Volker Dierschke

Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free‐living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual‐based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro‐invertebrates. Intake rate is measured as the ash‐free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (‘disc equation’) formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.

Effects of ship traffic on seabirds in offshore waters: implications for marine conservation and spatial planning

Most anthropogenic influences on marine ecosystems, except for river- or terrestrial-borne pollution, involve some sort of vessel activity. Increasing anthropogenic activities mean that many countries are being forced to develop spatial planning schemes, while at the same time implementing conservation sites for sensitive species at sea. The effects of ship traffic on seabirds sensitive to human disturbance are currently too poorly understood to allow for the development of proper planning and conservation guidelines. We therefore used aerial surveys and experimental disturbance to elucidate the effects of passing ships on the distribution patterns, habitat loss, and species-specific flight reactions of birds, as well as the potential for habituation. Loons (Gavia spp.) showed clear avoidance of areas with high shipping intensity. Flush distances of four sea duck species differed significantly, with the longest distances recorded for Common Scoters (Melanitta nigra) and the shortest for Common Eiders (Somateria mollissima). Flush distance was positively related to flock size. Among all the sea duck species studied, the duration of temporary habitat loss was longest for Common Scoters. We found indications of habituation in sea ducks within areas of channeled traffic. However, it is questionable if habituation to free-ranging ships is likely to occur, because of their unpredictable nature. We therefore recommend that spatial planning should aim to channel ship traffic wherever possible to avoid further habitat fragmentation and to allow for habituation, at least in some species. Information on the effects of shipping on other seabird species and during different periods of the year is urgently needed, together with information on the effects of different types of boats, including recreational and fishing vessels.

Time allocation in migrating Northern Wheatears (Oenanthe oenanthe) during stopover: Is refuelling limited by food availability or metabolically?

According to full-day observations (scan sampling), Northern Wheatears (Oenanthe oenanthe) stopping over on the island of Helgoland (SE North Sea) during spring and autumn migration spent 51–67 % of the daylight period foraging. Large parts of the day were used for resting or being vigilant, whereas flying, preening and aggressive behaviour were of minor importance. The density of Wheatears did not influence the time devoted to foraging and aggressive behaviour, and the time spent resting/vigilant was not correlated to predation risk (measured as the rate of raptor flying over). Several observations showed that refuelling on the beach, which presented the most favourable feeding conditions and allowed high rates of body mass gain, was metabolically limited. The total time devoted to foraging was independent of day length, and additional food (mealworms) offered in bowls was completely ignored, an indication that reduced foraging effort would not improve net energy gain within the physiological capacity. In contrast, Wheatears responded strongly to additional food supplied in the poorer grassland habitat. Although this strongly suggests that refuelling is limited by the amount of food available and the costs of obtaining it, foraging times were the same as on the beach. In grassland, the behaviour pattern of birds refuelling was probably distorted by a high proportion of transient and exploring individuals. Therefore, knowledge about foraging ecology and the response to additional food improves the value of the foraging time parameter in considerations of refuelling limitations. Ganztagsbeobachtungen an auf Helgoland während des Heim- und Wegzugs rastenden Steinschmätzern ergaben, dass 51–67 % der Tageslichtperiode für die Nahrungssuche genutzt werden. Große Zeitanteile entfielen auf Ruhen und Wachsamkeit, während Fliegen, Putzen und Revierverteidigung von geringerer zeitlicher Bedeutung waren. Die für Nahrungssuche und Revierverteidigung aufgewendeten Zeitanteile waren unabhängig von der Individuendichte rastender Steinschmätzer, und der Zeitanteil für Ruhen/Wachsamkeit war nicht mit dem Prädationsrisiko, das als Rate von Greifvogel-überflügen gemessen wurde, korreliert. Verschiedene Beobachtungen zeigten, dass die Nährstoffdeposition am Strand, wo sehr günstige Ernährungsbedingungen herrschten, durch den Stoffwechsel be grenzt wurde. Einerseits war die Dauer der Nahrungssuche unabhängig von der Tageslänge, andererseits wurde in Schalen angebotenes Zusatzfutter (Mehlwürmer) vollständig ignoriert. Innerhalb der physiologischen Grenzen hätte ein verringerter Aufwand bei der Nahrungssuche offenbar die Nettoenergieaufnahme nicht weiter steigern können. Im Gegensatz dazu nutzten Steinschmätzer das zusätzliche Futter im nahrungsärmeren Grünland sehr intensiv. Obwohl dieses Verhalten stark auf eine Limitierung der Nährstoffdeposition durch das verfügbare Nahrungsangebot und die Kosten, dieses zu nutzen, hinweist, unterschied sich die Dauer der Nahrungssuche nicht von jener am Strand. Wahrscheinlich wurde im Grünland das Verhalten von tatsächlich dort rastenden Individuen von solchen Vögeln überlagert, die dort nur durchwanderten bzw. auf der Suche nach geeigneten Rasthabitaten explorierten. Für die Frage, ob die Nährstoffdeposition durch Nahrungsverfügbarkeit oder physiologisch limitiert ist, sind deshalb Kenntnisse über die Ernährungsökologie und die Reaktion auf Zusatzfutter als wichtige Ergänzung zu Zeitbudgets anzusehen.

Morphometrics and stable isotopes differentiate populations of Northern Wheatears (Oenanthe oenanthe)

Linking events of breeding, wintering and stopover areas has important ecological and conservation implications for migratory species. To find a tool to connect these different events in a long-distance migrating songbird, the Northern Wheatear Oenanthe oenanthe, we applied a discriminant analysis based on morphometrics and analysed stable isotope values (δ13C, δ15N, δD) in feathers. Morphometric differences were additionally analysed with respect to wing shape as an adaptation to migration routes. Discriminant analysis 100% separated a group of long-winged migrants passing the German offshore island of Helgoland from Icelandic and Norwegian breeding birds, as well as from Northern Wheatears passing the Baltic Sea coast on migration. This clear assignment suggests a Greenlandic origin of these long-winged Northern Wheatears. The most likely Greenlandic origin was further supported by depleted δD values in feathers of these birds grown on the breeding grounds. We found a relatively high proportion of presumed Greenlandic birds on Helgoland and especially on Fair Isle (Scotland) during spring migration. Morphometric differences were based mainly on wing morphology and could be successfully connected with migration routes. Presumed Greenlandic Northern Wheatears showed more pointed wings than birds from other European breeding areas. Such wings might be natural selection’s solution for the long obligatory non-stop flights during the Atlantic crossings.ZusammenfassungFür ziehende Tierarten hat der Zusammenhang von Ereignissen in Brut-, Winter- und Rastgebieten wichtige Konsequenzen für ökologische Aspekte und den Artenschutzes. Um im Falle eines typischen Langstreckenziehers, des Steinschmätzers (Oenanthe oenanthe), ein Werkzeug zu finden, um Ereignisse in den verschiedenen Aufenthaltsgebieten verbinden und verschiedene Populationen ansprechen zu können, haben wir eine Diskriminanzanalyse aufgrund von morphometrischen Daten durchgeführt und Stabile Isotope (δ13C, δ15N, δD) aus Federn analysiert. Morphometrische Unterschiede wurden zusätzlich in Hinsicht auf Adaption der Flügelform aufgrund verschiedener Zugrouten untersucht. Anhand der Diskriminanzanalyse ließ sich eine Gruppe von besonders langflügeligen Durchziehern auf Helgoland vollständig sowohl von Isländischen und Norwegischen Steinschmätzern unterscheiden, als auch von Steinschmätzer die auf dem Zug an der baltischen Ostseeküste erscheinen. Diese klare Abgrenzung lässt einen Grönländischen Ursprung dieser langflügeligen Steinschmätzer vermuten. Eine Vermutung, die weiterhin durch deutlich abgereicherte δD Werte in Federn, die im Brutgebiet gewachsen waren, unterstützt wird. Wir fanden während des Frühjahrszuges einen relativ hohen Anteil an vermutlich Grönländischen Vögeln auf Helgoland und besonders auf Fair Isle (Schottland). Morphometrische Unterschiede basierten hauptsächlich auf Unterschieden in der Flügelform und konnten mit den unterschiedlich Anforderungen während des Zuges in Verbindung gesetzt werden. Steinschmätzer mit vermutlich Grönländischen Ursprung zeigten spitzere Flügel als Vögel von anderen Europäischen Brutgebieten. Diese Flügel scheinen das Ergebnis natürlicher Selektion innerhalb dieser Population zu sein, die besonders lange nonstop Flüge zur Überquerung des Nordost-Atlantiks bewältigen muss.

Prevalence and intensity of excretion of coccidial oocysts in migrating passerines on Helgoland

Prevalence and intensity of excretion of coccidial oocysts in actively migrating passerines (Turdus philomelos, Fringilla coelebs, Sylvia borin, Phoenicurus phoenicurus) were assessed in spring and autumn 2001 during stopovers on Helgoland. All examined species showed low prevalences of oocysts in spring (28–33%) and high prevalences in autumn migration (66–92%). In autumn, there are significant species-specific differences in prevalence and, partially, in intensity of excretion. Ground-feeding Chaffinches most commonly and most intensively shed oocysts (maximal 2,7×106 oocysts per g fresh faeces) indicating that feeding style may influence the parasite load. No significant correlation between parasite load, age, and body condition (fat depots, breast muscle shape, body mass) was found. The connection between feeding style, age, migration, immunocompetence, and survival of passerine hosts on one hand and the particularities of coccidial pathogenicity on the other are discussed.

Seasonal differences in at-sea activity of seabirds underline high energetic demands during the breeding period

We assessed seasonal differences in at-sea activity of Lesser Black-backed Gulls Larus fuscus, Black-legged Kittiwakes Rissa tridactyla and Common Guillemots Uria aalge in the south-eastern North Sea. The three species correspond to different ecological groups, with Lesser Black-backed Gulls representing omnivorous generalists, Kittiwakes representing surface-feeding pelagic seabirds, and Guillemots representing pursuit-diving pelagic seabirds. Using data from aerial surveys, we differentiated between active (flying or scavenging at fishing vessels) and inactive behaviour (swimming). We estimated the activity budgets of all three species for the different seasons and tested for differences in activity between different seasons. All species exhibited significant seasonal differences in activity, with the highest levels of activity observed during the breeding season. Numbers of flying auks were, however, exceptionally low in autumn due to moult and guarding of not-yet fledged chicks at sea. Our results underline the high energetic demands of the breeding season that lead to increased foraging and travelling activity.

Exploitation of resources modulates stopover behaviour of passerine migrants@@@Ausbeutung des Nahrungsangebotes beeinflusst das Rastverhalten ziehender Singvögel

The seasonal variation in the supply of ripe Elderberries (Sambucus nigra) on the offshore island of Helgoland (North Sea) was examined from the point of view of the effect of food supply and exploitation on the stopover behaviour of migrating passerines. Elderberries were completely exploited by birds by mid October, three weeks earlier than the last berries disappeared from exclosures. In five out of seven frugivorous passerine species the percentage of trapped birds settling (i. e. recaptured on days subsequent to initial capture) was significantly higher during the period of high availability of ripe Elderberries (early September to early October) than it was earlier (berries still unripe) and later (berries exhausted). No such pattern was observed in fuel deposition rates because rates of body mass change in recaptures differed between periods in only one out of five species. Therefore, in conjunction with the shortage of alternative food, berry supply had an impact on the stopover decision, but did not affect the refuelling itself. The energy demand of migrants as calculated from the number of birds present could not be covered by Elderberries from mid October onwards. Hence, exploitation by earlier migrants reduced the carrying capacity of the stopover site for later migrants, especially when high bird densities were involved. Continent-wide patterns of fuel deposition may have evolved in order to avoid such shortages of food for refuelling just before the crossing of ecological barriers. Passerine migrants would benefit from dispersed resources which allow the avoidance of crowding and exhaustion of supply. Die jahreszeitliche Variation im Angebot an Holunderbeeren (Sambucus nigra) auf der Insel Helgoland wurde gewählt, um die Effekte des Nahrungsangebots und dessen Ausbeutung auf das Rastverhalten ziehender Singvögel zu untersuchen. Holunderbeeren wurden von Vögeln bis Mitte Oktober vollständig ausgebeutet, so dass drei Wochen früher als in Exklusionsversuchen keine Holunderbeeren mehr vorhanden waren. In fünf von sieben untersuchten Singvogelarten war die Wiederfangrate von im Fanggarten beringten frugivoren Vögeln zur Zeit der Holunderreife (Anfang September bis Anfang Oktober) signifikant höher als in Zeiten davor (Beeren noch unreif) bzw. danach (Beeren ausgebeutet). In der Nährstoffdepositionsrate ließ sich beim Vergleich dieser Zeiträume allerdings nur bei einer von fünf Arten ein signifikanter Unterschied finden. Im Einklang mit dem Fehlen quantitativ bedeutender Alternativnahrung hatte das Nahrungsangebot also einen Einfluss auf die Entscheidung über Rast oder Abzug, beeinflusste aber offenbar nicht die Nährstoffdeposition selbst. Der Energiebedarf auf Helgoland beobachteter frugivorer Zugvögel (16 Arten) konnte ab Mitte Oktober nicht mehr durch Holunderbeeren gedeckt werden. Die Ausbeutung durch früher ziehende Vögel hat demnach die Umweltkapazität des Rastgebietes für später ziehende Vogel erheblich reduziert, besonders für Tage mit hoher Vogeldichte. Kontinentweite Muster der Nährstoffdeposition haben sich möglicherweise entwickelt, um solche Engpässe vor der Überquerung von ökologischen Barrieren zu vermeiden. Ubertragen auf isolierte Habitate zeigen die Ergebnisse von Helgoland, dass ziehende Singvögel von weit verteilten Ressourcen, die hohe Dichte und starke Ausbeutung verhindem, profitieren können.

Possible Conflicts between Offshore Wind Farms and Seabirds in the German Sectors of North Sea and Baltic Sea

In conclusion, it is obvious that depending on avoidance behaviour and collision risk as well as on the proportions of populations affected, the impact of offshore wind farms in the German sector of the North Sea on seabird populations differs considerably. The example of Red-throated Divers and Common Guillemots shows that large parts of the German Bight would be excluded from use by these species. This has to be taken into account in the process of commissioning by authorities and should lead to the application of threshold levels in order to select wind farm sites which have least impact on seabird populations. The above examples demonstrate that in the assessment of the effects of a single wind farm, the habitat loss in addition to the habitat already lost before due to other wind farms must be considered. Therefore, this underscores the need for a cumulative approach when assessing impacts on seabird populations.