Barn Owl Tyto Alba Research Articles

HIGH FREQUENCY OF BATS IN THE DIET OF THE BARN OWL TYTO ALBA IN A LOWLAND DRY FOREST IN TOLIMA, COLOMBIA

The cosmopolitan distribution of the Barn Owl Tyto alba and its relatively well documented diet through the study of pellets have provided evidence of the broad diversity of prey it relies on. In most studies, both in tropical and temperate regions, rodents are its main prey, with other small mammals and vertebrates, and insects complementing its diet. Although bats have been recorded in the diet of T. alba at many localities, they seldom represent an important part of its diet. This study describes the diet of a small colony of T. alba from a mosaic of agricultural fields, pastures for cattle ranching and tropical dry forests in Tolima, central Colombia. Overall, we collected 516 pellets between 2016 and 2017 in a non-systematic manner, and recovered 335 skulls of rodents (67%), bats (29%), birds (3%) and shrews (1%), as well as invertebrate prey. The large proportion of bats in the diet of T. alba in this study is only matched by a few other studies, and further documents the wide range of prey included in the diet of this nocturnal raptor. This is one of the first studies on the diet of T. alba in tropical dry forests in Colombia and complements the existing data on its diet in the American tropics and worldwide.

Collection methods of wild barn owl pellets at low environmental contamination and proposals of microbiological and ecological investigations

BackgroundThe barn owl Tyto alba is a medium-sized nocturnal raptor, predator of small mammals, birds, reptiles, and insects, distributed all over the world. Environmentally uncontaminated wild barn owl pellets are hard and rare to find and provide a natural matrix for biological investigations.ResultsDifferent solutions for collecting wild barn owl pellets at low environmental contamination are proposed. These solutions are based on a daily sanitized surface for sample retrieval, followed by rapid analysis. As an example of the possible investigations that can be carried out on these matrices, with the aim to highlight the relevance of the obtained results also from a biotechnological perspective. Two rare pellet samples from wild barn owl T. alba, appropriately collected in a rural area of central Italy, were analyzed using culture-dependent, molecular (Next-Generation Sequencing), and enzyme profile analysis techniques. The osteological observations of the bone remains provided useful information for identifying the prey, mainly rodents (Apodemus sp.). Under our experimental conditions, the results revealed both a wide heterogeneity between the pellet microbiota and a great percentage of uncultured bacteria not classified at the species level. Furthermore, microbial cultures of Malbranchea albolutea, Debaryomyces hansenii, and Lactobacillus sp. were isolated and studied.Finally, we quantified the environmental impact of our experimental work in terms of CO2 equivalent release. To compensate for the release of 300 kg/CO2 eq., three Taxus. baccata L. were planted.ConclusionsThis work provides a starting point for developing an effective strategy to study and characterize wild barn owl pellets at low environmental contamination. It presents a simple and easy technique/protocol for collecting the pellets. The microbiota heterogeneity found in the two analyzed samples suggests that barn owl pellets can represent a potential natural reservoir for the discovery of new microorganisms to be used in various biotechnological applications. This could open the way for further large-scale studies on a greater number of samples and populations.Graphical

Individual and environmental factors influencing preen gland's morphology and physiology in the barn owl Tyto alba

Avian preen gland helps birds cope with their environment, although its overall functioning remains unclear. We shed light on the complexity of the preen gland's functioning by studying how multiple factors associate with gland morphology (size and shape) and physiology (wax secreted) in barn owls Tyto alba. Individual factors (sex, breeding stage, body condition) were more important predictors of preen gland than environmental factors (temperature, humidity, brood size). Sex, depending on breeding stage in adults, influenced preen gland traits, pointing to the preen gland's regulation by sex hormones and a greater pressure on females to protect their eggs, offspring and themselves throughout reproduction. Adults and fledglings in better condition had larger glands, pointing to the existence of physiological costs. Temperature and humidity, in interaction with plumage coloration, also influenced but to a lesser extent preen gland traits, suggesting that melanin pigmentation and preen gland act as superseding mechanisms when protecting plumage against microorganisms. Finally, fledglings living in larger broods had larger glands, suggesting a role for the social environment in preen gland's functioning. Overall, our study supports the idea that preen gland functions in diverse biological contexts within the same species and is thus subject to multiple selective pressures.

Melanin-based plumage coloration and melanin content in organs in the barn owl

Although the evolutionary ecology of melanin pigments and melanin-based coloration has been studied in great details, particularly in birds, little is known about the function of melanin stored inside the body. In the barn owl Tyto alba, in which individuals vary in the degree of reddish pheomelanin-based coloration and in the size of black eumelanic feather spots, we measured the concentration in melanin pigments in seven organs. The eyes had by far the most melanin then the skin, pectoral muscle, heart, liver, trachea, and uropygial gland. The concentration in eumelanin was not necessarily correlated with the concentration in pheomelanin suggesting that their production can be regulated independently from each other. Redder barn owls had more pheomelanin in the skin and uropygial gland than white owls, while owls displaying larger black feather spots had more eumelanin in the skin than small-spotted owls. More data are required to evaluate whether melanin-based traits can evolve as an indirect response to selection exerted on melanin deposition in organs.

Historical changes in mortality patterns of diurnal and nocturnal raptors in the Czech Republic, Central Europe: 1913–2017

Raptors are apex predators and essential environmental indicators of ecosystem changes, with anthropogenic mortality heavily influencing their population trends. However, mortality factors can change over time, potentially diverting attention from the underlying drivers of raptor declines. Therefore, a broader perspective beyond studying contemporary mortality patterns for individual species is necessary to prioritize conservation efforts across raptor species. Our study analyzed data from six databases spanning 1913–2017 for the recoveries of 24,443 dead birds, including 24 diurnal and nocturnal raptor species in the Czech Republic. Our results showed that direct persecution historically dominated raptor mortality, but contemporary patterns are shaped by different sources of anthropogenic mortality. Mortality due to vehicle collisions, electrocution or collision at powerlines, and other anthropogenic sources gradually increased from 1913 until 2000 but began to decline in the last two decades. After 2000, vehicle collisions caused higher mortality rates than mortality linked to powerlines and other anthropogenic causes. Natural mortality was lower than average anthropogenic mortality during 1913–1990 but prevailed over anthropogenic mortality after 2000. Species-specific variation in raptor mortality causes was also detected. Direct persecution was the most common mortality factor for diurnal species with higher human-wildlife conflict. Powerline mortality affected mainly farmland and larger raptors, while vehicle collisions were the most significant mortality factor for most owl species and other anthropogenic mortality for barn owls Tyto alba. Raptor conservation efforts should prioritize reducing direct persecution, but coordinated actions to mitigate vehicle collision mortality are also necessary for nocturnal raptors.

Exclusion of barn owls Tyto alba from a greater horseshoe bat Rhinolophus ferrumequinum roost in Devon, UK

The greater horseshoe bat Rhinolophus ferrumequinum is a rare species in the UK that relies heavily on undisturbed stone buildings in which to breed. Barn owls Tyto alba are also a protected species that roost and raise their broods in similar places. This overlap in roosting requirements can lead to barn owls moving into buildings containing well-established greater horseshoe bat maternity colonies. This in turn could result in disturbance and abandonment of the building by the bats. Such an event occurred at one of the largest greater horseshoe bat roosts in the UK in 2018 when the colony deserted the roost after barn owls moved in. We describe measures used to exclude the owls while retaining access for the bats, to encourage the colony to return. The suite of modifications consisted primarily of wooden baffles and smooth surfaces on the entrances and doorways. Monitoring showed that the interventions were effective in excluding barn owls whilst allowing bats to access the building. Exclusion of the owls resulted in the return of the greater horseshoe bat colony in similar numbers to 2017 when owls were absent (2022: 92% of the adults) and breeding resumed (2022: 93% of the juveniles in 2017). This paper is the first to evaluate the before and after effects of protecting a bat roost from disturbance or predation by other species.

Distribution of Four Vole Species through the Barn Owl Tyto alba Diet Spectrum: Pattern Responses to Environmental Gradients in Intensive Agroecosystems of Central Greece.

Voles are the most common vertebrate pests in European agriculture. Identifying their distribution and abundance patterns provides valuable information for future management. Barn Owl diet analysis is one of the optimum methods used to record small mammal distribution patterns on large spatial scales. From 2003 to 2005, a total of 10,065 Barn Owl pellets were collected and analyzed from 31 breeding sites in the largest agroecosystem in Greece, the Thessaly plains. A total of 29,061 prey items were identified, offering deep insight into small mammal distribution, specifically voles. Four discrete vole species (Harting's vole Microtus hartingi, East European vole Microtus levis, Thomas's pine vole Microtus thomasi, and Grey dwarf hamster Cricetulus migratorius) comprised 40.5% (11,770 vole prey items) of the total Barn Owl prey intake. The presence and abundance of the voles varied according to underlying environmental gradients, with soil texture and type playing a major role. M. levis showed no significant attachments to gradients, other than a mild increase in Mollisol soils. It was syntopic in all sites with M. hartingi, which was the dominant and most abundant small mammal species, preferring non-arable cultivated land, natural grasslands, set-aside fields, and fallow land. M. thomasi was strictly present in western Thessaly and strongly associated with a sandy-clay soil texture and Alfisol soils. C. migratorius was the least represented vole (162 items), exclusively present in eastern Thessaly and demonstrating a stronger association with cereals, Mollisol soils, and an argillaceous-clay soil texture. This is the first study in Greece at such a large spatial scale, offering insights for pest rodents' distribution in intensive agroecosystems and their response to environmental gradients including soil parameters.

JENIS BURUNG PEMANGSA DI RESORT TEGALLEGA TAMAN NASIONAL GUNUNG GEDE PANGRANGO

The presence of birds of prey at the ecosystems in Gunung Gede Pangrango National Park (TNGGP) is very important as the top predator that helps stabilizing the pyramid or food chain. This study purpose to examine the diversity of raptors at Tegallega Resort in TNGGP. Fieldwork had been conducted from June 2021 to Maret 2022 and Oktober 2022 at this resort. Data collection used 1 km line transect where the point counts applied. Three line transects with 1 km in between distance were visited 11 times during the field data collection. The result shows that Tegallega Resort has five raptor species: crested serpent-eagle Spilornis cheela, changeable hawk-eagle Nisaetus limnaeetus, javan hawk-eagle N. bartelsi, Eurasian sparrowhawk Accipiter nisus, and barn owl Tyto alba. The eagles and hawk are protected by Indonesian regulations, while the owl is least concern species according to IUCN Redlist. When found the eagles flies in circles (soaring) with a sound when the weather is sunny and hot.

The analysis of the diet composition of the barn owl Tyto alba (Scopoli, 1769) from the region of Brzeg, Opolskie Voivodeship (Poland)

The analysis of the diet composition of the barn owl Tyto alba (Scopoli, 1769) from the region of Brzeg, Opolskie Voivodeship (Poland)

Nesting associations and breeding output of Barn Owls Tyto alba and Red-billed Choughs Pyrrhocorax pyrrhocorax sharing nest boxes

ABSTRACT Capsule Barn Owls Tyto alba and Red-billed Choughs Pyrrhocorax pyrrhocorax show differential breeding outputs as a consequence of sharing nest boxes. Aims To describe for the first time a commensal relationship between the Red-billed Chough and the Barn Owl during the breeding period. Methods During systematic monitoring of 90 nest boxes installed for Barn Owls in Navarre, northern Spain, it was found that in some boxes, Barn Owls and Red-billed Choughs nested simultaneously (inside and on top of the box, respectively). The clutch size, breeding success, and the number of fledglings of each species were compared when nesting together versus separately during the breeding period of 2015 and 2017. Results About one-third of the nest boxes observed during the study were simultaneously used by both Barn Owls and Red-billed Choughs. No relationship was found between the composition of the habitat around the boxes and the degree of occupancy for either species. Nest box sharing had no significant effect on the breeding output of Barn Owls, but improved the breeding performance of Red-billed Choughs by increasing the number of fledglings in relation to clutch size. Conclusion Barn Owls can provide benefits to Red-billed Choughs by controlling the populations of rodents, which could act as nest predators. Given that Barn Owls apparently do not obtain benefits nor incur any costs by the association with Red-billed Choughs, this interaction may be defined as commensalism. To our knowledge, this is the first documented nesting association between a corvid and an owl species.

Effects of a forest landscape on the prey composition of the Common Barn-owl Tyto alba in southern Bulgaria

The diet of the Common Barn-owl in a forest- and shrub-dominated hunting area in the Strandzha Mountains, southern Bulgaria, was identified from 516 prey specimens. Shrews (52.9% by number, 26.7% by biomass) and rodents (42.1% N, 71.5% B) were prevalent. Among them, White-toothed shrews, Criocidura sp., (45.3% N, 21.4% B) was the most numerous prey genus. Mice, Apodemus sp., (15.7% N, 29% B) contributed with the largest share to the food biomass due to high predation of Striped field mice, A. agrarius, (12.2% B). The proportions of forest species in diet (Apodemus sp, Sorex sp., and dormice liridae) increased with the higher proportion of forest habitats (forests and shrublands cover more than 25% of the area) in most Barn Owl hunting territories in southern Bulgaria.

Difference in small mammal assemblages in the diet of the Common Barn-Owl Tyto alba between two landscapes

As an opportunistic predator, the Common Barn-owl (Tyto alba) proved to be an appropriate model organism to survey the composition of small mammal assemblages. This study analysed barn owls’ pellet samples from 14 localities containing 34 animal taxa and 4,088 prey items in two years (2015–2016). Two groups of samples (7–7 localities) were separated based on the dominance of semi-natural habitats and agricultural lands. Rarefaction analysis proved that the species richness and diversity of barn owls’ diet were significantly higher in semi-natural landscapes. The multiple regression analysis between PCA scores showed that in the agrarian landscape the abundance of generalist species was influenced by the proportion of forests, while the value of the trophic level index was determined by the size of arable fields. In the case of semi-natural landscapes, the abundance of the synantrop guild and generalist species, especially S. araneus and A. agrarius, was influenced by the proportion of urban areas, the number of habitats and the size of arable fields. The results of this study suggested that the small mammal consumption of the Common Barn-owl is significantly different in the two landscapes, which reflects the impact of habitat heterogeneity and agricultural activity on prey availability.

Home range size and habitat quality affect breeding success but not parental investment in barn owl males

Life-history theory predicts that parents should balance their limited resources to maximize lifetime fitness, limiting their investment in current reproduction when the fitness value of current progeny is lower than that gained by producing offspring in the future. Here, we examined whether male barn owls (Tyto alba) breeding in low-quality habitats increased their parental effort to successfully complete offspring rearing or limited their investment by paying a fitness cost while saving energy for the future. We equipped 128 males with GPS devices between 2016 and 2020 to collect information on home range size, habitat composition, food provisioning rate to the brood and nightly distances covered. We also recorded nestlings’ growth and survival, as well as males’ body mass variation and future reproductive success. Males living in lower-quality habitats exploited bigger home ranges compared to individuals whose nests were settled in prey-rich habitats. They fed their brood less frequently, while covering longer nightly distance, resulting in a slower growth of late-hatched nestlings and ultimately in a lower fledging success. As males did not differ in body mass variation or future reproductive success our findings suggest that males hunting in home ranges with less prey-rich structures do not jeopardize future reproduction by investing disproportionately larger resources to compensate for their current low home range quality.

High trophic niche overlap in mixed‐species colonies using artificial nests

Although successful at recovering endangered populations, conservation actions based on nest provisioning seldom consider how they shape the composition of communities and alter interspecific interactions. Specifically, the extent to which dietary overlap within these communities may affect the conservation of target species has rarely been assessed. In Southern Europe, sites of large‐scale nest‐site provisioning aimed at recovering Lesser Kestrels Falco naumanni populations have attracted non‐target bird species, resulting in mixed breeding assemblages that might promote interspecific competition for resources during breeding. Here we used stable isotope analysis (δ15N and δ13C) to assess inter‐ and intraspecific dietary segregation in these assemblages and investigate the mechanisms allowing species coexistence. We examined resource partitioning and trophic niche overlap among Lesser Kestrels, Common Kestrels Falco tinnunculus, European Rollers Coracias garrulus, Western Barn Owls Tyto alba, Little Owls Athene noctua and Spotless Starlings Sturnus unicolor; and within species between parents and their offspring. Similar isotope ratios and highly overlapped niches, particularly among Lesser Kestrels, Rollers and Starlings, suggest limited dietary segregation and use of similar prey. Within species, parent–offspring segregation was marked across all species. Our results indicate that species breeding in these assemblages occupy similar ecological niches, despite a potential increase in competition. High resource availability in the area may permit coexistence but the viability of mixed‐species groups may be compromised in areas with limited resources, which are predicted to expand with ongoing human‐ and climate‐induced changes. Conservation practices based on nest provisioning should consider the ecological niches of target and non‐target species as well as their interactions.

Role of The Barn Owl Tyto alba javanica as a Biological Agent For Rat Pest Control in The Oil Palm Plantation of Bumitama Agri Ltd.

The Rat was one of the main pests in oil palm plantations cause able to significantly reduce oil palm yield. This research aimed to know the role of T. alba in controlling rat pest attacks in oil palm plantations. Observations were made on estates that had different ratios/hectare of T. alba nest boxes such as Pantai Harapan Estate (1:54), Kruing Estate (1:80), and Sungai Mirah Estate (1: 162). The presence of T. alba was observed every month at each nest box. The attack rate of rats was observed every month during January – December 2020 through a census attack in each plant block. Kruing estate was the highest nest box occupancy rate of T. alba of 37 %. The occupancy rates of Pantai Harapan Estate and Sungai Mirah Estate were 31% and 6%, respectively. While the rate of rat attacks in Pantai Harapan Estate, Kruing Estate, and Sungai Mirah Estate were 0.70 % a, 0.60 % a, 2,30 % b, respectively. The barn owl T. alba can control the attack of rats naturally and reduce the use of rodenticide. Rodenticide applications at Bumitama Agri Ltd for rat pest control in 2020 also decreased by 45% when compared to applications in 2019.

Diet of sympatric Barn Owls Tyto alba and Short-eared Owls Asio flammeus in an agricultural landscape in south-east Spain

The diets of the Barn Owl Tyto alba and the Short-eared Owl Asio flammeus have been extensively studied worldwide over the past few years, especially on the Iberian Peninsula. Nevertheless, very few studies have examined the diets and the trophic niche overlap in areas where these two raptor species occur in sympatry. As such, in this study we compared the diets of the Barn Owl and the Short-eared Owl inhabiting agricultural landscapes of the Vega de Granada, south-east Spain, based on pellet analysis. The diets were very similar, as both owls preyed mainly on small mammals, the Algerian Mouse Mus spretus being the prey most commonly found in pellets from both species. Although the diet of the Barn Owl was more diverse than that of the Short-eared Owl, the food niche overlap was very high, thus indicating a low interspecific trophic segregation. Despite the similarities between both diets, the frequency of the Mediterranean Pine Vole Microtus duodecimcostatus was much higher in pellets from the Barn Owl, thus suggesting that the Barn Owl may exert pest control in years when the Mediterranean Pine Vole occurs in high numbers.

Small mammals from barn owl <em>Tyto alba</em> pellets in a Mediterranean agroforestry landscape of central Italy

In order to investigate diversity patterns and similarities in the small mammal communities of an agroforestry landscape in western central Italy (Maremma of Lazio), we analyzed, in a multivariate setting (Cluster analysis, DCA-Detrended Correspondence Analysis), the prey content of barn owl Tyto alba pellets collected along one year in five sampling sites. Small mammal communities were composed by guilds typical of habitats included in agroforestry landscapes (croplands and mosaics, forests and ecotones, wet habitats and synanthropic ones). Since landscape matrices were characterized almost everywhere by croplands, typical agro-ecosystem species (Apodemus cfr. sylvaticus, Microtus savii, Mus domesticus and Soricidae) dominated in the majority of the collecting sites. The statistical analyses show how small changes in land use and cover can explain the faunal differences between sites, with the occasional presence of Arvicola italicus in wet habitats, and of Muscardinus avellanarius and Sorex samniticus in sites dominated by forest or agroforestry ecotones. Communities recorded in sites characterized by wet and forest habitats showed a higher distance from the others, dominated by croplands. Communities occurring in landscapes with the lowest habitat diversity showed also the lowest species diversity.

The 100,000\u201377,000-Year Old Middle Stone Age Micromammal Sequence from Blombos Cave, South Africa: Local Climatic Stability or a Tale of Predator Bias?

In this paper, we present a case study of the micromammal sequence from Marine Isotope Stage 5 (130,000–71,000 YBP) at Blombos Cave on the southern Cape coast of South Africa. Our analyses of the micromammal assemblage from 100,000 to 76,000 YBP shed light on micromammal taxonomic distributions, local palaeoenvironments, and site formation processes at this renowned Middle Stone Age site. Taphonomic analyses indicate that spotted eagle owls (Bubo africanus) were the main predator species responsible for accumulating the micromammals, but with contributions from barn owls (Tyto alba). In addition, the micromammal bones have been subjected to a range of post-depositional processes, some of which are associated with microbial actions likely resulting from human or animal activities in the cave. We have recorded three species in the archaeological assemblage that do not occur in the Blombos area today. These are the Hottentot golden mole (Amblysomus hottentotus), Duthie’s golden mole (Chlorotalpa duthieae), and Laminate vlei rat (Otomys laminatus). The biodiversity indices based on micromammal species suggest that local vegetation consisted of different habitats that could sustain a diverse small mammal population. During MIS 5c/5b, the diversity of species declined, but there was still a mosaic of vegetation habitats present in the local area. On a larger temporal scale, climate conditions were slightly more humid than at present, and winter rainfall was seemingly greater. The amount of winter rainfall would have been similar to locations currently c. 50–150 km further west of Blombos Cave. However, based on micromammal proxies, there were seemingly no major fluctuations visible in climate or vegetation composition during the entire 24,000-year period. We suggest that the explanation could be multifaceted, two potential factors being predator bias derived from the owls’ preference for generalist micromammal species or an actual reflection of local stable climatic and environmental conditions in the Blombos area.

Geographic variation in body size and plumage colour according to diet composition in a nocturnal raptor

Predator–prey interactions are amongst the strongest selective forces that promote the evolution of local phenotypes in both predators and prey. However, intraspecific spatial covariation in phenotypic traits between predators and prey has been rarely investigated, especially at a large geographic scale. Here, we studied the covariation between prey composition and some phenotypic traits, such as wing length, bill length and plumage colour, of a widely‐distributed nocturnal predator, the western barn owl Tyto alba. By using 3100 specimens collected across its entire range of distribution, spanning from Europe to Middle East and Africa, we showed that wing length positively covaries with prey size, but not with taxonomic composition. This finding suggests that larger prey might have selected for larger body size and/or that larger individuals might be more selective in hunting large prey. In addition, we also found that paler‐plumaged populations generally hunt larger prey. Paler barn owls might be thus better specialized in capturing averagely larger prey and/or mainly hunt in habitats where larger prey are more abundant. In addition, considering that paler individuals are generally larger than brownish ones, it is possible that paler plumage colour might have evolved as a by‐product of selection towards a large body size, which in turn have emerged in response to prey size composition. However, irrespectively of the direction of causality and the phenotypic target of selection, we showed that predator–prey interactions can affect spatial phenotypic variation by promoting the evolution of local adaptations.

Juvenile Barn Owl Tyto alba dispersal: a radio-tracking study of roost site selection in relation to landscape features

ABSTRACT Capsule Radio-tracking of 19 juvenile Barn Owls Tyto alba alba in England revealed different spatial patterns for males and females, that tree branches were frequent roost sites, and that altitudinal distribution changed after owls acquired home ranges. Aims To gain new understanding of the natal dispersal of Barn Owls by identifying roost sites and analysing their spatial pattern, site type, altitude, and location in relation to linear features. Methods Diurnal roost sites of 19 juvenile Barn Owls were located by radio-tracking in southwest England. We evaluated the possibility of sex-bias in natal and between-roost dispersal distances and tested for change in the site types used as natal sites and daytime roosts. We tested for differences in the altitude of roosts used during the dispersal phase and those used once the birds acquired a home range, and we analysed the location of roost sites in relation to major roads, railways, and rivers. Results Barn Owls commenced dispersal when they were between 63 and 122 days after hatching (median = 94). Roost sites used by females were more widely scattered than those used by males. Owls roosted most frequently in trees, despite most natal sites being in buildings. The altitude of roost sites during dispersal in elevated landscapes was higher than those on their acquired home range. We could find no evidence that owls followed linear features during dispersal, nor that hills were barriers to dispersal. Conclusion Post-fledging roosting behaviour of female and male Barn Owls differed spatially and was congruent with a wider pattern of sex-bias in avian natal dispersal. Frequent roosting on tree branches had implications for exposure to adverse weather. Some juveniles roosted at altitudes that were higher than subsequent roosts on their acquired home range. We found no evidence to support the proposal that linear landscape features constituted dispersal corridors.