Päiväkirja-arkisto kohteelle joulukuu 2018

13. joulukuuta 2018

The diet of a pair of Northern Goshawks in the urban center of Berlin

Introduction

Berlin has an urban goshawk population of around 100 pairs (Kenntner, 2018). Such populations exist in several German cities, notably Hamburg and Cologne. Monitoring of the goshawk population in Hamburg suggests that the abundance of urban goshawks can be linked, among other factors, to the high availability of prey, particularly feral pigeons (Rutz, 2008). The observations annotated here confirm this, however, goshawk diet is not constrained to pigeons. Variations in diet include crows, small passerines and other raptors.

Data and Methods

The observations annotated here are the result of monitoring a couple of northern goshawks (Accipiter gentilis) nesting in a Berlin cemetery. The location of the cemetery has been obscured, to protect the birds from poachers. The area was visited 46 times from December 2017 until December 2018. The pair of goshawks occupied the area during all this time. Three chicks were raised, hatching in May and dispersing in August.

The pluckings were photographed in the nesting area. Plucking analysis is a standard method to obtain predation data for raptors, however it should be noted that plucking analysis can be biased by two factors: selective prey transportation by the predator and varying detectability of feathers of the prey species (Rutz, 2003).

The pictures were uploaded to iNaturalist and added to the "Found Feathers" group. For difficult identifications, the feathers were collected and kept as reference. This resulted in 38 "research grade" observations, i.e. the observation have correct metadata (date and coordinates), and the species id is reviewed by the online community.

Results

Table 1 shows the diet composition, in prey count and relative biomass. Biomass estimation was obtained from Dunning (2007), see additional material for details.

Table 1: Diet of a pair of Northern Goshawks in the urban center of Berlin.

Species N=38 N % Biomass %
Columba livia domestica 16 42.10 42.98
Columba palumbus 9 23.68 33.47
Corvus cornix 3 7.89 12.98
Turdus merula 2 5.26 1.71
Pica pica 2 5.26 3.13
Turdus philomelos 1 2.63 0.52
Fringilla montifringilla 1 2.63 0.18
Garrulus glandarius 1 2.63 1.19
Falco tinnunculus 1 2.63 1.40
Pyrrhula pyrrhula 1 2.63 0.17
Asio otus 1 2.63 2.27

Discussion

A comparison to previous work by Rutz (2003, 2006) in Hamburg found similarities and differences in diet, however feral pigeons clearly dominate the diet of northern goshawks in both urban areas. Columbidae are the most important prey group of goshawks in Berlin and in Hamburg, by number of prey items and by biomass. The second prey group are Corvidae. The diet of urban goshawks is however more diverse, as passerines the size of blackbirds or smaller, as well as diurnal and nocturnal raptors are also preyed upon.

Acknowledgements

The iNaturalist "group" feature, specifically the "found feathers" group, proved particularly helpful for identifying prey items. I wish to thank Amanda Janusz (@featherenthusiast) the founder of the "found feathers" group, as well as all iNaturalist contributors who identified these observations: @alexis_orion, @audun, @bluejay2007, @brennafarrell, @chwillbill68, @crapfou, @dendzo, @jakob, @karoopixie, @ldacosta, @martingrimm, @peterwijnsouw, @richardjaybee, @thekat, @tlaloc27, @solokultas, @stephen54

References

Dunning Jr, J. B. (2007). CRC handbook of avian body masses. CRC press.

Kenntner, N. (2018). Urbaner Habicht in Berlin, http://habicht-berlin.de/ (13 December 2018)

Rutz, C. (2003). Assessing the breeding season diet of goshawks Accipiter gentilis: biases of plucking analysis quantified by means of continuous radio-monitoring, 259, 209–217. https://doi.org/10.1017/S0952836902003175

Rutz, C. (2008). The Establishment of an Urban Bird Population. Source: Journal of Animal Ecology Journal of Animal Ecology, 77(77), 1008–1019. https://doi.org/10.1111/j.l365-2656.2008.01420.x

Additional material

Average biomass estimation, based on Dunning (2007), total biomass in goshawk diet (grams).

Species Average mass Total mass in diet
Columba livia 354 5664
Columba palumbus 490 4410
Turdus philomelos 68 68
Fringilla montifringilla 24 24
Corvus cornix 570 1710
Garrulus glandarius 157 157
Turdus merula 113 226
Pica pica 206 412
Falco tinnunculus 184 184
Pyrrhula pyrrhula 22 22
Asio otus 299 299
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