Klaus-Michael Exo

When and where does mortality occur in migratory birds? Direct evidence from long‐term satellite tracking of raptors

Information about when and where animals die is important to understand population regulation. In migratory animals, mortality might occur not only during the stationary periods (e.g. breeding and wintering) but also during the migration seasons. However, the relative importance of population limiting factors during different periods of the year remains poorly understood, and previous studies mainly relied on indirect evidence. Here, we provide direct evidence about when and where migrants die by identifying cases of confirmed and probable deaths in three species of long-distance migratory raptors tracked by satellite telemetry. We show that mortality rate was about six times higher during migration seasons than during stationary periods. However, total mortality was surprisingly similar between periods, which can be explained by the fact that risky migration periods are shorter than safer stationary periods. Nevertheless, more than half of the annual mortality occurred during migration. We also found spatiotemporal patterns in mortality: spring mortality occurred mainly in Africa in association with the crossing of the Sahara desert, while most mortality during autumn took place in Europe. Our results strongly suggest that events during the migration seasons have an important impact on the population dynamics of long-distance migrants. We speculate that mortality during spring migration may account for short-term annual variation in survival and population sizes, while mortality during autumn migration may be more important for long-term population regulation (through density-dependent effects).

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.

Migratory connectivity and population-specific migration routes in a long-distance migratory bird

Knowledge about migratory connectivity, the degree to which individuals from the same breeding site migrate to the same wintering site, is essential to understand processes affecting populations of migrants throughout the annual cycle. Here, we study the migration system of a long-distance migratory bird, the Montagus harrier Circus pygargus, by tracking individuals from different breeding populations throughout northern Europe. We identified three main migration routes towards wintering areas in sub-Saharan Africa. Wintering areas and migration routes of different breeding populations overlapped, a pattern best described by ‘weak (diffuse) connectivity’. Migratory performance, i.e. timing, duration, distance and speed of migration, was surprisingly similar for the three routes despite differences in habitat characteristics. This study provides, to our knowledge, a first comprehensive overview of the migration system of a Palaearctic-African long-distance migrant. We emphasize the importance of spatial scale (e.g. distances between breeding populations) in defining patterns of connectivity and suggest that knowledge about fundamental aspects determining distribution patterns, such as the among-individual variation in mean migration directions, is required to ultimately understand migratory connectivity. Furthermore, we stress that for conservation purposes it is pivotal to consider wintering areas as well as migration routes and in particular stopover sites.

Migratory herbivorous waterfowl track satellite-derived green wave index

Many migrating herbivores rely on plant biomass to fuel their life cycles and have adapted to following changes in plant quality through time. The green wave hypothesis predicts that herbivorous waterfowl will follow the wave of food availability and quality during their spring migration. However, testing this hypothesis is hampered by the large geographical range these birds cover. The satellite-derived normalized difference vegetation index (NDVI) time series is an ideal proxy indicator for the development of plant biomass and quality across a broad spatial area. A derived index, the green wave index (GWI), has been successfully used to link altitudinal and latitudinal migration of mammals to spatio-temporal variations in food quality and quantity. To date, this index has not been used to test the green wave hypothesis for individual avian herbivores. Here, we use the satellite-derived GWI to examine the green wave hypothesis with respect to GPS-tracked individual barnacle geese from three flyway populations (Russian n = 12, Svalbard n = 8, and Greenland n = 7). Data were collected over three years (2008–2010). Our results showed that the Russian and Svalbard barnacle geese followed the middle stage of the green wave (GWI 40–60%), while the Greenland geese followed an earlier stage (GWI 20–40%). Despite these differences among geese populations, the phase of vegetation greenness encountered by the GPS-tracked geese was close to the 50% GWI (i.e. the assumed date of peak nitrogen concentration), thereby implying that barnacle geese track high quality food during their spring migration. To our knowledge, this is the first time that the migration of individual avian herbivores has been successfully studied with respect to vegetation phenology using the satellite-derived GWI. Our results offer further support for the green wave hypothesis applying to long-distance migrants on a larger scale.

Satellite tracking of two Montagu's Harriers (Circus pygargus): Dual pathways during autumn migration

Autumn migration routes of two Dutch female Montagu’s Harriers (Circus pygargus) were documented for the first time using satellite telemetry. Both migrated to their African wintering area—one via the Straits of Gibraltar through the Mediterranean and the other via Italy/Tunisia. The rate of travel was comparable to values reported for larger raptor species.

Change detection in animal movement using discrete wavelet analysis

Abstract Animals moving through a complex environment change their movement patterns frequently. Such transitions from one movement behavior to another are a result of multiple factors such as weather, habitat and inter-specific interactions occurring either individually or in tandem. Understanding where and when these changes occur is the first step towards understanding the limiting or key factors acting at different scales. We applied discrete wavelet analysis to find change points in the movement behavior of a Lesser Black-backed gull ( Larus fuscus ). We compared our results with results from the residence method proposed by Barraquand and Benhamou (2008) and the statistical method proposed by Lavielle (1999, 2005). Discrete wavelet analysis allows the identification and localization of change points by decomposing the frequency content of two time series consisting of step length and residence time data, while omitting the problem of redundancy of coefficients consistent with continuous wavelet transformation of movement data. We show that the novel use of discrete wavelet analysis in animal movement studies is widely applicable on different focus variables, allowing changes in the movement behavior and precise change points to be detected.

Can differences in incubation patterns of Common Redshanks Tringa totanus be explained by variations in predation risk

The effects of predation on Common Redshanks Tringa totanus incubation patterns and behaviour was investigated in 2006 and 2007 at three mainland study sites at the Jadebusen and at one study site on the island Wangerooge, Wadden Sea National Park Lower Saxony (Germany), using temperature data loggers. At these sites, breeding Redshanks were naturally exposed to different predation pressures (as revealed by different hatching success) that varied between 10 and 90% amongst study sites. In areas with a higher predation risk, incubating Redshanks showed a lower nest attendance than in areas where predation risk was low. On the mainland, two behavioural strategies could be distinguished. Some nests were unattended during the night for several hours whereas other nests were incubated for most of the night. Nocturnal absence behaviour was found only on the mainland, and then only in areas easily reached by ground predators. Since ground predators are often active during the night, and dangerous for the incubating birds, this behaviour has probably become established to avoid direct mortality of adult birds rather than avoidance of egg predation. This results from a trade-off between current and future reproduction. By leaving the nest unattended during the night, these birds probably put their own survival before reproduction to maximise their lifetime reproductive success. Since such long incubation recesses may have negative consequences for embryo development, the results of this investigation provide an example of an indirect effect of predation. However, the behavioural trait of nocturnal absence from the nest may directly reduce reproduction and thus fitness of these individuals.ZusammenfassungUm den Einfluss von Prädation auf das Inkubationsverhalten von Rotschenkeln Tringa totanus zu untersuchen, wurden in den Jahren 2006 und 2007 die Bebrütungsmuster von Rotschenkeln im Nationalpark „Niedersächsisches Wattenmeer” auf drei Untersuchungsflächen am Festland, im Jadebusen, sowie auf der Insel Wangerooge mit Hilfe von Thermologgern analysiert. Infolge Prädation variierte der Schlupferfolg zwischen den Gebieten zwischen ca. 10 und 90%. Die Dauer der Nestbesetzung war in Gebieten mit höherem Prädationsrisiko im Durchschnitt signifikant niedriger als in Gebieten mit geringerer Prädation. Die Brutvögel am Festland konnten in zwei Gruppen unterschiedlichen Verhaltens eingeteilt werden. Einige Nester wurden nachts für mehrere Stunden verlassen, während andere Nester durchgehend bebrütet wurden. Mehrstündige nächtliche Brutunterbrechungen konnten nur am Festland beobachtet werden und hier nur in Bereichen, welche leicht für Bodenprädatoren zugänglich waren. Da Bodenprädatoren meist nachtaktiv sind, und sie zudem auch die brütenden Altvögel gefährden, scheint dieses Verhalten eher der Vermeidung der direkten Prädation der Altvögel zu dienen, weniger der Vermeidung von Nestprädation und resultiert vermutlich aus dem „Trade-off” zwischen dem aktuellen und zukünftigem Reproduktionserfolg. Beim nächtlichen Verlassen der Nester stellen die Altvögel ihr eigenes Überleben möglicherweise über die Reproduktion von Nachkommen, um so ihren Lebensbruterfolg zu erhöhen. Mehrstündige Brutunterbrechungen können jedoch negative Auswirkungen auf die Embryonenentwicklung haben und zudem das Risiko von Nestprädation erhöhen. Die durch das Risiko der Altvogelprädation induzierte Verhaltensänderung der nächtlichen Brutunterbrechungen ist somit ein Beispiel für einen indirekten Effekt von Prädatoren, wobei die Abwesenheit vom Nest sich direkt auf den aktuellen Reproduktionserfolg auswirken kann und damit auf die individuelle Fitness.

Reproductive performance and nest-site selection of White-naped Cranes Grus vipio in the Ulz river valley, north-eastern Mongolia

North-eastern Mongolia is an important breeding area for the globally threatened White-naped Crane Grus vipio . We studied reproductive performance and the influence of factors related to livestock grazing on reproduction and nest-site selection of the White-naped Crane population in the Ulz river valley, north-eastern Mongolia in 2000 and 2001. In 2001, 42 territorial pairs were found in 270 km of river basin. Recruitment was 18–22% resulting in 0.5–0.6 juveniles per territorial pair. The years 2000 and 2001 were within a dry period and recruitment might have been below the long-term average. White-naped Crane nests were significantly associated with river basin sections containing shallow water-bodies. The number and extent of shallow water-bodies are likely to be a limiting factor for breeding population size. Nest-sites in water-bodies were selected for high nest concealment, but nests were initiated even when concealing vegetation was not available. Only 26% of nests were >90% concealed suggesting that appropriate cover was often not available. Nest-sites were also selected for low grazing intensity. No correlation was found between hatching or fledging success and nest concealment or grazing intensity. Brood-size of successful pairs was higher if the home range was ungrazed, but further investigations are needed to clarify whether this was caused by absence of livestock grazing or other factors. Both because of internationally important numbers and because of a potentially high reproductive output, the study area is of high importance for the species.

Brutbestand und Dichteabhängigkeit des Bruterfolges der Silbermöwe (Larus argentatus) auf Mellum

In the mid-nineties the German Wadden Sea island Mellum (53°N 43′E 08°09′E) harboured about 30% of the Herring Gull (Larus argentatus) population breeding along the German North Sea coast. In 1995 we estimated the actual population size as well as reproductive success and chick growth in relation to population density and food choice. Furthermore, the methods of monitoring population size and breeding success recommended for the trilateralJoint Monitoring Project for Breeding Birds in the Wadden Sea were evaluated. To survey population size gull nests were counted on 50 randomly selected plots of 0.25 ha each (’stratified sampling‘). For 1995 the estimation indicated a colony size of 13,449 Herring Gull and 1846 Lesser Black-backed Gull (Larus fuscus) paris settling on an area of about 245 ha. Highest densities were recorded in the northern dune area, where mean density was 126 Herring Gull nests/ha. Breeding biology was studied on two plots with different population density. One plot with 134 pairs/ha was located in the north of the island, and a second with only 65 pairs/ha in the south. Nests were visited every 3 days. In the southern dune area clutch initiation started significantly later than in the northern part of the island (p<0.01; median: 11.–15.5. vs. 6.5.–10.5.). Whereas mean clutch size (2.8 eggs/clutch) was similar in the two sites, reproductive success was different. On the southern plot reproductive success was 1.5 fledged young/breeding pair, compared to 1.1 on the northern plot. The difference was mainly caused by greater predation by conspecifics, in particular predation on chicks in the more densely populated north of the island. During the first 4 weeks after hatching, the growth rate of chicks was nearly identical in the 3 years of investigation (1994–1996). Between day 5 and day 25 (linear phase of growth) body mass increased by 26.3 g/d. From day 30 onwards, the body mass of most chicks decreased, thus causing different body masses of the fledglings. At the age of 45 days the highest body masses were recorded on the northern plot in 1995, on average 781.7 g, the lowest masses in the southern dune area 1994 (552.5 g). The methods recommended for monitoring population size and breeding success in the international Wadden Sea seem also to be appropriate for recording spatial differences as well as temporal changes. Diet composition was examined by pellet analyses. The birds of both study plots fed predominantly on mussels. WhereasMacoma balthica was the main prey species of birds breeding in the north,Mytilus edulis dominated in pellets of birds breeding in the south of the island. Interestingly, birds of both plots, but in particular from the southern plot fed on terrestrial food, too. About 25% of the pellets from birds breeding in the south contained earthworms and/or miscellaneous refuse. Direct observations and radio tracking studies showed that birds from both plots regularly fed at the mainland coast, especially on refuse tips. A further increase of the Herring Gull population seems to be possible. Though reproductive success decreased with increasing population density, even a density of ≥100 pairs/ha resulted in a high breeding success. In 1995 mean fledging success was estimated at 1.3 fledged chicks/pair. Only about 0.6 chicks/pair are necessary to maintain population size. On only about 20% of the colonized area has a density of ≥100 pairs/ha been reached as yet. Furthermore, there are some potential breeding grounds on Mellum not yet colonized. Though feeding conditions were good, it can be assumed for the future that feeding conditions can became one of the main factors limiting population size. Additionally, interspecific competition with the ever increasing population of Lesser Black-backed Gulls has to be taken into consideration. Die Nordseeinsel Mellum (53°43′N 08°09′O) beherbergte Mitte der 1990er Jahre annähernd 30% des gesamten Silbermöwenbrutbestandes der deutschen Nordseeküste. 1995 wurden neben einer Bestandserfassung brut- und ernährungsbiologische Untersuchungen in zwei räumlich getrennten Koloniebereichen mit unterschiedlicher Siedlungsdichte durchgeführt. Darüber hinaus wurden die im trilateralenJoint Monitoring Project for Breeding Birds in the Wadden Sea für das Brutbestands- und Bruterfolgsmonitoring vorgeschlagenen Erfassungsmethoden evaluiert. Die Hochrechnung von Gelegezählungen auf 50 nach „geschichteter Zufallsauswahl“ ausgewählter Probeflächen von je 0,25 ha Größe ergab für 1995 einen Brutbestand 13.449 Silbermöwen- und 1846 Heringsmöwen-Brutpaaren. Die zur Brut genutzte Fläche betrug 245 ha. Siedlungsschwerpunkt mit im Mittel 126 BP/ha war die Norddüne. In dreitägigem Abstand vorgenommene Nestkontrollen ergaben, daß Vögel der mit 65 BP/ha besiedelten Süddüne 1995 signifikant später mit der Brut begannen als Brutvögel der mit 134 BP/ha besiedelten Norddünen-Probefläche (p<0,01; Legegipfel: 11.–15.5. bzw. 6.–10.5.). Bei auf beiden Probeflächen identischer mittlerer Gelegestärke von 2,8 Eiern/Gelege flogen im Süden 1,5 Küken/BP aus, im Norden nur 1,1. Hauptverlustursache, primär im Bereich hoher Siedlungsdichte, waren Ei- und Kükenraub durch Artgenossen. In den ersten 28 Lebenstagen ergab sich in drei Untersuchungsjahren (1994–96) eine annähernd identische Körpermassenentwicklung. Vom 5.–25. Lebenstag (Phase linearer Massenzunahme) nahmen die Jungvögel im Mittel 26,3 g/Tag zu. Etwa ab dem 30. Lebenstag nahmen die meisten Küken wieder ab. Jährlich bzw. flächenspezifisch unterschiedliche Ausflugmassen resultierten aus unterschiedlichen Entwicklungen ab dem 30. Tag. Die höchsten Körpermassen erreichten 1995 im Norden Mellums aufgezogene Jungvögel mit im Mittel 781,7 g am 45. Lebenstag; am leichtesten waren 1994 auf der Süddüne aufgezogene Küken mit nur 552,5 g. Die für das wattenmeerweite Brutbestands- und Bruterfolgsmonitoring vorgeschlagenen Methoden sind zur Dokumentation zeitlicher und räumlicher Unterschiede geeignet. Brutvögel beider Probeflächen fraßen überwiegend Muscheln, wobei im NordenMacoma balthica und im SüdenMytilus edulis dominierte. Auffallend waren die hohen Anteile terrestrischer Nahrung im Süden: Lumbriciden und „Müllkomponenten“ hatten im Mittel Anteile von jeweils ca. 25%. Direktbeobachtungen und radiotelemetrische Untersuchungen belegen für Brutvögel beider Koloniebereiche ein regelmäßiges Aufsuchen des ca. 10 km südwestlich Mellums gelegenen Festlands, insbesondere von Mülldeponien. Auch wenn dichteabhängige Regulationsprozesse des Bruterfolgs nachgewiesen werden konnten, scheint eine weitere Zunahme des Silbermöwen-Brutbestands Mellums möglich. Der Bruterfolg war mit im Mittel 1,3 Küken/BP weit höher als zur Bestandserhaltung notwendig (0,6 Küken/BP). Auf nur ca. 20% der zur Brut genutzten Fläche wurden Dichten von ≥100 BP/ha ermittelt. Zudem wurden weite Bereiche potentiell zur Brut geeigneter Gebiete noch nicht genutzt. Auch wenn die Nahrungssituation gut war, könnte der Populationszuwachs langfristig am ehesten durch das Nahrungsangebot bzw. ggf. auch durch Konkurrenz mit der Heringsmöwe limitiert werden.

Die akustische Unterscheidung von Steinkauzmännchen und -weibchen(Athene noctua)

The primary call „Guhk“ of male Little Owls functions in pair formation and territory defense. According to literature this call is uttered exclusively by the male. A similar call given by female Little Owls has been recorded on tape; both calls were analysed by means of a sound spectrograph. In comparison to the call of the female the males call lasted longer, the pitch being higher in the main part. The call was uttered louder and clearer by the male than by the female. During the main time of courtship the male repeated the Guhk-call often monotonously for several minutes, whereas spacing between the notes was irregular and the number of calls given in a group varied only between three and five, or the notes occured singly when the call was uttered by the female. Because of these differences it should be possible to distinguish male and female Little Owls by their vocalizations. This can be important for census work.