Counting birds on farmland habitats in winter

Capsule Perimeter counts underestimate the number of birds using agricultural fields.

Aims To determine the degree to which farmland birds may be undercounted on field surveys and the factors influencing this.

Methods In a study of 96 fields, birds in the field were counted during a walk of the perimeter. Afterwards transects were walked across the field to determine the number of birds missed. Additionally in a national survey we looked at the birds seen in field transects expressed as a proportion of the total seen in the transect and on perimeter counts.

Results To obtain an accurate count of birds on a field requires the use of the whole-area search methodology especially for species such as Grey Partridge Perdix perdix, Snipe Gallinago gallinago, Skylark Alauda arvensis and Meadow Pipit Anthus pratensis. However, for a large number of species, including thrushes, flocking finches and buntings, over 90% of individuals can be recorded using the much less time-consuming perimeter count method.

Conclusions Perimeter counts are likely to be reliable for most species but special efforts may be required for a few cryptic species.     

Trophic niche width and overlap of two sympatric gulls in the southwestern mediterranean

The diets of two potential competitor species, Audouin's Larus audouinii and yellow-legged gulls Larus cachinnans, were examined while they bred at the Chafarinas Islands during 1993, 1994, and 1995. Data were collected during two commercial fishing regimes: (1) trawling and purse seine fisheries, and (2) diurnal trawlers only. Since the food supply for the gulls in this area was heavily reliant on the activity of purse seine fisheries, these contrasting situations allowed us to analyze short-term effects, induced by daily changes in food supply, on niche width, dietary shift, and niche overlap between the two species. Overall, both species relied mainly on fish for food, especially Clupeiforms, in the case of Audouin's gull irrespective of the fishing situation, and in the case of the yellow-legged gull, only when purse seine fishing was in operation. When purse seine boats did not operate (food shortage), yellow-legged gulls broadened their niche, consuming equal amounts of all the feeding resources, and they showed a dietary shift toward a greater consumption of prey from refuse tips. In contrast, Audouin's gulls did not change their niche width, but showed a slight dietary shift away from the consumption of epipelagic fish, compensated by an increase in reliance on benthic-mesopelagic resources. Niche overlap was clearly higher on days when both fishing fleets operated, probably because a superabundant food resource facilitates high overlap without affecting coexistence between the two species. Since our study was developed on the basis of daily variations in food supply, and competition effects are to be expected on a longer-term basis, these changes can be seen as the outcome of the coexistence of two species in stable competitive equilibrium. Received: 4 October 1996 / Accepted: 20 May 1997     

On the trophic fate of Phaeocystis pouchetii (Hariot). II. Grazing rates of Calanus hyperboreus (Krøyer) on diatoms and different size categories of Phaeocystis pouchetii

Experiments were conducted with CIV and C V copepodites of Calanus hyperboreus (Krøyer) to determine if they would feed on the prymnesiophyte Phaeocystis pouchetii (Hariot). We used analysis of gut pigment to estimate ingestion and clearance rates. In applying this methodology we have demonstrated that pigments can be completely extracted from whole animals within 90 min, and that laborious procedures of tissue homogenization and centrifugation are not required. We conducted two experiments. In the first experiment Stage IV copepodites were exposed to ≈1 mg C·1⁻¹ of either P. pouchetii flagellates, small colonies (25–200 μm), large colonies (> 200 μm) or mixed diatoms > 25 μm (primarily Chaetoceros socialis Lauder and Nitzschia grunowii Hasle). Ingestion rates and daily rations were almost four times greater on both sizes of colonies than on either Phaeocystis pouchetii flagellates or mixed diatoms. Daily rations of copepodites feeding on colonies ranged from 8.1 to 12.4% · day⁻¹, well within the range previously reported for Calanus hyperboreus or sympatric copepods of similar size. From the second experiment we determined that Stage V copepodites obtained a daily ration of 6.2 to 10.8% · day⁻¹ when feeding on small colonies of Phaeocystis pouchetii. We conclude that a diet of P. pouchetii colonies should sustain the metabolic and growth requirements of Calanus hyperboreus copepodites.     

Intercolony variations in movement patterns and foraging behaviors among herring gulls (Larus argentatus) breeding in the eastern Wadden Sea

Herring gulls (Larus argentatus) are opportunistic predators that prefer to forage in the intertidal zone, but an increasing degree of terrestrial foraging has recently been observed. We therefore aimed to analyze the factors influencing foraging behavior and diet composition in the German Wadden Sea. Gulls from three breeding colonies on islands at different distances from the mainland were equipped with GPS data loggers during the incubation seasons in 2012–2015. Logger data were analyzed for 37 individuals, including 1,115 foraging trips. Herring gulls breeding on the island furthest from the mainland had shorter trips (mean total distance = 12.3 km; mean maximum distance = 4.2 km) and preferred to feed on the tidal flats close to the colony, mainly feeding on common cockles (Cerastoderma edule) and shore crabs (Carcinus maenas). In contrast, herring gulls breeding close to the mainland carried out trips with a mean total distance of 26.7 km (mean maximum distance = 9.2 km). These gulls fed on the neobiotic razor clams (Ensis leei) in the intertidal zone, and a larger proportion of time was spent in distant terrestrial habitats on the mainland, feeding on earthworms. δ¹³C and δ¹⁵N values were higher at the colony furthest from the mainland and confirmed a geographical gradient in foraging strategy. Analyses of logger data, pellets, and stable isotopes revealed that herring gulls preferred to forage in intertidal habitats close to the breeding colony, but shifted to terrestrial habitats on the mainland as the tide rose and during the daytime. Reduced prey availability in the vicinity of the breeding colony might force herring gulls to switch to feed on razor clams in the intertidal zone or to use distant terrestrial habitats. Herring gulls may thus act as an indicator for the state of the intertidal system close to their breeding colony.     

Haldane's rule revisited: do hybrid females have a shorter lifespan? Survival of hybrids in a recent contact zone between two large gull species

Haldane's rule predicts that particularly high fitness reduction should affect the heterogametic sex of interspecific hybrids. Despite the fact that hybridization is widespread in birds, survival of hybrid individuals is rarely addressed in studies of avian hybrid zones, possibly because of methodological constraints. Here, having applied capture–mark–recapture models to an extensive, 19‐year‐long data set on individually marked birds, we estimate annual survival rates of hybrid individuals in the hybrid zone between herring (Larus argentatus) and Caspian (Larus cachinnans) gulls. In both parental species, males have a slightly higher survival rate than females (model‐weighted mean ± SE: herring gull males 0.88 ± 0.01, females 0.87 ± 0.01, Caspian gull males 0.88 ± 0.01, females 0.87 ± 0.01). Hybrid males do not survive for a shorter time than nonhybrid ones (0.88 ± 0.01), whereas hybrid females have the lowest survival rate among all groups of individuals (0.83 ± 0.03). This translates to a shorter adult (reproductive) lifespan (on average by 1.7–1.8 years, i.e. ca 25%) compared with nonhybrid females. We conclude that, in line with Haldane's rule, the lower survival rate of female hybrids may contribute to selection against hybrids in this hybrid zone.     

Responses of Black-headed Gulls Larus ridibundus to conspecific brood parasitism

Conspecific brood parasitism in birds occurs when a female inserts her egg into the clutch of her own species. If successful, i.e. the parasitic egg is accepted by the host, then the host female or pair rears the offspring of the parasite. In the present study, we studied natural conspecific brood parasitism in Black-headed Gulls (Larus ridibundus), and conducted series of the experiments with mimetic (conspecific) and non-mimetic (conspecific painted light blue) eggs to explore responses of the tested pairs towards these alien eggs. The natural parasitism rate was 10% and the probability of being parasitized significantly increased with nest density. Experimentally parasitized pairs rejected both types of experimental eggs at a similar rate: 14.3 % for mimetic and 25.5% for non-mimetic within 2 days. Non-mimetic eggs were more selectively rejected than mimetic eggs. The relationships between the probability of egg rejection (dependent variable) and predictor (independent) variables were examined by fitting generalized linear models. Contrast and intraclutch variation in ground color and spotting pattern and the volume of the egg had no significant effect on rejection behavior in either non-mimetic or mimetic eggs. However, nest density significantly positively affected rejection behavior of the Black-headed Gulls in both non-mimetic and mimetic treatments.     

Ultraviolet photopigment sensitivity and ocular media transmittance in gulls,with an evolutionary perspective

Gulls (Laridae excluding Sternidae) appear to be the only shorebirds (Charadriiformes) that have a short wavelength sensitive type 1 (SWS1) cone pigment opsin tuned to ultraviolet (UV) instead of violet. However, the apparent UV-sensitivity has only been inferred indirectly, via the interpretation that the presence of cysteine at the key amino acid position 90 in the SWS1 opsin confers UV sensitivity. Unless the cornea and the lens efficiently transmit UV to the retina, gulls might in effect be similar to violet-sensitive birds in spectral sensitivity even if they have an ultraviolet sensitive (UVS) SWS1 visual pigment. We report that the spectral transmission of the cornea and lens of great black-backed Larus marinus and herring gulls L. argentatus allow UV-sensitivity, having a λ_T0.5 value, 344 nm, similar to the ocular media of UV sensitive birds. By molecular sequencing of the second α-helical transmembrane region of the SWS1 opsin gene we could also infer that 15 herring gulls and 16 yellow-legged gulls L. michahellis, all base-pair identical, are genetically UV-sensitive.     

Untersuchungen an Regio olfactoria und Nervus olfactorius der Silbermöve (Larus argentatus)

Zusammenfassung Nach Perfusionsfixierung mit Glutaraldehyd und anschließender Nachfixierung mit OsO₄ erscheint die Regio olfactoria vom Vogel (Larus argentatus) in Alkohol bei schräg einfallendem Licht in einer goldbraunen Interferenzfarbe. Sie ist von der blaugrünen Regio respiratoria scharf abgegrenzt und bedeckt in einer Nasenhälfte 71 mm². Unter der Stereolupe ist ein fellartiges Faserwerk der oberflächlich ausgerichteten Riechgeißelbündel erkennbar. Der olfaktorische Saum ist in zwei funktionell-morphologische Zonen unterteilt. 1. In eine Innenzone, die die Riechkolben und die Mikrozotten der Stützzellen beherbergt und von der intervillösen Schleimschicht erfüllt wird. 2. In eine Außenzone, die aus der Schicht der oberflächlich ausgebreiteten Riechgeißeln und dem sie bedeckenden Terminalfilm gebildet wird. Als Produkt der Bowmanschen Drüsen ist der Terminalfilm mit osmiophilen Sekrettropfen versehen, die sich in ihm auflösen oder zu einem 500 Å feinen, osmiophilen Oberflächenfilm zerfließen. Durch positive PAS-Reaktionen konnte der Mucopolysaccharidgehalt beider Schleimschichten nachgewiesen werden.Die Riechkolben der Vögel erfahren durch etwa 200 Mikrovilli eine Oberflächenvergrößerung um rund 800% und sind mit 7–13 Riechgeißeln versehen, deren Länge 90–130 . beträgt. Das 9+2-Fibrillenmuster der 20–30 langen proximalen Geißelabschnitte geht in den 60–100 langen distalen Segmenten in eine regellose Formation von 8–16 Tubuli über, die in einer kurzen Endzuspitzung auf 2–6 abnehmen. Die von den Riechkolben zentralwärts ziehenden Mikrotubuli nehmen von anfänglich 270–210 im Sinnesfortsatz kontinuierlich an Zahl ab, um sich im apikal vom Kern gelegenen Golgiapparat zu verlieren. Außer durch Schleimpakete ist das Stützzellplasma durch knäuelförmige Endoplasmaformationen mit konzentrisch geschichteten Endoplasmaspalten charakterisiert.Der Axongehalt des rechten Riechnerven eines jungen Männchens vonLarus argentatus beträgt 3,7·10⁶, der eines alten Weibchens 2,9·10⁶; man kann auf eine Gesamtzahl der Riechrezeptoren von 7,4·10⁶ und 5,8·10⁶ schließen. Die Axone verlieren auf ihren Weg von der Lamina propria zum rostralen Riechnervendrittel etwa 2–3 Mikrotubuli. Neben Axonen mit 2–14 Mikrotubuli, die den Riechfaseranteil darstellen, enthält der Riechnerv noch einige Axone mit rund 20 und 40 Tubuli. Diese letztgenannte Gruppe tritt in synaptischen Kontakt mit Ganglienzellen im rostralen Riechnervenviertel. Die Ganglienzellen verkörpern das diffuse Ganglion terminale, dessen Bedeutung für eine nervöse Steuerung der Bowmanschen Drüsen diskutiert wird.Die funktionelle Bedeutung der Rezeptorzotten, des osmiophilen Oberflächenfilmes und der intervillösen Schleimschicht wird erörtert. Der Gehalt der Regio olfactoria der Silbermöve an Riechrezeptoren wird mit bereits vorliegenden Untersuchungen an Makrosmatikern verglichen.

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On the regio olfactoria and nervus olfactorius in the gull,Larus argentatus

Summary After perfusion with glutaraldehyde and subsequent fixation with OsO₄ the regio olfactoria of the gull (Larus argentatus) shows in alcohol with incident illumination a golden-brown interference colour. It is distinctly defined against the bluegreen regio respiratoria and covers in one side of the nasal cavity 71 mm². The olfactory region demonstrates under a magnifier a furlike structure, produced by the superficial bundles of olfactory cilia. The olfactory border can be subdivided into two layers, namely: 1. An inner layer, which contains the olfactory vesicles and microvilli of supporting cells and is covered by the inner mucous layer. 2. An outer layer, which is composed by the zone of the superficially spread olfactory cilia and the covering terminal film. As a product of the Bowman glands the terminal film is supplied with osmiophilic secretion droplets, which dissolve or rise up and flow into a 500 Å thick superficial osmiophilic coating. The contents of mucopolysaccharids of both mucous layers were proved by positive PAS-reaction.The free surface of the avian olfactory vesicle is increased to 800% by about 200 microvilli and has 7–13 olfactory cilia, the length of which measures 90–130 . The 9+2 fibrillar pattern of the 20–30 long proximal segments of the olfactory cilia changes into an irregular formation of 8–16 tubules of the 60–100 long distal segments, which contain in their short endings only 2–6 tubules. The number of microtubules in the sensory processes decreases continuously on the way from the olfactory vesicle to the soma from initially 270–210 until they disappear in the Golgi apparatus, located apical from the nucleus. Apart from copious drops of mucous the supporting cells are characterized by voluminous convolutes of endoplasmatic membranes.Concluding from the contents of axons of the right n. olfactorius of a young male with 3,7·10⁶ and an adult female with 2,9·10⁶ the total number of receptor cells is estimated to amount to 7,4·10⁶ and 5,8·10⁶. From the lamina propria to the distal quarter of the n. olfactorius the axons loose about 2–3 microtubules. In addition to the axons with 2–14 microtubules representing the olfactory nerve fibers the n. olfactorius also consists of some axons with about 20 and 40 tubules. The latter fibers come into synaptic contact with scattered nerve-cells in the distal quarter of the n. olfactorius, apparently forming the ganglion terminale, which probably regulates secretory activities of the Bowman glands.The receptor microvilli, the osmiophilic superficial mucous coating, and the inner mucous layer are discussed in respect to their olfactory function. The quantity of olfactory receptor cells in the Herring Gull is compared with data on macrosmatic animals in earlier investigations.

Unter dankenswerter Anleitung von Prof. Dr. med. K. H. Andres.     

A comparison of Herring Gull Larus argentatus and Lesser Black-backed Gull Larus fuscus nest sites: their characteristics and relationships with breeding success

Nest sites used by Herring Gulls and Lesser Black-backed Gulls in a mixed colony were compared. Correlations between features surrounding a nest and breeding success at different stages of the breeding cycle are presented. Lesser Black-backed Gulls nested on more vegetated and flatter areas than did Herring Gulls, even though the latter species had a lower hatching success at less vegetated sites. The difference in the general topography of nest sites between the species suggests that the Lesser Black-backed Gull, through an individual defence strategy, may be better adapted to use sites which are accessible to ground predators than the Herring Gull. The fledging success of Lesser Black-backed Gulls tended to increase with an increased proportion of relatively tall vegetation close to the nest. The Lesser Black-backed Gull may therefore be able to leave its young unattended and rely on their concealment for protection against predators. In contrast, the Herring Gull may rely more on parental vigilance to protect young. More frequent attendance by adult Herring Gulls at the nest site during chick rearing compared with Lesser Black-backed Gulls supports this hypothesis.