We found mean occupation periods of 82.25 and 65 months and annual turnover rates of 19.3% and 13.65% for males and females, respectively. Our survey, using vocalizations, is unique since we use data collected over a 12‐year period to derive estimates of population turnover in Wood Owls, and consider what questions can be addressed in similar studies.

Female Owls: First to Advertise Good Genes

Female Owls: First to Advertise Good Genes

Censusing Tawny OwlsStrix alucoby the use of imitation calls

Tawny Owls respond well to imitation or playback of conspecific calls. In 94% of known territories a response to imitation was obtained within 30 min. No measured temporal or weather variable had any effect on response time. The only factor that appeared to influence response time was population density, with birds in open farmland responding faster and more vigorously than those in continuous woodland. This is considered to be due to lack of exposure to other owls in open farmland. Female owls respond more to imitated male calls in autumn than in winter and spring. This may be due to the fact that autumn is when pair formation occurs, and females may be investigating new males. Alternatively they may be defending their territory at a time when juveniles are searching for vacancies. The findings suggest that censusing should be conducted for 30 min, at any time throughout the winter and in any reasonable weather conditions.

Nest Predation May Not Explain Poor Reproductive Success of Polygynously Mated Female Tengmalm's Owls

Nest Predation May Not Explain Poor Reproductive Success of Polygynously Mated Female Tengmalm's Owls

Effects of Radio Tags on Spotted Owls

Fifteen of 47 radio-tagged northern spotted owls (Strix occidentalis caurina) died during a 2-year study in northern California, and 11 of 33 radio-tagged California spotted owls (S. o. occidentalis) died in 2.5 years in the Sierra Nevada. Female owls with 19-g radio tags had significantly lower annual survival rates than color-banded females (P=0.006) after a forest fire in 1987 in the Klamath Mountains. There were no differences in annual survival rates between males with and without radio tags. In 1988, fewer radio-tagged pairs (17%) fledged young than color-banded pairs (88%) (P<0.001), and in 1989 radio-tagged pairs nested (7%) less ofthen than color-banded owls (64%) (P=0.0002). We suggest that spotted owl researchers avoid using backpack-mounted radio tags

Summer Habitat Ecology of Northern Spotted Owls in Northwestern California

We studied the summer habitat ecology of 12 Northern Spotted Owls (Strix occidentalis caurina) in two areas of northwestern California. Spotted Owls used mature or old-growth conifer forests significantly more than expected relative to their availability within their home ranges. In contrast, Spotted Owls used forests of intermediate and young age significantly less than expected relative to their availability within their home ranges. Eighty four percent of 616 Spotted Owl radiotelemetry locations were recorded in mature or old-growth forests. Spotted Owls used forests of complex structure and old age. There were significant differences in habitat structure (e.g., canopy closure, shrub cover, herb cover, old-growth conifer basal area, and hardwood tree density) among habitats used for frequent foraging, infrequent foraging, and roosting. In addition, male and female owls appeared to select habitats with different structure for foraging. Male owls which are smaller than female owls foraged in habitats which had higher tree density than female owls. The mean summer home-range size was 413 ha (SD = ?+196 ha) with males having smaller mean home-range size than females (338 ha and 538 ha, respectively).

A radio tracking study of post‐fledging mortality and movements of Tawny Owls in Argyll

The post‐fledging mortality and behaviour of Tawny Owls was investigated in an upland conifer forest in Argyll, in a year of poor food supply. Twelve unsexed juvenile and 6 adult female owls were fitted with back‐mounted radios. Eleven (91.7%) of the juveniles died between fledging and independence. Deaths occurred in two peaks, initially in the period immediately following fledging when they were vulnerable to mammalian predators, and later on when they died mostly of starvation. While dependent on their parents the corrected home range size of the juveniles varied from 0.8 to 3.5 ha. Two broods showed sudden movements of 400–500 m from coniferous habitat around the nest site to predominantly broadleaved areas. The one juvenile that successfully left its parents territory did so 81–86 days after fledging. These results are discussed in relation to the method of radio tracking used.

Incestuous breeding and abnormal movement by a female Barn OwlTyto alba

Incestuous breeding and abnormal movement by a female Barn Owl<i>Tyto alba</i>

Die akustische Unterscheidung von Steinkauzmännchen und -weibchen(Athene noctua)

The primary call „Guhk“ of male Little Owls functions in pair formation and territory defense. According to literature this call is uttered exclusively by the male. A similar call given by female Little Owls has been recorded on tape; both calls were analysed by means of a sound spectrograph. In comparison to the call of the female the male's call lasted longer, the pitch being higher in the main part. The call was uttered louder and clearer by the male than by the female. During the main time of courtship the male repeated the Guhk-call often monotonously for several minutes, whereas spacing between the notes was irregular and the number of calls given in a group varied only between three and five, or the notes occured singly when the call was uttered by the female. Because of these differences it should be possible to distinguish male and female Little Owls by their vocalizations. This can be important for census work.

Nest Defence and Evolution of Reversed Sexual Size Dimorphism in Snowy Owls Nyctea scandiaca

Trivers' (1972) definition of parental care includes any investment that benefits the young at the cost of the parent's ability to invest in other offspring. Defence of offspring is one example of parental care (Barash 1975) since it reduces the parent's probability of survival and future production of young. Monogamous birds often have some division of labour, and this also applies to birds with reversed, sexual size dimorphism. In these birds the larger size of the female decreases her vulnerability to predators compared with the male. Storer (1966) used this argument and proposed that as a secondary effect the female would be predisposed to being the most important defender of the nest and the offspring. Andersson and Norberg (1981) suggested that the nest defence function was among the most important factors selecting for large female size. The Snowy Owl, a large arctic owl, has a long breeding cycle and is dependant on microtine prey for successful breeding (Pitelka 1959). The owls are predominantly monogamous and have partitioned the roles of parental care so that only the female broods and the male does most of the hunting (Tulloch 1969). The male hunts within an exclusive territory, which is defended against conspecifics (Watson 1957). There is also a well pronounced reversed, sexual size dimorphism in this species (males 1320-2013 g, females 1550-2690 g, Earhart and Johnson 1970). Here we report on differences in defence behaviour between male and female Snowy Owl against a human and a few other intruders near the nest. The aim is to (1) describe and compare nest defence of males and females, (2) test male and female defence responses in relation to brood size and (3) briefly discuss selective forces for sexual size dimorphism.

Territoriality and habitat selection of female snowy owls (Nyctea scandiaca) in winter

Territoriality of snowy owls (Nyctea scandiaca) was investigated near Calgary, Alberta, in the winters of 1976–1977 and 1977–1978. Males appeared to be nomadic, although some individuals remained in small areas for periods of up to 17 days. Many females defended territories of 150–450 ha for periods of up to 80 days. Territories of females contained some habitats in excess of their proportion on the study area. This represents selection of habitats with high prey availability. Owls spent more time in such habitats than in resource-poor habitats within their territories. Territory size was inversely related to the proportion of stubble field and the amount of "edge" habitat bordering stubble fields, on the territories. These findings are consistent with an earlier hypothesis that the size of feeding territories is regulated to provide the territory holder with a constant level of food. Juvenile females defended larger territories than did adult females, with higher proportions of nonpreferred habitats. We suggest that these differences result from the inexperience of juveniles in selecting and defending optimal winter territories.

Plasma concentrations of oestradiol-17 beta, oestrone, progesterone and testosterone during the ovarian cycle of the owl monkey (Aotus trivirgatus).

Oesterone, oestradiol-17 beta, progesterone and testosterone were measured by radioimmunoassay in daily plasma samples throughout the ovarian cycle in 4 female owl monkeys. Clearly defined peaks of oestradiol-17 beta occurred at intervals of 15.5 +/- 0.56 days and confirmed the length of the cycle reported previously. Progesterone rose on the day on the day of the oestradiol-17 beta to reach a maximum 4--6 days later, thereafter decreasing gradually to low levels before the onset of the next cycle. On the basis of these data the follicular and luteal phases were estimated to be 6 and 10 days respectively. Osterone and testosterone peaks preceded that of progesterone by 24 and remained elevated throughout the luteal phase. Plasma concentrations of all steroids were markedly higher than for other primate species. Vaginal cytology was considered unsuitable as an indicator of the stage of the ovarian cycle.

The excretion of gonadal steroids during the reproductive cycle of the owl monkey ( Aotus trivirgatus)

The major ovarian steroid metabolites in the urine of the female owl monkey have been identified. Quantitatively, oestrone was the major oestrogen excreted while the predominant progesterone metabolite was 6β-hydroxypregnanolone. Oestradiol-17β, pregnanediol, allo pregnanediol, and two isomers of 16-hydroxypregnanolone were excreted in significant quantities. Urinary steroid metabolites were measured in daily urine collections throughout the reproductive cycle using gas chromatography and by radioimmunoassays for oestrone, oestradiol-17β and pregnanediol-3α-glucuronide. The results indicated a 16-day reproductive cycle with peak levels of oestrone occurring about 1–2 days before the maximum excretion of progesterone metabolites. Although pregnanediol was not the major progesterone metabolite, radioimmunoassay results for pregnanediol-3α-glucuronide paralleled gas chromatographic estimations of 6β-hydroxypregnanolone. It was concluded that measurement of urinary pregnanediol-3α-glucuronide served as a reliable index of luteal function.

Cyclic changes in the circulating and urinary levels of ovarian steroids in the adult female owl monkey (Aotus trivirgatus).

Circulating levels of oestrone and progesterone were measured by radioimmunoassays in plasma samles from 5 female owl monkeys on 60 consecutive days. Both steroids exhibited cyclic fluctuations and based on nadir to nadir intervals the ovarian cycle was estimated to be 15.92 +/- 0.26 days. Levels of oestrone and pregnanediol-3 alpha-glucuronide were also measured in daily urine samples. The fluctuations of urinary steroids reflected those observed in plasma. Ketamine sedation had no effect on the length of the cycle. Peak values of plasma progesterone and oestrone were 250.48 +/- 11.37 and 3.59 +/- 0.066 ng/ml respectively. There was no clear hormonal distinction between the follicular and luteal phase of the cycle in these owl monkeys.

Renal adenocarcinoma in an owl monkey (Aotus trivirgatus).

A renal adenocarcinoma in the right kidney of a 5-year-old female owl monkey is reported.