Fieldwork
Portrait of an adult male northern goshawk; this hawk was fitted with a radio-tag and followed throughout the entire breeding season in 1997.
In 1995, I founded a long-term research project (ongoing) on the biology of an expanding urban population of northern goshawks Accipiter gentilis in the city of Hamburg, Germany. Goshawks are medium-sized, powerful raptors with a broad diet spectrum – they occasionally kill prey as small as a goldcrest or mouse, but are capable of subduing animals much larger than themselves, including herons or even full-grown hares.
For two main reasons, urban-breeding goshawks are an attractive study system. Firstly, urban hawks experience unusual ecological circumstances (e.g., high food abundance, unusual food sources, reduced interspecific competition, increased habitat fragmentation), and investigating their responses to these conditions can shed light on some fundamental aspects of predator ecology. Secondly, particularly favourable observation conditions in the city (stress tolerance of hawks, excellent urban infrastructure, high density of keen birdwatchers) enable the collection of data which are difficult, or even impossible, to obtain in such detail in typical woodland habitats.
Goshawk nestling of perfect ringing age; so far I have marked more than 250 nestlings during my monitoring program.
Amongst other things, I have investigated: (i) the foundation and expansion of Hamburg’s urban goshawk population; (ii) the hypothesis that the city constitutes an ‘ecological trap’ for this predator; (iii) the foraging ecology of urban and exurban hawks; (iv) factors shaping brood sex ratio variation in urban goshawks; and (v) changes in movement patterns of foraging hawks along an urban-rural habitat gradient.
Other Work
Breeding goshawks can be trapped with a tame eagle owl – if the researcher survives handling the owl…
Apart from collecting my own data on goshawk biology, I have teamed up with other researchers to (re-)analyse existing datasets. This offers exciting opportunities, as I can work with datasets that span exceptionally long time periods (which is necessary for addressing certain questions in population ecology), and it also enables replication at the population level (which is important for generalising results).
For example, together with Rob Bijlsma, I have recently investigated how food-supplies have limited a local goshawk breeding population in the central Netherlands, which he had studied in detail for 26 years. In another project, I compiled with three collaborators data on goshawk biology from the published literature to explore variation in goshawk demography and diet across the species’ entire European breeding range.