Browsing by Author "Garthe, Stefan"
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- Data packageData from: Effects of agricultural practices on foraging habitats of a seabird species in the Baltic Sea(2022-11-28) Garthe, Stefan; Schwemmer, Philipp; Kubetzki, Ulrike; Heinze, BerndOmnivorous and opportunistic species may be good indicators of food availability. Gulls often use human-impacted landscapes and may respond to changes by altering their feeding ecology. We investigated the foraging behavior of individual common gulls (Larus canus), focusing on their distribution during foraging and their selected habitat types. We tracked adult common gulls using GPS telemetry at their largest breeding colony in the southwestern Baltic Sea, Germany. Foraging habitats were analyzed from tracking data for three breeding seasons 2016, 2017, and 2019 and were compared with potentially available foraging habitats. Most breeding birds flew toward terrestrial areas. Feeding sites were located on average 11.7–14.3 km from the colony (range 0.9–36.5 km). Corn and sugar beet fields were used significantly and extensively compared with their availability in 2016 and 2017, while wheat, rape, and barley fields were used significantly less. Data from 2019 suggested seasonal shifts in habitat use. Birds spent between 30 and 1300 min per week at their preferred feeding sites, with significant differences between the major habitats selected. We found a stable, clear, multiyear pattern in common gull foraging behavior in relation to agricultural practices. Fields with little or no crop cover and thus access to the soil were preferred over fields with high crop cover. These results suggest that local food availability may be limiting further population increases in this species.
- Data packageData from: High foraging site fidelity and spatial segregation among individual great black-backed gulls(2019-11-24) Borrmann, Rahel M.; Phillips, Richard A.; Clay, Thomas A.; Garthe, StefanIndividual foraging site fidelity, whereby individuals repeatedly visit the same foraging areas, is widespread in nature, and likely benefits individuals through higher foraging efficiency and potentially, higher breeding success. It may arise as a consequence of habitat or resource specialisation, or alternatively, where resources are abundant or predictable, the partitioning of space might guarantee individuals exclusive foraging opportunities. We tracked seven adult great black‐backed gulls Larus marinus at a North Sea colony from early incubation to the end of the breeding season in 2016, providing a total of 1,170 foraging trips over a mean ± SD tracking period of 67 ± 16 days. There was clear spatial segregation between individuals, with almost no overlap of their core areas (50% utilisation distribution) during incubation and chick‐rearing. Core areas were relatively small and there was high repeatability (R ± SE) in foraging parameters, including initial departure direction (0.73 ± 0.11), foraging range (0.41 ± 0.14) and cumulative distance travelled (0.19 ± 0.1) throughout the breeding season. Despite the low spatial overlap, there was little evidence of differential habitat use by individuals. The near‐exclusive individual foraging areas of this species, usually considered to be a generalist, indicate that where there is high resource availability throughout the breeding season and a small local population, individuals appear to adopt a territorial strategy which likely reduces intraspecific competition.
- Data packageData from: Migrating curlews on schedule: departure and arrival patterns of a long-distance migrant depend on time and breeding location rather than on wind conditions(2021-03-16) Schwemmer, Philipp; Garthe, StefanBackground: Departure decisions in long-distance migratory bird species may depend on favourable weather conditions and beneficial resources at the destination location, overarched by genetic triggers. However, few studies have tried to validate the significance of these three concepts simultaneously, and long-term, high-resolution tagging datasets recording individual movements across consecutive years are scarce. We used such a dataset to explore intraspecific and intra-individual variabilities in departure and arrival decisions from/to wintering grounds in relation to these three different concepts in bird migration. Methods: We equipped 23 curlews (Numenius arquata) wintering in the Wadden Sea with Global Positioning System data loggers to record their spatio-temporal patterns of departure from and arrival at their wintering site, and the first part of their spring migration. We obtained data for 42 migrations over 6 years, with 12 individuals performing repeat migrations in consecutive years. Day of year of departure and arrival was related to 38 meteorological and bird-related predictors using the least absolute shrinkage and selection operator (LASSO) to identify drivers of departure and arrival decisions. Results: Curlews migrated almost exclusively to Arctic and sub-Arctic Russia for breeding. They left their wintering site mainly during the evening from mid- to late April and returned between the end of June and mid-July. There was no difference in departure times between the sexes. Weather parameters did not impact departure decisions; if departure days coincided with headwind conditions, the birds accounted for this by flying at higher altitudes of up to several kilometres. Curlews breeding further away in areas with late snowmelt departed later. Departures dates varied by only <4 days in individual curlews tagged over consecutive years. Conclusions: These results suggest that the trigger for migration in this long-distance migrant is largely independent of weather conditions but is subject to resource availability in breeding areas. The high intra-individual repeatability of departure days among subsequent years and the lack of relationship to weather parameters suggest the importance of genetic triggers in prompting the start of migration. Further insights into the timing of migration in immatures and closely related birds might help to further unravel the genetic mechanisms triggering migration patterns.
- Data packageData from: Terrestrial and marine foraging strategies of an opportunistic seabird species breeding in the Wadden Sea(2016-08-19) Garthe, Stefan; Schwemmer, Philipp; Paiva, Vitor H.; Corman, Anna-Marie; Fock, Heino O.; Voigt, Christian C.; Adler, SvenLesser black-backed gulls Larus fuscus are considered to be mainly pelagic. We assessed the importance of different landscape elements (open sea, tidal flats and inland) by comparing marine and terrestrial foraging behaviours in lesser black-backed gulls breeding along the coast of the southern North Sea. We attached GPS data loggers to eight incubating birds and collected information on diet and habitat use. The loggers recorded data for 10–19 days to allow flight-path reconstruction. Lesser black-backed gulls foraged in both offshore and inland areas, but rarely on tidal flats. Targets and directions were similar among all eight individuals. Foraging trips (n = 108) lasted 0.5–26.4 h (mean 8.7 h), and ranges varied from 3.0–79.9 km (mean 30.9 km). The total distance travelled per foraging trip ranged from 7.5–333.6 km (mean 97.9 km). Trips out to sea were significantly more variable in all parameters than inland trips. Presence in inland areas was closely associated with daylight, whereas trips to sea occurred at day and night, but mostly at night. The most common items in pellets were grass (48%), insects (38%), fish (28%), litter (26%) and earthworms (20%). There was a significant relationship between the carbon and nitrogen isotope signals in blood and the proportional time each individual spent foraging at sea/land. On land, gulls preferentially foraged on bare ground, with significantly higher use of potato fields and significantly less use of grassland. The flight patterns of lesser black-backed gulls at sea overlapped with fishing-vessel distribution, including small beam trawlers fishing for shrimps in coastal waters close to the colony and large beam-trawlers fishing for flatfish at greater distances. Our data show that individuals made intensive use of the anthropogenic landscape and seascape, indicating that lesser black-backed gulls are not a predominantly marine species during the incubation period.