De opkomst van de Havik Accipiter gentilis in de stad Groningen

The city of Groningen (50 km², 5% forested) is situated in the northeastern part of The Netherlands. It is surrounded by farmland with few scattered woodlots, where Goshawks started to breed in small numbers in the 1980s. The first breeding pairs in the city of Groningen were recorded in 1996, with no indications of previous nesting (old nests lacking). Between 1996 and 2004, seven territories were identified of which the occupation rate in 1996-2004 varied between 11 and 100% (on average 59%; Table 1). One of these territories (U6) may have involved a switch from U4 (Fig. 1), but our collection of moulted feathers did not suffice to (dis)prove this suggestion. A maximum of six territories was occupied in any one year (1999-2000), and the initial population growth has presently stabilised (for the time being?) at 3-4 occupied territories in 2002-2004. All territories so far have been confined to woodlots, a park, a camping, a cemetery and a garden in the outskirts of the city; the inner city is not yet colonised (Fig. 1). Nesting success (% occupied nests with at least one fledgling) of territories occupied for 8-9 years and for 1-5 years did not differ significantly (see Appendix 1 for basic data). Among age-identified breeding birds, based on findings of moulted feathers and visual observations, 4% and 4% of the males (n = 25) and 8.7% and 13.0% of the females (n=23) were in their 1st and 2nd year of life respectively. The proportion of females breeding in Ist-year plumage was similar to the overall Dutch mean in 2003 (8.4%, n=154 females; Bijlsma 2004). Onset of laying, clutch size, number of fledglings per successful pair nor nesting success differed significantly between suburban Goshawks and those breeding nearby in farmland with scattered woodlots (Table 2). Choice of food during the breeding season, based on findings of prey remains on nests and pluckings near nests, was also similar for suburban (n = 92, at least 23 bird species) and rural Goshawks (n = 141, at least 18 bird species), with a rather high proportion of farmland birds (ducks, waders, woodpigeons, corvids) (Table 3). Mammals were not recorded as prey. The three most commonly captured prey species accounted for 40.2% in the prey list of suburban Goshawks (Numerical Top 3: racing pigeon, starling and woodpigeon), and for 53.9% in rural Goshawks (Top 3: racing pigeon, thrushes, starling). These data implicate that the Goshawks in the city of Groningen are suburban dwellers, presumably leading a life quite similar to those nesting in farmland nearby with hunting forays in farmland and woodlots but not yet in the inner city (notice absence of feral pigeons in the prey list of suburban goshawks). However, this is mostly based on circumstantial evidence and hard data on habitat use and activity pattern is still lacking.