Owl Nests Research Articles

Importance of agricultural landscapes to nesting burrowing owls in the Northern Great Plains, USA

Anthropogenic habitat loss and fragmentation are the principle factors causing declines of grassland birds. Declines in burrowing owl (Athene cunicularia) populations have been extensive and have been linked to habitat loss, primarily the decline of black-tailed prairie dog (Cynomys ludovicianus) colonies. Development of habitat use models is a research priority and will aid conservation of owls inhabiting human-altered landscapes. From 2001 to 2004 we located 160 burrowing owl nests on prairie dog colonies on the Little Missouri National Grassland in North Dakota. We used multiple linear regression and Akaike’s Information Criterion to estimate the relationship between cover type characteristics surrounding prairie dog colonies and (1) number of owl pairs per colony and (2) reproductive success. Models were developed for two spatial scales, within 600 m and 2,000 m radii of nests for cropland, crested wheatgrass (Agropyron cristatum), grassland, and prairie dog colonies. We also included number of patches as a metric of landscape fragmentation. Annually, fewer than 30% of prairie dog colonies were occupied by owls. None of the models at the 600 m scale explained variation in number of owl pairs or reproductive success. However, models at the 2,000 m scale did explain number of owl pairs and reproductive success. Models included cropland, crested wheatgrass, and prairie dog colonies. Grasslands were not included in any of the models and had low importance values, although percentage grassland surrounding colonies was high. Management that protects prairie dog colonies bordering cropland and crested wheatgrass should be implemented to maintain nesting habitat of burrowing owls.

The Importance of Being Spatial (and Reserved): Assessing Northern Spotted Owl Habitat Relationships with Hierarchical Bayesian Models

Regional conservation planning increasingly draws on habitat suitability models to support decisions regarding land allocation and management. Nevertheless, statistical techniques commonly used for developing such models may give misleading results because they fail to account for 3 factors common in data sets of species distribution: spatial autocorrelation, the large number of sites where the species is absent (zero inflation), and uneven survey effort. We used spatial autoregressive models fit with Bayesian Markov Chain Monte Carlo techniques to assess the relationship between older coniferous forest and the abundance of Northern Spotted Owl nest and activity sites throughout the species' range. The spatial random-effect term incorporated in the autoregressive models successfully accounted for zero inflation and reduced the effect of survey bias on estimates of species-habitat associations. Our results support the hypothesis that the relationship between owl distribution and older forest varies with latitude. A quadratic relationship between owl abundance and older forest was evident in the southern portion of the range, and a pseudothreshold relationship was evident in the northern portion of the range. Our results suggest that proposed changes to the network of owl habitat reserves would reduce the proportion of the population protected by up to one-third, and that proposed guidelines for forest management within reserves underestimate the proportion of older forest associated with maximum owl abundance and inappropriately generalize threshold relationships among subregions. Bayesian spatial models can greatly enhance the utility of habitat analysis for conservation planning because they add the statistical flexibility necessary for analyzing regional survey data while retaining the interpretability of simpler models.

Maternal effects on offspring Igs and egg size in relation to natural and experimentally improved food supply

1. Maternal effects have been suggested to function as a mechanism for transgenerational plasticity, in which the environment experienced by the mother is translated into the phenotype of the offspring. In birds and other oviparous vertebrates where early development is within the egg, mothers may be able to improve the viability prospects of their offspring at hatching by priming eggs with immunological and nutritional components. 2. We studied how resource availability affects maternal investment in offspring by feeding Ural owl (Strix uralensis, Pall.) females prior to egg-laying in 3 years of dramatically different natural food conditions. 3. Supplementary feeding prior to laying increased body mass and the level of Igs of females measured at clutch completion. Supplementary fed Ural owl females laid larger eggs than control females, and had offspring with higher levels of Igs at hatching compared to offspring of control females. 4. We found variation in maternal allocation of resources to the eggs in response to environmental conditions: during a year of rapidly declining food abundance, maternal Igs in hatchlings were higher, whereas egg size was smaller compared to years with a more stable food supply. 5. Egg size had a positive effect on offspring body mass at fledging, whereas Igs at hatching did not affect Igs at fledging. 6. We conclude that maternal body condition and maternal Igs, as well as hatchling Igs and egg size are limited by food resources during egg production. Hatchlings rely on maternally derived Igs and, hence, our results suggest that mothers with high levels of Igs passively transfer higher Igs levels to their eggs instead of active manipulation of Igs levels in eggs. Ural owl egg size appears to be highly sensitive to short-term changes in food abundance, with important consequences for nestling growth.

Brent goose colonies near snowy owls: Internest distances in relation to abundance of lemmings and arctic foxes

Brent goose colonies around snowy owl nests have been studied near Medusa Bay (73°21′ N, 80°32′ E) and in the lower reaches of the Uboinaya River (73°37′N, 82°10′E), the northwestern Taimyr Peninsula, from 1999 to 2006. All brent nests within 680 m from an owl nest have been regarded as an individual colony. The results show that the area of the colony is always larger than the protected area around the owl nest. In years of low abundance of lemmings, brent geese nest generally closer to the owl nest than in years of high abundance. When arctic foxes are abundant, however, brent geese nest significantly closer to owls than when the foxes are scarce, irrespective of lemming abundance. The mechanism of brent colony formation around owl nests is based on a number of stimuli.

Factors Affecting Detection of Burrowing Owl Nests During Standardized Surveys

Abstract: Identifying causes of declines and evaluating effects of management practices on persistence of local populations of burrowing owls (Athene cunicularia) requires accurate estimates of abundance and population trends. Moreover, regulatory agencies in the United States and Canada typically require surveys to detect nest burrows prior to approving developments or other activities in areas that are potentially suitable for nesting burrowing owls. In general, guidelines on timing of surveys have been lacking and surveys have been conducted at different times of day and in different stages of the nesting cycle. We used logistic regression to evaluate 7 factors that could potentially affect probability of a surveyor detecting a burrowing owl nest. We conducted 1,444 detection trials at 323 burrowing owl nests within 3 study areas in Washington and Wyoming, USA, between February and August 2000–2002. Detection probability was highest during the nestling period and increased with ambient temperature. The other 5 factors that we examined (i.e., study area, time of day, timing within the breeding season, wind speed, % cloud cover) interacted with another factor to influence detection probability. Use of call‐broadcast surveys increased detection probability, even during daylight hours when we detected >95% of owls visually. Optimal timing of surveys will vary due to differences in breeding phenology and differences in nesting behavior across populations. Nevertheless, we recommend ≥3 surveys per year: one that coincides with the laying and incubation period, another that coincides with the early nestling period, and a third that coincides with the late nestling period. In northern latitudes, surveys can be conducted throughout the day. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):688–696; 2008)

Composition of the body mass overshoot in European barn owl nestlings (Tyto alba ): insurance against scarcity of energy or water?

European barn owl chicks (Tyto alba) show a body mass overshoot prior to fledging that has been predicted to serve as an energy reservoir during periods of stochastic food availability. However, the composition of the mass overshoot has heretofore not been directly examined in nestlings of this or any other species displaying a body mass overshoot during growth (e.g., raptors and seabirds). To experimentally determine whether the overshoot in body mass in juvenile European barn owls (Tyto alba) may act as an energy reservoir, we compared the body composition of owl chicks raised on an ad libitum diet to those fed a restricted diet designed to eliminate the overshoot. Chicks raised on the two diets were also compared for differences in maturation of diverse functions (e.g., locomotion) and tissues (e.g., skeletal development). Contrary to expectations, our results on body composition in juvenile barn owls indicate that the mass overshoot prior to fledging is primarily comprised of an increased water compartment. Thus, we suggest that the mass overshoot in owls (and possibly in other species) does not serve as an energy reservoir but, rather, may function as an insurance against dehydration when hot in-nest conditions force chicks to rely on evaporative cooling: temperatures in barn owl nests can reach up to 43 degrees C. We found no significant differences in maturation indexes between diet treatments at the time of fledging.

Breeding Dispersal and Nesting Behavior of Burrowing Owls Following Experimental Nest Predation

Nest predation is considered a primary factor affecting the life-history characteristics and particularly dispersal of many avian species. We tested the hypothesis that nest predation would increase dispersal probability, dispersal distance and the frequency of renesting. We removed eggs from burrowing owl (Athene cunicularia) nests to simulate nest predation in southeastern California. Owls responded to egg removal with increased dispersal probability, nesting attempts and egg production. We found that nest predation tended to increase dispersal probability (50% depredated nests vs. 14% control nests), which occurred fairly soon after nest predation (0–25 d). Dispersal distance was highly variable among owls (range: 148–13,012 m). Following experimental nest predation, burrowing owls increased the number of nesting attempts and thus the total number of eggs produced in a season, regardless of dispersal. Clutch size, however, decreased as the number of breeding attempts increased. Despite large initial clutch size, burrowing owls in the Imperial Valley may have adapted to nest predation by both dispersal and the ability to renest frequently.

Cavity dynamics and breeding success of the Lanyu Scops Owl (Otus elegans)

The Lanyu Scops Owl (Otus elegans) breeds in natural tree cavities on Lanyu (Orchid Island), a subtropical island off the southeastern coast of Taiwan in the western Pacific. There are no cavity-making birds or mammals on the island. This study attempted to understand the relationship between this owl’s cavity use and breeding success. In a 10-ha study site, all cavities on all trees with a diameter breast height larger than 18 cm were documented. There were 257 cavities in 119 trees, and 94 of them were used by owls at least once during the study period. The detailed location and size of these 94 cavities in 74 trees were recorded and monitored. From 1999 to 2006, more than 80% of breeding owls were color-marked and their cavity use and breeding success were tracked. Nest trees were taller than other trees, and cavities used by owls were higher above the ground, and larger than unused cavities. Unused cavities were mostly unsuitable due to flooding or decay, or were on trees already containing a nest. Regardless of how many cavities there were on a tree, a single tree never had more than one Lanyu Scops Owl nest at a given time. More than 60% of the 94 cavities were used multiple times. Cavity occupancy frequently showed gaps, usually following breeding failure. Cavity quality (the proportion of years a cavity was used during the study) and female quality (the proportion of years an owl bred out of the total number of years it competed for cavities) significantly contributed to the average fledging success of owls, but male quality did not.

Multiscale Habitat Selection by Burrowing Owls in Black‐Tailed Prairie Dog Colonies

ABSTRACT Some populations of western burrowing owls (Athene cunicularia hypugaea) have declined in recent decades. To design and implement effective recovery efforts, we need a better understanding of how distribution and demographic traits are influenced by habitat quality. To this end, we measured spatial patterns of burrowing owl breeding habitat selection within black‐tailed prairie dog (Cynomys ludovicianus) colonies in northeastern Wyoming, USA. We compared burrow‐, site‐, colony‐, and landscape‐scale habitat parameters between burrowing owl nest burrows (n = 105) and unoccupied burrows (n = 85). We sampled 4 types of prairie dog colonies: 1) owl‐occupied, active with prairie dogs (n = 16); 2) owl‐occupied, inactive (n = 13); 3) owl‐unoccupied, active (n = 14); and 4) owl‐unoccupied, inactive (n = 14). We used an information‐theoretic approach to examine a set of candidate models of burrowing owl nest‐site selection. The model with the most support included variables at all 4 spatial scales, and results were consistent among the 4 types of prairie dog colonies. Nest burrows had longer tunnels, more available burrows within 30 m, and less shrub cover within 30 m, more prairie dog activity within 100 m, and were closer to water than unoccupied burrows. The model correctly classified 76% of cases, all model coefficients were stable, and the model had high predictive ability. Based on our results, we recommend actions to ensure persistence of the remaining prairie dog colonies as an important management strategy for burrowing owl conservation in the Great Plains of North America.

Coexistence of a generalist owl with its intraguild predator: distance-sensitive or habitat-mediated avoidance?

Intraguild predation is increasingly reported as a population-limiting factor for vertebrate predators. However, long-term coexistence of the intraguild prey with its predator is a common occurrence usually maintained by some form of predator avoidance, which may be achieved through distance-sensitive avoidance (selection of sites as far as possible from the intraguild predator), and/or habitat-mediated avoidance (avoidance of habitats associated with high predation risk). The former is expected when the distribution of the predator is heterogeneous, leaving gaps which can be exploited by the prey, while the latter is expected at high predator densities, when few predation refugia are available. To date, few studies have focused on such switch in predator avoidance under changing scenarios of intraguild predator density. To test this hypothesis, we censused tawny owls (Strix aluco, body mass ∼0.4–0.7 kg) and their intraguild predator, the eagle owl (Bubo bubo, ∼1.5–4 kg), in 12 areas of the Alps. As predicted, tawny owls were indifferent to predator distance in an area of low predation risk, they switched to distance-sensitive avoidance in an area of medium predator density and to habitat-mediated avoidance in an area of high predator density with few available refugia. Actual predation rates were low, but increased with proximity to the intraguild predator nest. Similarly, tawny owl breeding output declined with closeness to an eagle owl nest. Habitat loss associated with predator avoidance translated into population effects, leading to a negative relationship between the densities of the two owl species. The spatial gaps in tawny owl distribution caused by eagle owls indirectly favoured other owl species, resulting in higher diversity of the overall owl community and suggesting that eagle owls acted as keystone predators. Our results suggest that intraguild predation may alter habitat choices and affect density, productivity and guild structure of vertebrate mesopredators. Such effects are probably more common than previously thought.

FACTORS INFLUENCING BURROWING OWL REPRODUCTIVE ;PERFORMANCE IN CONTIGUOUS SHORTGRASS PRAIRIE

We analyzed Burrowing Owl (Athene cunicularia) reproductive performance (i.e., clutch size, brood size, and number of young fledged) in relation to nest and colony factors in Buffalo Gap National Grassland, South Dakota. We monitored 129 pairs of Burrowing Owls in 43 prairie dog (Cynomys ludovicianus) colonies in 1999 and 143 pairs in 45 colonies in 2000. Mean clutch size was 7.2 and ranged from 3–10 eggs. Pairs averaged 2.6 fledglings per nesting attempt and 76% of the nesting pairs were successful in fledging at least one young. Abandonment appeared to be the primary cause of nest failures. Variables measured at the colony level explained more (34–70%) of the variability in reproductive performance than did variables measured at the nest level (7–33%). In general, Burrowing Owls that arrived early, initiated clutches sooner, and either nested at greater distances from nearest neighbors or had fewer owl nests within 250 m of the nest burrow, had larger clutches, larger broods, and more fledglings. Seventy percent of prairie dog colonies were occupied by owls; most (85%) unoccupied colonies were <10 ha in size. Larger prairie dog colonies supported greater numbers of owl nests and consequently produced more total fledglings than did smaller colonies (P < 0.001).

BARRED OWL HABITAT AND PREY: A REVIEW AND SYNTHESIS OF THE LITERATURE

Barred Owls (Strix varia) historically inhabited the forests of eastern North America. During the last century, they expanded their range to include forests throughout the southern provinces of Canada, southeastern Alaska, British Columbia, Washington, Oregon, and northern California. To date, there has been no synthesis of the varied habitats or prey used by Barred Owls in their expanded range. Here I review and synthesize studies concerning habitat (N = 114) and prey (N = 43) of Barred Owls throughout North America north of Mexico. Barred Owls nested in wilderness areas and national parks as well as small suburban woodlots. They typically preferred old or mature, mixed deciduous/coniferous forests with fairly high canopy closure. Barred Owl nests in natural locations (N = 341) were in cavities (74.8%), hawk nests (12.9%), tops of hollow trees (8.2%), squirrel nests (2.1%) and other locations (2.1%). Trees used by Barred Owls for nesting (N = 169) included 22 genera, although one-fourth of all reported nest trees were Populus. Barred Owls used significantly more coniferous nest trees than deciduous nest trees in their expanded range vs. their historic range. They also nested in nest-boxes (N = 103), in buildings (N = 6), on nesting platforms (N = 2), on the ground (N = 2), in a creek bank (N = 1), and under a bridge (N = 1). Prey individuals (N = 7077) obtained from pellets (N = 5504) were mammals (74.7%), birds (8.3%), amphibians (6.4%), insects and spiders (5.6%), crayfish (3.0%), fish (1.5%), reptiles, snails and slugs, and earthworms (<1.0% each). The distribution of prey types documented by other means (e.g., observations, prey remains; N = 1573) was significantly different, due especially to more earthworms in non-pellet samples: mammals (62.0%), birds (13.5%), insects and spiders (9.5%), amphibians (4.8%), earthworms (4.5%), fish (3.3%), reptiles (1.1%), snails, and crayfish (<1.0% each). The distributions of prey types in all samples that could be placed into winter or non-winter months (N = 4631) differed seasonally, with more mammals in the winter diet (98.4%, vs. 58.9% in non-winter months) and more birds (11.9%), insects or spiders (11.4%), amphibians (10.7%), earthworms (2.3%), fish (2.0%), and crayfish (2.0%) in non-winter months.

January initiation of suburban great horned owl nests in Manitoba

January initiation of suburban great horned owl nests in Manitoba

An Analysis of Spatial Clustering and Implications for Wildlife Management: A Burrowing Owl Example

Analysis tools that combine large spatial and temporal scales are necessary for efficient management of wildlife species, such as the burrowing owl (Athene cunicularia). We assessed the ability of Ripley's K-function analysis integrated into a geographic information system (GIS) to determine changes in burrowing owl nest clustering over two years at NASA Ames Research Center. Specifically, we used these tools to detect changes in spatial and temporal nest clustering before, during, and after conducting management by mowing to maintain low vegetation height at nest burrows. We found that the scale and timing of owl nest clustering matched the scale and timing of our conservation management actions over a short time frame. While this study could not determine a causal link between mowing and nest clustering, we did find that Ripley's K and GIS were effective in detecting owl nest clustering and show promise for future conservation uses.

Dark‐bellied brent geese Branta b. bernicla breeding near snowy owl Nyctea scandiaca nests lay more and larger eggs

Several studies have demonstrated that snowy owls Nyctea scandiaca defend an area around their nests against predators, hereby inadvertently creating safe havens for breeding dark‐bellied brent geese Branta b. bernicla. However, studies investigating brent goose breeding ecology within the predator‐exclusion zones of the snowy owls are absent. In 1999 and 2005, years of high lemming abundance Lemmus sibiricus and Dicrostonyx torquatus, brent geese were primarily breeding in association with snowy owls in the Medusa river catchment on western Taimyr, Russia. Goose nest failure, either as a result of nest abandonment by the adult birds or of nest depredation, increased with increasing distance from the owl nests. Within the brent goose colonies, clutch size as well as egg size increased with decreasing distance from the snowy owl nest, indicating an increasing adult quality closer to owl nests. However, as a result of the abandonment of eggs and goslings, the increasing clutch size did not result in a higher nest success during this study. Apparently brent geese compete for breeding sites close to owl nests, but details of this process remain unknown.

First record of Lepidoptera from Ural owl nests in Japan

Four species of Tineidae (Niditinea baryspilas (Meyrick), Monopis pavlovskii (Zagulajev), Mon. flavidorsalis (Matsumura), Mon. sp.) and one species of Oecophoridae (Martyringa ussuriella Lvovsky) were reared from the detritus found in four Ural owl (Strix uralensis Pallas) (Strigidae) nest boxes that had been abandoned by young birds in Wakayama, Osaka, and Shiga Prefectures, Japan. This is the first report of moths from owl nests in Japan. Tineid larvae feed on keratin sources in the detritus of the nest boxes that include pellets, fur, and feathers; the oecophorid possibly feed on decaying wood chips. Biological information regarding N. baryspilas and Mon. pavlovskii are presented for the first time, accompanied by photographs of their immature stages. It is likely that owl nests provide a suitable habitat for keratophagous tineids in nature because of a wide variety of keratin sources in the nests.

NEST DESTRUCTION ASSOCIATED WITH MORTALITY AND DISPERSAL OF BURROWING OWLS IN THE IMPERIAL VALLEY, CALIFORNIA

Abstract We documented the effects of inadvertent nest destruction from road maintenance activities on the survivorship, reproductive success, and breeding dispersal of burrowing owls (Athene cunicularia) breeding in natural burrows along the water delivery system in the Imperial Valley of California. The activities affected 4 nests (7 adult owls) along an 800-m section of road, filling in or destroying all of the burrows. Three of 7 adult owls in the impacted area were killed, 2 of 2 active nests failed, 2 nests that had previously failed were destroyed and might have led to the dispersal of the surviving adults. We suggest that artificial burrows will reduce the conflict between maintenance and burrowing owl nests, which will benefit both owls and landowners.

Breeding Range Extension of the Northern Saw-whet Owl in Quebec

Abstract Although the breeding range of the Northern Saw-whet Owl (Aegolius acadicus) is restricted to North America, the northern limits of its range are still unclear. In Quebec, the most northerly confirmed breeding records had come from the Saguenay area (Chicoutimi; 48° 25′ N, 71° 03′ W) in balsam fir- (Abies balsamea) white birch (Betula papyrifera) forest and on the Gaspe Peninsula (Amqui; 48° 28′ N, 67° 25′ W) in balsam fir-yellow birch (B. alleghaniensis) forest. Between 1998 and 2003, however, we documented nine Northern Saw-whet Owl nests in balsam fir-black spruce (Picea marina) forest in boreal Quebec on the Mingan Terraces. These records extend the species' known breeding range northward to >50° N.

New Coleoptera records from owl nests in Nova Scotia, Canada

A preliminary study of boreal owl, Aegolius funereus richardsoni (Bonaparte), and northern sawwhet owl, Aegolius acadicus acadicus (Gmelin), nests from Nova Scotia, Canada revealed the presence of 14 species of Coleoptera, several of which represent significant extensions of their known ranges. Atheta (Datomicra) celata (Erichson) is newly recorded for North America; Atheta irrita Casey is newly recorded for Canada and eastern North America; Atheta pseudocrenuliventris Klimaszewski, and Hylota ochracea Casey are newly recorded for Nova Scotia; and Gnathoncus barbatus Bousquet and Laplante, Carcinops pumilo (Erichson), and Phyllodrepa floralis (Paykull) are newly recorded for Cape Breton Island. Gnathoncus barbatus, H. ochracea, A. irrita, A. pseudocrenuliventris, and Corticarina cavicollis (Mannerheim) are all recorded for the first time from bird nests. One new synonym Atheta (Datomicra) celata (Erichson, 1837) [= Datomicra wrangeli Casey 1911. syn nov. ] is designated. Additional new records of A. irrita from Québec are also reported. Attention is drawn to the limited state of knowledge of this specialized beetle fauna and to the high proportion (50%) of adventive species which were found in these environments.

New Coleoptera records from owl nests in Nova Scotia, Canada

New Coleoptera records from owl nests in Nova Scotia, Canada