Trends, broedresultaten en voedsel van roofvogels in Nederland in 2023

Bijlsma R.G. 2024. Trends, breeding performance and diet of raptors in The Netherlands in 2022. De Takkeling 32: 5-61. A total of 3195 nest record cards of raptors were submitted in 2023 (Appendix 1, up to and including 10 January 2024), for 13 species but not including Montagu’s Harrier which is covered separately (www.grauwekiekendief.nl). The winter preceding the breeding season was extremely mild (frost index of 4.0 on a scale of 0-100), the summer itself was extremely warm (summer index 87.2, on a scale of 0-100) with heavy rainfall in March/April and July/August, a very dry and sunny June and a summer storm in July which destroyed many raptor nests in the western and northern parts of the country. Food abundance on various trophic levels was scarce (social wasps in woodland), moderate (birds) or abundant (voles). Voles (especially Microtus arvalis) and mice (Apodemus sylvaticus/flavicollis) showed a peak in abundance, although not as high as in 2014/2015 and 2019. Social wasps were almost completely absent from woodland, but apparently moderately abundant in farmland given the food remains recorded on nests of Honey Buzzards. Honey Buzzard Pernis apivorus: onset of laying averaged 25 May (range 12 May- 14 June, n=29). Clutch size was 23x 2 eggs. Brood size was 3x 1 and 24x 2 young. In 24 nests, prey remains constituted Vespula vulgaris (79, combs), V. germanica (17), unidentified combs (66, of grey texture), Dolichovespula media (3), D. saxonica (5), Vespa crabro (14), Polistes dominula (1), Bombus spp. (1), Woodpigeon Columba palumbus (1, squab), Blackbird Turdus merula (1), Song Thrush T. philomelos (2, nestlings), Bullfinch Pyrhhula pyrrhula (1, nestling), and Rana temporaria (5). Comb diameter of depredated Vespula vulgaris increased from 60 mm in early July to 110- 135 mm in early and mid-August. Honey Buzzards nests were found in 5 species of coniferous trees (n=22) and 6 species of deciduous trees (n-=12). Nest height averaged 16.1 ± 4.5 m (range 7-22.6 m). At least 50% of 30 nests were built by Honey Buzzards (possibly 77%, when old nests were included, most of which had been occupied by Honey Buzzards in previous years), and some nests were reused that had been built by Goshawk (3), Buzzard (2), Red Kite (1) and Carrion Crow (1). The Dutch Honey Buzzard population is in decline, by at least 30% in the last few decades in its core breeding area on the Veluwe. Finding successful nests has become increasingly difficult, related to a high proportion of pairs refraining from egg-laying, and high predation in egg and nestling stages (including predation of adult birds). Several factors are probably involved, such as declining food supply (notably of Vespula germanica), intraguild predation and large-scale habitat destruction by State Forestry Service and conservation societies (like Natuurmonumenten) which specifically targets mature forest (money) and exotic tree species (notably larch Larix spp. and douglas fir Pseudotsuga menziesii) which are preferred by nesting raptors. Systematic clear-felling not just destroys prime nesting habitat, but also facilitates predation via fragmentation of prime nesting habitat. Red Kite Milvus milvus: 35 nests were located, in 28 of which egg-laying commenced. Chick production was 5x 0, 3x 1, 7x 2 and 13x 3 fledglings per successful pair. Black Kite Milvus migrans: two breeding pairs in the southeastern Netherlands (possibly two more, but no details); both raised 3 fledglings. White-tailed Eagle Haliaeetus albicilla: of 36 pairs, 32 produced a clutch of which four failed to hatch. 24 successful pairs raised a total of 45 fledglings. Marsh Harrier Circus aeruginosus: after a steep increase in the 1970s and 1980s, and peak numbers in the late 1990s, the species is now in decline, from 1300-1450 pairs in 1998-2000 to <1000 pairs in the 2020s. Habitat degradation (e.g. desiccation of reedbeds in the wake of intensification of farmland practices), nest disturbance and declines of vole populations may be involved. Of 137 nests, 93 were successful in raising at least one fledgling. Mean start of egg laying was 24 ± 11.2 April (range 4 April-26 May, n=70), mean clutch size 4.7 ± 0.9 eggs (2-7, n=72), mean brood size at ringing 3.5 ± 1.1 young (1-5, n=74). The above-average vole density resulted in above-average breeding success. Sex ratio among nestlings on 48 nests was 72 males and 72 females. Among nesting failures human-related losses were most common (13x, either intentionally or as a byproduct of farming, recreation or construction). Hen Harrier Circus cyaneus: three nests on the Wadden Sea Island of Texel, from 2 males and three females (the same as in 2022). Clutch size was 4, 5 and 6 eggs, of which 2, 2 and 4 chicks fledged (3 males and 5 females). Mean start of egg laying was 29 April (25 April-1 May. Six nestlings received a GPS GSM transmitter, but three already had died by October 2023. Of the nestlings equipped with a sender in 2022, a single female survived, which ranged widely in the northern Netherland and northern Germany before (probably temporarily) settling down in the province of Groningen in October 2023. Apart from the three pairs on Texel, three pairs were located on Terschelling (one successful, with 2 fledglings) and two in Groningen. Montagu’s Harrier Circus pygargus: 62 pairs were detected, raising a total of 156 fledglings (the best result since 1990, the all-time low in The Netherlands, from when on breeding pairs have been monitored and protected (most pairs breed in cereals and alfalfa in large-scale farmland) (for more information, www.grauwekiekendief.nl). Pallid Harrier Circus macrourus: a male teamed up with a female Marsh Harrier in Groningen, raising four chicks (of which three unmistakably in hybrid plumage). Goshawk Accipiter gentilis: apart from a decline in woodlands on sandy soils in the eastern half of The Netherlands, high mortality was registered in the last two years (2022 and 2023) in the wake of Avian Influenza. Out of 12 dead Goshawks checked in 2022, eight tested positive for HPAI (results for 2023 not yet available). High mortality was indirectly registered by raptorphiles checking nesting sites for occupancy: 22.3% of 300 nesting sites in 2023 failed to produce a clutch (with many apparently not occupied, or only shortly so in early spring), compared to 5.5% of 380 nests in 2019 (preceding the outbreak of Avian Influenza in 2020-21). Non-breeding was recorded throughout the country. Overall, Goshawks in The Netherlands are in decline. Local trends suggest decreases of 50% or more. Trends of Goshawks in the western half of the country (colonized in the 1990s and 2000s) are still more or less stable but refer to smaller numbers. Declines are assiociated with steep declines in food supply, and are exacerbated by large-scale clear-felling of mature forest and non-indigenous tree species by State Forestry Service and conservation societies like Natuurmonumenten. The impact of Eagle Owls, on the increase as a breeding bird in the eastern Netherlands, is yet to be assessed. The increasing frequency and severity of pinching-off followed by death in nestling Goshawks was found to be probably associated with tembotrione, a post-emergence herbicide increasingly used in crops like maize. Mean start of laying was 3 April (n=138, of which 41% started in March, range 14 March-20 April), clutch size averaged 3.5 eggs (with 4x C/5 among 135 clutches), brood size (at ringing/fledging age) 2.7 (169 nests, none containing 5 chicks). Sex ratio in 116 nests was 179 males and 148 females. For the period 1996-2023 sex ratio (% male) ranged between 50.7% (1999, 2022) and 60.0% (2015), on average being 55.1% (5931 nests with 15,780 nestlings). Causes of failure were 5x human-related and 7x natural (mostly clutch desertion, and predation of eggs or nestlings). Prey remains (n=520) collected near nests showed 96.0% birds in 45 species (especially pigeons and corvids including Jay, with Starling, woodpeckers and thrushes being important but in much smaller numbers), as well as mammals in 3 species (mostly rabbits, some hares and red squirrels). Sparrowhawk Accipiter nisus: mean onset of laying was 29 April (range 11 April-27 May, n=128), with 62.5% of pairs starting in April. Clutch size averaged 5.0 (n=111), the number of fledglings per successful pair 3.9 (n=150). The secondary sex ratio among fledglings was 227 males and 205 females on 112 fully sexed nests. In 1996-2023, 15,263 nestlings were sexed on 3978 nests (51.5% male), shifting from an equal sex ratio in the 1990s to an excess of males in later years. Causes of failure in the breeding season ranged from human-related (2x) to predation of eggs (4x), of nestlings (14x) and of breeding birds (2x), and adverse weather (2x). Forest and nature managers are responsible for clear-felling and fragmentation of woodland (between August-March), with concomittant losses and degradation of breeding sites (underrepresented on nest cards, as it happens in advance of the breeding season). Buzzard Buteo buteo: mean start of egg laying was calculated at 5 April (range 13 March-5 May, n=267; 29.9% of all pairs started laying in March). Mean clutch size was 2.6 (n=368, with 24x C/4 and 1x C/5), mean brood size of successful pairs was 2.1 (n=496, with 16 B/4). The early start and relatively large clutch and brood sizes match the vole peak in 2023. Secondary sex ratio in 65 nests was 78 males and 56 females (58.2% male), i.e. much stronger male-biased than averaged for 1996-2023 (4552 nests, 8662 nestlings, 53.9% male). Deliberate disturbances caused by humans accounted for 33% of all failures (out of 45 with known cause), the rest was attributed to desertion (7x), egg predation (3x), chick predation (12x), adverse weather causing nest destruction (5x) and take-overs by Egyptian Geese (3x). Prey found on nests included 40 bird species (21.8% of 497 identified prey) and 14 mammal species (69.7% of all prey items; lagomorphs, moles and voles most important in terms of biomass), plus some reptiles and amphibians. Among mammalian prey, voles accounted for 18.3% in numbers. Osprey Pandion haliaetus: the Dutch population comprised three successfully nesting pairs, two of which in Biesbosch and another on the northern Veluwe. Several solitary males and a non-breeding pair complemented the summer population. Kestrel Falco tinnunculus: based on local and regional trends, a decline of Kestrels has been apparent for many decades, amounting to >50% since the 1960s and 1970s. The ample supply of nestboxes may have slowed the decline, but long-running nestbox schemes show that declines have been inevitable. Degradation of farmland resulted in declining food supply, notably of voles. Increased predation may have played a subsidiary role, as evident from the regions where nestboxes had been provided (a slower decline, with pairs shifting from predation-prone open nests to boxes, the latter providing protection from most avian predators though not from martens). A moderate vole peak resulted in early laying (mean onset 24 April, range 18 March-10 June, n=591), large clutch size (mean 5.3, n=739) and brood size (mean number of fledglings/successful nest 4.6, n=829), and high occupancy of nestboxes (716 of 929 boxes). Local occupancy rates differed fivefold, probably reflecting regional variations in food supply. Almost all recorded pairs nested in nest boxes, i.e. 917 out of 921 nests, compared to only one using a tree nest of Carrion Crow and three using cavities in buildings. Voles were the most important prey species, e.g. 73% of 639 preys recorded in nestboxes and in pellets, supplemented mostly by (song)birds (13%). Hobby Falco subbuteo: mean start of laying was 12 June (range 28 May-2 July, n=25). Clutch size was 2x 2 and 9x 3, the number of fledglings/successful nest 11x 1, 17x 2, 37x 3 and 1x 4. Sex ratio in 7 nests was 8 males and 10 females. In 1996-2023, 230 nests contained 254 males and 301 females (45.8% male). All pairs used old nests of corvids, i.e. 89x Carrion Crow, 3x Raven and 1x Rook. The proportion of pairs nesting on old nests in electricity pylons amounted to 52.9% (out of n=104 nests; average height in pylons was 39.2 ± 8.9 m, n=37). Corvid nests in large variety of tree species were used, with nest height averaging 20.0 ± 6.0 m (n=41). Peregrine Falcon Falco peregrinus: most recorded nests were in nestboxes, but breeding on nests of corvids in electricity pylons in farmland and cavities/niches in industrial buildings is now widespread (though underrecorded). In 2023 80 out of 86 nests were in nestboxes, 4 on corvid nests in electricity pylons, 2 on the ground and 1 on a viaduct. At many sites dead Peregrines were found, sometimes several lying in nestboxes (once a female with four nestlings) or nearby, with an incubating female in situ. Of dead Peregrines, 10 out of 14 tested positive on HPAI in 2022, and (preliminary) 13 out 14 in 2023. High mortality from Avian Influenza resulted in many take-overs of nest sites by floaters, sometimes coinciding with intraspecific aggression of up to 3 males and 4 females at a single site. Mean laying date was 15 March (range 24 February-9 April, n=69), mean clutch size 3.6 (n=59) and mean brood size at ringing age 2.8 (n=75). Sex ratio of older nestlings was 98 males and 82 females on 62 nests in 2023, but over 1997- 2023 sex ratio varied between years with male bias (15 years), years with female bias (8 years) and years with equal sex ratio (6 years), overall 770 males and 700 females on 512 nests (52.4% male). Diets of Peregrines in the breeding season were dominated by racing/feral pigeons (from 42-54% at nest sites in the SW Netherlands), complemented mostly with waders and small/medium-sized passerines.