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This species of owl nests on the ground, building a scrape on top of a mound or boulder. A site with good visibility is chosen, such as the top of a mound with ready access to hunting areas and a lack of snow. Gravel bars and abandoned eagle nests may be used. The female scrapes a small hollow before laying the eggs. Breeding occurs in May to June, and depending on the amount of prey available, clutch sizes range from 3 to 11 eggs, which are laid singly, approximately every other day over the course of several days.
Hatching takes place approximately five weeks after laying, and the pure white young are cared for by both parents. Although the young hatch asynchronously, with the largest in the brood sometimes 10 to 15 times as heavy as the smallest, there is little sibling conflict and no evidence of siblicide. Both the male and the female defend the nest and their young from predators, sometimes by distraction displays. Males may mate with two females that may nest about a kilometre apart. This powerful bird relies primarily on lemmings and other small rodents for food during the breeding season, but at times of low prey density, or during the ptarmigan nesting period, they may switch to favoring juvenile ptarmigan.
They are opportunistic hunters and prey species may vary considerably, especially in winter. They feed on a wide variety of small mammals such as meadow voles and deer mice , but will take advantage of larger prey, frequently following traplines to find food.
Some of the larger mammal prey includes hares , muskrats , marmots , squirrels , rabbits , raccoons , prairie dogs , rats , moles and entrapped furbearers. Birds preyed upon include ptarmigan, ducks , geese , shorebirds , pheasants , grouse , coots , grebes , gulls , songbirds , and even other raptors, including other owl species. Most of the owls' hunting is done in the "sit and wait" style; prey may be captured on the ground or in the air, or fish may be snatched off the surface of bodies of water using their sharp talons.
Each bird must capture roughly 7 to 12 mice per day to meet its food requirement and can eat more than 1, lemmings per year. Unlike most owls that hunt at night, snowy owls are also diurnal and hunt during the day and night.
Snowy owls, like other carnivorous birds, swallow their small prey whole. Strong stomach juices digest the flesh, while the indigestible bones, teeth, fur, and feathers are compacted into oval pellets that the bird regurgitates 18 to 24 hours after feeding. Regurgitation often takes place at regular perches, where dozens of pellets may be found. Biologists frequently examine these pellets to determine the quantity and types of prey the birds have eaten. When large prey are eaten in small pieces, pellets will not be produced.
Although snowy owls have few predators, the adults are very watchful and are equipped to defend against any kind of threat towards them or their offspring. During the nesting season, the owls regularly defend their nests against Arctic foxes , corvids and swift-flying jaegers ; as well as dogs , gray wolves , and avian predators. Males defend the nest by standing guard nearby while the female incubates the eggs and broods the young. Both sexes attack approaching predators, dive-bombing them and engaging in distraction displays to draw the predator away from a nest.
They also compete directly for lemmings and other prey with several predators, including rough-legged hawks , golden eagles , peregrine falcons , gyrfalcons , jaegers, glaucous gulls , short-eared owls , great horned owls , Eurasian eagle-owls , common ravens , wolves, Arctic foxes, and ermine. Unlike many sparrows, which are commonly associated with grassland communities, the Chipping Sparrow prefers open woodlands, the borders of natural forest openings, edges of rivers and lakes, and brushy, weedy fields. Its preference for nesting in the groves and open glades of coniferous forests, and for foraging in brushy open areas, suit this sparrow to human-modified habitats.
The Chipping Sparrow is a common summer resident in towns and gardens and around more isolated human habitations in many parts of North America. Even though common and abundant, the Chipping Sparrow is surprisingly under-studied. For example, until recently it was widely accepted that the Chipping Sparrow was a typically territorial and monogamous species, but evidence from Ontario now challenges this assumption.
Observations of color-banded birds show that once nesting has begun, males move through neighboring territories, where they may copulate with several different females. It is not known, however, if this behavior is characteristic of all Chipping Sparrow populations. While this study considered predator vision, it only focused on one species that moves around an area actively, and whose appearance is also complicated by the need for socio-sexual signals 28 , Generally, we still know comparatively little about how animals achieve successful camouflage in complex real environments.
Here, we studied nine species of ground-nesting birds nightjars, plovers and coursers in Zambia Fig 1 , and used image analysis and modelling of predator vision to test whether individual appearance the eggs from all nine species, and the plumage of adult nightjars that sit tightly on their eggs better matched their chosen backgrounds than other potential backgrounds. The different species all nest in the hot Zambian dry season, sitting on their eggs and fleeing when a predator approaches see below. They utilise different visual backgrounds, ranging from dry leaf litter to scorched bare ground, with little to no nest structure built.
This shows the variation in camouflage match among species, and some of the different colours and markings found in the eggs and adults used for concealment. Images of one adult nightjar, one nightjar clutch, and one courser clutch to predator vision. These images see Methods correspond to a dichromatic mammal, which sees colours that to humans are yellows and blues, a trichromatic primate with equivalent colour perception to humans, and a tetrachromatic bird.
The latter has colour vision involving four cone types, including ultraviolet, and because there is no standard way to illustrate the range of colours birds may see conventional images being restricted to three channels , we here present separate trichromatic images based on the avain LW, MW, and SW cones , and greyscale UV violet; VS cone images whereby brighter pixels correspond to greater UV information.
British Birds with their Nests and Eggs. Vol. 1-6
We studied background nest site selection at three spatial scales corresponding to adjacent backgrounds approximately 5 m from the nest and 5 cm from the nest, and the backgrounds of nest sites chosen by conspecific individuals Fig 2. The first m and second cm spatial scales allow us to test whether individuals refine their nest site selection towards a given substrate area or patch, and then choose a specific fine-scale point within that patch.
The third spatial scale allows us to determine whether individuals choose nest sites that improve their own individual camouflage compared to other potential nest sites chosen by other individuals. Analysis of egg camouflage between chosen nest sites and potential other sites. We compared the camouflage of eggs or adult nightjars at three spatial scales. Finally, we compared the camouflage of targets to the chosen nest background of each individual to the sites chosen by other conspecifics. Our past work has shown that the degree of camouflage in these birds directly influences the likelihood of nest survival against wild predators Specifically, nest survival in nightjars is predicted by adult camouflage since adults sit tightly on the nest until a threat is very close , whereas nest survival in plovers and coursers which flee early when a threat arises depends on egg camouflage.
Furthermore, we have also recently shown that the birds modify their escape distance based on their individual level of camouflage: Coupled with this evidence, our study system is ideal for investigating camouflage and microhabitat choice because the background environment where the birds nest is unambiguous, and because each species nests in areas that differ with respect to the range of substrates encountered and the type of visual background.
Based on camera-trap recordings of predation events at our monitored nests, we modelled camouflage with respect to the visual systems of the main predator groups, specifically tetrachromatic birds, trichromatic primates, and dichromatic mammals. We chose these predator visual systems based on recordings of egg predation by diurnal predators including grey-headed bush-shrikes Malaconotus blanchoti , vervet monkeys Chlorocebus pygerythrus , and banded mongooses Mungos mungo Modelling of predator vision and image analysis allowed us to quantify several metrics of camouflage, including colour, luminance perceived lightness , and pattern 39 — We used a range of metrics that have previously been shown to predict survival of nests in the field and escape behaviour by adults, as well as predicting human detection times of camouflaged artificial targets see Methods.
Data for plovers and coursers were analysed separately from data for nightjars because plovers and coursers Charadriiformes flee the nest early in response to a potential threat, exposing the eggs to potential detection for long periods.
In contrast, adult nightjars Caprimulgiformes sit tight on their nest until a predator is close, such that adult plumage camouflage is also crucial for egg survival First, since there is variation in egg and plumage appearance among individuals, the hypothesis that nesting birds choose microhabitats that improve their camouflage predicts that birds should choose nest sites that improved their own specific camouflage compared to sites selected by conspecific individuals. Second, the hypothesis predicts that background selection for camouflage should occur at two further spatial scales: Furthermore, based on past work 37 , 38 the hypothesis predicts that microhabitat choice would be based on egg camouflage in plovers and coursers, and on both egg coloration and adult plumage camouflage in nightjars.
As predicted, individuals of a given species chose nest sites that were a better match to their own eggs than to the nest sites of other individuals of the same species all results summarized in Table 1 ; Fig 3. Plover and courser egg luminance and colour matches did not significantly differ between their own versus conspecific nest sites. Nightjar egg colour matches did not differ between their own and conspecific nest sites.
Model estimates of planned comparisons comparing the match between each subject egg or adult bird and its chosen nest background versus its match to backgrounds chosen by its conspecifics. Estimates above zero indicate that the chosen background is a better fit than non-chosen conspecific backgrounds, supporting the hypothesis that females select backgrounds that match their own camouflage, rather than a species-specific habitat preference.
The y-axis model estimates shows the model estimate multiplier for pattern, luminance, or colour matching. Summary of results comparing camouflage match to the chosen versus non-chosen locations for all three metrics. The majority of results show support for microhabitat selection, especially for pattern. Where a model did not retain zone as a predictor, Chi-square model comparison results are shown. F-statistics are shown for conspecific comparisons as the variable has two levels same or different nest. T-values for planned contrasts are shown for fine scale and local scale results Tukey Post-hoc test.
Next, we tested whether individual egg camouflage was a better match to i the chosen background versus comparable areas 5 m away, and ii the chosen background within ca. This resulted in four zones, with two comparisons planned between these zones i. In all cases bar one, eggs were better matched to the chosen background than to non-chosen backgrounds a few cm away and 5 m away Fig 4 ; see statistical results in Table 1. However, eggs matched the luminance of their chosen background less closely than the background a few cm away.
This suggests a potential trade-off resulting in pattern-matching being selected over luminance-matching. This finding shows that pattern and luminance matching are independent of each other in randomly selected patches of habitat. However, the areas chosen as nest sites matched the patterns of the eggs better than expected, and matched luminance worse than expected.
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This finding is consistent with the hypothesis that plovers and coursers select nest sites that match the patterns of their eggs at the expense of matching the luminance. Egg colour match did not differ between chosen and nearby non-chosen backgrounds at either spatial scale.
Estimates greater than zero show that the zone nearer the clutch matches the eggs or adult better than the zone further away. Estimates below zero show the opposite effect, matching the more distant zone better than the nearer zone. Colour matching for plover and courser eggs did not differ between zones, so is not included. An exception was nightjar egg luminance, which was better matched to non-chosen backgrounds 5 m away than to their chosen nest site. However, nightjar eggs were less well matched to the chosen background under trichromatic and dichromatic mammalian vision, whereas they were better matched under avian tetrachromatic vision.
Finally, since nightjars flee from the nest only when a predator is nearby our data show a mean flush distance across all three nightjar species of 1. The plumage pattern of Mozambique and pennant-winged nightjars matched their chosen nest backgrounds better than the adjacent background a few cm away, whereas fiery-necked nightjars did not; individuals of all nightjar species matched their chosen background for pattern better than other potential backgrounds 5 m away.
For luminance, nightjars were better matched to chosen backgrounds than to adjacent backgrounds both a few cm away and 5 m away. Plumage colour was a better match to chosen nest sites at the metre scale, but not the cm scale. First, they were able to choose a suitable nesting patch in the general habitat at a scale of approximately five metres, and second, they refined their nest site selection within that patch at a very fine scale within a few cm.
Moreover, because females selected sites that matched their own eggs and plumage better than those of their conspecifics, decisions were made with reference to their own individual phenotype rather than following a general species-wide strategy.
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Our findings are consistent with those of a recent laboratory study on substrate selection in nesting quail 27 , and also tie in with our recent study of escape distances in the present study system. The latter showed that individuals modulate escape behaviour based on their level of camouflage, providing further evidence that nesting birds can modify their behaviour in response to perceived levels of concealment These findings also concur with a study of island populations of wall lizards, which found that individuals were more likely to be found sitting on backgrounds that provide better camouflage than on other potential sites Our work here shows that the benefits of microhabitat choice and behavioural changes based on assessment of individual camouflage extend across a wide range of avian species, several spatial scales, and two life history stages adults and eggs.
The above results demonstrate that behavioural choice of substrates and backgrounds may offer a major route to enhancing camouflage, and suggest that studies that simply compare the camouflage of individuals against random background samples may sometimes yield inaccurate findings if individuals vary both in appearance and substrate preference. Many camouflaged species show either discrete polymorphisms or high levels of continuous phenotypic variation 3 , 9 , 19 , 42 , In such cases it may be particularly beneficial for individuals to have corresponding substrate preferences, both to improve their level of camouflage and perhaps to increase the range of microhabitats exploited.
Microhabitat choice may also have important broader evolutionary consequences. For example, some insect species comprise several morphs that occur on different host plants, and disruptive selection against intermediates may potentially drive speciation through reproductive isolation of populations in sympatry We cannot entirely rule out the possibility that our results could be partially explained by predation having eliminated poorly-camouflaged nests from our dataset before we could record them.
While this would itself be an important finding given that individual variation in camouflage matching in wild animals has rarely been directly demonstrated to affect predation risk , we feel that this explanation is unlikely to fully explain our results. Second, a proximate mechanism of background selection based on individual egg coloration has already been established in controlled laboratory experiments with ground-nesting birds, implying that this is a more parsimonious explanation for our results They do not preclude other factors, beyond the scope of this study, influencing nest site selection in birds; these may include habitat visibility for detecting predator approaches, thermal considerations, and vicinity to other nesting birds.
This is, however, highly unlikely to have affected the majority of our findings see Supplementary Information. These factors may add to a rich complexity of factors influencing background selection in birds and other animals. Beyond the question of how widespread background choice for camouflage might be, there is much to be gained from trying to disentangle the mechanisms involved. In birds, egg coloration appears to be strongly heritable, with relatively little environmental influence Exactly how birds make appropriate decisions is not yet clear but we suggest it could arise through two not mutually exclusive mechanisms.
First, if background choice is also heritable, a genetic correlation could allow individuals with a given egg phenotype to also inherit the appropriate substrate preference. Alternatively, behavioural preferences could develop with experience as birds learn what their eggs look like, and so to make appropriate decisions.
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The latter mechanism seems more likely because inherited behavioural choice would offer little flexibility, and also because there is good evidence that birds learn their egg appearances in other contexts. For example, hosts of brood parasites appear to learn what their own eggs look like in initial breeding attempts, and then reject any subsequent parasitic eggs that deviate from this template of appearance A further and related potential mechanism could involve chicks imprinting on specific backgrounds after hatching, and basing nest site choice on this when they later become breeders.
Overall, camouflage can be enhanced not only through genetic or developmental changes in individual appearance, but also through individual behavioural choices. Thus, in many species the value and tuning of animal camouflage may result from a complex mixture of morphology, behaviour, and environment.
More broadly, our study underlines that animals possess sensory and potentially cognitive mechanisms that allow them to improve the adaptive value of their own individual phenotype by choosing appropriate backgrounds. We should further look for individual background choice in the many other contexts where signalling success is affected by aspects of the environment, such as conspicuous warning coloration and sexual signals 48 — The study system, general methods, and quantification of camouflage closely followed our past work including that demonstrating how our camouflage metrics predict survival of the nests of the birds we study here and a range of past and recent methodological approaches 37 , 38 , Our dataset here overlaps with our previous work with respect to the individual birds recorded and measured, and to some of the images of the natural backgrounds used to assess camouflage, with the addition of further comparison background images taken at 5 m scales used only for this study.
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The study site comprised c. Fieldwork was undertaken during the hot dry season, when an open understorey affords nesting habitat for the ground-nesting bird species we studied. The field sites are in an agricultural region, but the cultivated areas primarily maize and tobacco crops are comparatively small and occur within a greater area of natural habitat deciduous miombo woodland and grassland. As such, predator communities should not differ greatly from conditions occurring before than human impact on the region. Most nests were found by local farm workers, detected when the birds flushed on approach, or through nocturnal eye-shine from torchlight.
Our sample of nests may lack the extremes of camouflage matching if we were unable to find the most camouflaged nests, and if some of the least camouflaged nests were attacked by predators first. However, our resulting sample should remain ecologically representative of the surviving nests, and indeed there was considerable variation in survival and camouflage among them We took digital images with Nikon D cameras, fitted with mm Micro-Nikkor lenses, which transmit ultraviolet UV light.
The cameras had undergone a quartz conversion Advanced Camera Services Limited, Norfolk, UK to allow sensitivity to both human-visible and ultraviolet wavelenghts, involving replacing the UV and IR blocking filter with a quartz sheet to allow visual analysis throughout the avian-visible spectrum 51 , For photographs in the human-visible part of the spectrum, the lens was fitted with a Baader UV-IR blocking filter transmitting to nm.
UV photographs were taken using a Baader UV pass filter transmitting to nm. During the brief crepuscular periods at our study site, there would be changes in ambient light spectra, background contrast, and shadows, but we cannot test those effects with our current dataset. To quantify adult nightjar camouflage, we closely followed past work on the same system 37 , Images of nightjars sitting on their nests were taken from a standing position from 5 m distance and the flank least obscured by vegetation.
If both sides were clearly visible, images were taken so as to avoid directly facing the sun. Acquiring images of adult plovers and coursers was not possible because these birds frequently flush at long distances. This enabled us to control for lighting conditions in the adult bird images without the standard needing to be in the same photograph the sequential method Images of plover, courser, and nightjar eggs were acquired in situ from 1.
We chose 5 m as the photography distance for the meter scale because adult nightjars could reliably be photographed at this distance without fleeing their nests, and because control photographs taken 5 m on either side of the nest did not overlap with one another. For the fine cm scale, we photographed from directly above the clutch because this included both the largest clutches and surrounding nest site area. To calibrate the images, all photos were linearized to control for the non-linear response of the camera to light intensity, and then standardized against the grey standard to remove effects of the light conditions