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Food supplementation can potentially provide additional energy and/or liberate time usually devoted to foraging, enabling birds to invest more in activities, such as nest construction, that provide long-term fitness benefits. Although nest construction can represent large temporal and energetic investments in a breeding attempt, few studies have investigated how it is influenced by food supplementation. We studied the effects of food supplementation on nest construction by Blue Tits (Cyanistes caeruleus) and Great Tits (Parus major). We found that food supplementation advanced nest construction and egg laying in both species. Food supplementation also resulted in truncation of the nest construction period of Blue Tits, but not Great Tits. The duration of the nest-lining period was not influenced by food supplementation in either species. Unexpectedly, food supplementation resulted in significantly shallower nests in Blue Tits but not in Great Tits. We conclude that food availability can constrain nest construction behaviour and that these constraints can be released by the provision of supplementary foods prior to and throughout nest construction.

Masked boobies Sula dactylatra, like many other species of Sulidae, do not construct elaborate nests. However, their nest sites produce a characteristic ‘nest signature’. We found that these nest signatures could apparently be seen in freely available satellite images (Google EarthTM) of the main island of Ascension in the south Atlantic. We verified that this was the case by comparing nest signatures detected on these satellite images with field reports of occupied nests. We found that the locations of these nest signatures determined from satellite images agreed closely with the coordinates of actual nests on the ground. We used this information to determine the position and size of a previously unreported masked booby colony on the island. Thus, we show that the presence and abundance of some species can be estimated using freely available satellite imagery if a suitable signature in the satellite image can be found. Regularly updated satellite imagery of target sites could also be used for population monitoring. While this would be expensive, initial evaluation of the technique for particular species or populations can be achieved using freely available images. We encourage wildlife managers to view their study sites on Google EarthTMfor evidence of their target species.

Sub-lingual oral fistulas are a condition first reported in New Zealand Stitchbirds (Notiomystis cincta) in which a lesion develops on the periphery of the mandible in the oral cavity and a fistula develops through which the tongue protrudes. We report that it arises in another species, the Sooty Tern (Onychoprion fuscata), breeding on Ascension Island in the south Atlantic. We discovered five adults with oral fistulas out of a total of 13,664 adults and chicks ringed during 11 breeding seasons over 13 years. Compared with other threats to the Ascension breeding population of over 180,000 pairs of Sooty Terns, we report that oral fistulas are a minor threat to colony stability, especially because afflicted birds were incubating eggs. Nevertheless, our findings suggest that further investigations of whether the condition is sex-linked, of its pathogenesis, and of its causation would augment our knowledge of the biology of Sooty Terns; such information might also have conservation implications for our understanding of the condition in endangered Stitchbirds. We call upon ornithologists to report oral fistulas in other species, because it would seem unlikely that these are the only two species predisposed to the condition.

The schematic eye ofStrix aluco, a nocturnal owl species, is described. A comparative and ecological context is used to examine the relationships between optical parameters of the eye and its light gathering and resolving powers. It is concluded that the essentially ‘nocturnal’ feature of the owl eye does not lie in either its light gathering power or the sensitivity of individual rod receptors. Differences in visual performance at low light levels between the owl and the diurnal pigeon appear to be attributable to differences in the retinal neural integration mechanisms of the two species. However, it is hypothesised that the neural mechanisms which mediate the extraction of spatial information from the retinal image throughout the nocturnal luminance range, can function in the owl eye only because of its absolutely large sized retinal image. Thus the primarily nocturnal feature of the owl eye is its absolutely large posterior nodal distance, retinal image brightness is maximised only as a secondary feature.

The European starling,Sturnus vulgaris, is a diurnal, ground feeding bird of the order Passeriformes. Aspects of its visual fields, eye movements and the optical structure of its eye are described.1.The magnitude of eye movements as a function of elevation in the median sagittal plane of the head was measured (Fig. 1). Maximum eye movement amplitude (32°) occurs in a plane which passes 50°–60° below the closed bill while the minimum amplitude (11°) occurs in a plane approximately orthogonal to this.2.The ways in which eye movements can alter the binocular and cyclopean retinal visual fields are described (Figs. 3 and 5). When the eyes are fully converged maximum binocular overlap equals 43° and this occurs approximately 40° below the bill. The bill intrudes into the visual field and when the bill is fully open, maximum binocular field width occurs between the mandibles. The eyes can also be swung backwards and upwards so that the margins of the monocular fields just coincide behind the head to give extensive visual coverage of the celestial hemisphere. When the eyes are in this position the frontal binocular field is almost abolished.3.The monocular retinal field is asymmetric about the optic axis and this can be attributed to asymmetry in the gross structure of the eye rather than asymmetry in the optical system (Figs. 4 and 9). From purely optical considerations the starling could have a maximum binocular field width twice that actually found.4.A schematic model of the starling eye optical system (Fig. 4 and Table 2) shows that optically this ‘flat-diurnal’ eye is a small scaled version of the larger ‘tubular-nocturnal’ eye of an owl (Figs. 6 and 7). In addition the calculated optical fields of these two eyes are very similar (Fig. 8), and these eyes differ in their maximum retinal image brightness by only 0.33 log10 (Table 4). However, although the rat and starling eyes are of similar total dioptric power they are found to be of quite different optical design (Fig. 7). It is concluded that no particular ‘diurnal’ feature can be discerned in the optical structure of the starling eye.5.The starling feeds mainly on the ground where it is known to employ vision to locate prey at short range. However, when foraging it is vulnerable to aerial predators. The visual problems which this presents are considered. How the bird's visual fields and eye movements may cope with these problems are discussed. There is evidence that the starling eye may embody a ‘ramp’ retina as a static accommodatory device which could facilitate the simultaneous detection of both close prey and distant predators (Fig. 9 and Table 5).

It is argued that accurate optical flow can only be determined if problems such as local motion ambiguity, motion segmentation, and occlusion detection are simultaneously addressed. To meet this requirement, a new multiresolution region-growing algorithm is proposed. This algorithm consists of a region-growing process which is able to segment the flow field in an image into homogeneous regions which are consistent with a linear affine flow model. To ensure stability and robustness in the presence of noise, this region-growing process is implemented within the hierarchical framework of a spatial lowpass pyramid. The results of applying this algorithm to both natural and synthetic image sequences are presented. Copyright 1994, 1999 Academic Press

Cormorants hunt both benthic (sedentary) and pelagic (motile) prey but it is not known if the energy costs of foraging on these prey differ. We used respirometry to measure the costs of diving in double-crested cormorants (Phalacrocorax auritus) foraging either for sedentary (fish pieces) or motile (juvenile salmon) prey in a deep dive tank. Short dives for sedentary prey were more expensive than dives of similar duration for motile prey (e.g. 20% higher for a 10s dive) whereas the reverse was true for long dives (i.e. long dives for motile prey were more expensive than for sedentary prey). Across dives of all durations, the foraging phase of the dive was more expensive when the birds hunted motile prey, presumably due to pursuit costs. The period of descent in all the dives undertaken appears to have been more expensive when the birds foraged on sedentary prey, probably due to a higher swimming speed during this period.

IT has long been assumed from considerations of the behaviour of owls that the vision of these species is much more sensitive than that of man. Experimental determinations of the absolute visual sensitivity of these birds have provided evidence in support of this view, suggesting that visual sensitivity in owls is of the order of 10 to 100 times (+1.0 log10to +2.0 log10units) that of man1,2. Owing to calibration problems, however, these values were probably inaccurate by at least a factor of 10 (refs 1, 2). I report here that in at least one highly nocturnal owl species, visual sensitivity is only slightly higher than that of man, and that the increased sensitivity can be attributed to optical factors rather than to any increase in sensitivity at the retinal level. This suggests that retinal mechanisms in both man and owl have reached the ultimate in sensitivity for the duplex retinas of terrestrial vertebrates.

THE function of the brightly coloured oil droplets situated within the cone receptors of the avian retina has long been the subject of debate1–4. Among other hypotheses it has been suggested that they reduce glare and chromatic abberation1, remove unwanted short-wavelength sensitivity2, and have a role in colour vision3–5. The transmission spectra of the droplets in the visible spectrum are typically those of short wavelength cut-off filters5,6–8, due to the presence in high concentration of carotenoids of dietry origin9–11. Recent evidence from microspectrophotometry suggests that the droplets filter the light reaching the visual pigment in the receptor's outer segment, resulting in the modification of the sensitivity functions of some of the individual receptors, from the broad band sensitivity of the visual pigment alone to a narrow band one12,13. It is also known that marked differences occur in the proportions of the variously coloured droplets between species, and also between different parts of the retina of a given species4,14–16. In the pigeon (Columba livia) the retina is divided into two distinct areas, known as the red and yellow fields, defined by their oil droplet populations. The red field is in the dorsal retina (frontal–ventral visual field), whereas the yellow field is in the ventral retina (dorsal visual field): the two fields differ in both the type and number of broad and narrow band receptors present, as well as in receptor density and the neural complexity of the retina13,17,18. We report here clear differences in the photopic spectral sensitivity of the two fields, determined behaviourally, and suggest that these reflect a difference in the colour vision mechanisms of the two areas.