Trends en broedresultaten van roofvogels in Nederland in 2021

A total of 3034 nest record cards of raptors were submitted in 2021 (Appendix 1, handed in up to and including 31 January 2022), covering 12 species (not including Montagu’s Harrier which is covered separately; www.grauwekiekendief.nl, nor Osprey, which is covered by personnel from the State Forestry Service). The preceding winter was mild (frost index of 13.7 on a scale of 1-100, with a short frost period and snow cover in the first half of February), April and May very cold and wet, the summer was warm (summer index 67.2, on a scale of 0-100). Voles (especially Microtus arvalis) and mice (Apodemus sylvaticus and increasingly A. flavicollis) were at a low ebb. Bird numbers in June-August were small, with a longer-term decline in birds belonging to weight classes of 200-500 g. Social wasps had a very poor year, with low numbers and small colonies (as determined from comb sizes found on nests of Honey Buzzards). Honey Buzzard Pernis apivorus: onset of laying averaged 27 May (range 21 May- 4 June, n=10). Clutch size was 3x 2 eggs. Brood size was 2x 1 and 8x 2 young. The small number of occupied nests found indicated a very poor breeding season for this species, presumably partly because poor weather in May and very poor wasp numbers. On 7 nests, only 49 prey remains were found, of which 48 combs of mostly common wasps Vespula vulgaris and (presumably mostly) German wasps V. germanica. Comb diameters averaged 75 mm in V. vulgaris (N=13) and 58 mm in grey wasp combs, indicating small wasp colonies in 2021. Other prey species consisted of Dolichovespula saxonica (N=4), Vespa crabro (N=4), pigeon Columba sp. (1) and Song Thrush Turdus philomelos (1). Red Kite Milvus milvus: 34 breeding pairs were located, of which 26 commenced egg-laying (resulting in 11x 0, 2x 1, 4x 2 and 5x 3 fledglings). This is a substantial increase compared to 2020, when 22 pairs were found. Black Kite Milvus migrans: again two breeding pairs in the southeastern Netherlands, raising respectively 2 and 3 fledglings. In the latter case, prey remains found on the nest included vertebrates like fish, rabbit, birds, and at least 31 Melolontha melolontha. White-tailed Eagle Haliaeetus albicilla: of 22 pairs, 16 produced a clutch. Twelve successful pairs raised a total of 17 fledglings (7x 1, 5x 2). Most large wetlands in The Netherlands have now been colonized, and some pairs settled beyond the obvious wetland regions. Marsh Harrier Circus aeruginosus: mean onset of laying was 27 April (range 11 April-25 May, n=93). Clutch size averaged 4.53 (N=90), mean number of fledglings per successful pair 2.9 (N=99), i.e. below the long-term averages. 76 broods had a secondary sex ratio of 123 males and 93 females (56.9% male). Out of 175 pairs that started egg-laying, 63% was successful in raising at least one young. Causes of nest failure were mainly human-related, i.e. 12x (compared to 7x natural causes). Food remains found on nests consisted of 41% birds (many young Pheasants), 58% mammals (mostly voles and hares) and 1% fish (123 prey collected on/near nests). Hen Harrier Circus cyaneus: two nests were found in cereals in mainland Groningen, six nests on Wadden Sea Islands (3 Terschelling, 3 Texel). Of nine chicks equipped with a sender, only one survived till December. The Dutch population is only barely surviving. Montagu’s Harrier Circus pygargus: for details, see www.grauwekiekendief.nl. A total of 58 pairs were recorded in the entire Netherlands (compared to 53 in 2019 and 80 in 2020). Just 44 fledglings were raised (compared to 142 in 2019 and 100 in 2020), reflecting poor vole numbers. Pallid Harrier Circus macrourus: one, possibly two, male Pallid Harriers visited Groningen and were observed displaying (but failed to attract a mate). Goshawk Accipiter gentilis: mean start of laying was 3 April (n=136, of which 42% started in March, range 18 March-27 April), clutch size averaged 3.2 eggs (of which 3x C/5 among 151 clutches), brood size (at ringing/fledgling age) 2.48 (194 nests). Sex ratio in 125 nests showed 167 males and 155 females (51.9% male). Causes of failure were 9x human-related, 10x natural. Out of 327 nests with full details on breeding performance, 86 failed to raise fledglings (26%). Prey remains collected near nests showed 97.5% birds in 49 species (especially pigeons and corvids including Jay, with respectively 40% and 18% of all prey, and with Starling, woodpeckers and thrushes in much smaller numbers), as well as some mammals in 3 species (mostly rabbits). Sparrowhawk Accipiter nisus: mean onset of laying was 2 May (range 13 April-27 May, n=78), with 38% of pairs starting in April. The late start is likely related to inclement weather in April and May (cold en wet). Clutch size averaged 4.8 (n=88), the number of fledglings per successful pair 3.8 (n=98). The secondary sex ratio among fledglings was 124 males and 103 females on 59 nests. Among 261 nests, 111 failed to produce fledglings; predation is by far the most common cause of failure (30 out of 33 failures with known natural cause of failure, occurring in all stages of life, i.e. as adult, egg and nestling). Buzzard Buteo buteo: mean start of egg laying was calculated at 9 April (range 18 March-7 May, N=259; only 14.5% of all pairs started laying in March). Mean clutch size was 2.35 (N=292, with 5x C/4), mean brood size of successful pairs was 1.68 (N=419, with 4 B/4). Late start of laying and small clutch and brood size were thought to have resulted from poor vole numbers. Secondary sex ratio in 66 nests was 59 males and 46 females. Of 286 nests in which at least one egg was laid, 218 were successful in raising one or more fledglings. Nest success increased from C/1 (9 out of 13 successful), to C/2 (122 out of 164 ditto) and C/3 (88 out of 103 ditto). In C/4 3 out of 5 nests were successful. Deliberate disturbances caused by humans accounted for 42.5% of all failures (out of 40 with know cause), the rest could be attributed to desertion (8x), egg predation (5x), chick predation (3x), death of parent bird (1x) and adverse weather causing nest destruction (4x). Diet was varied, with 41 bird species (36.6% of 803 prey items), 16 mammal species (61.2% of all prey items, lagomorphs, moles and voles most important in terms of biomass), and some reptiles, frogs/toads and a Procambarus clarkii. Among mammalian prey, voles only accounted for 25.6% in numbers, indicative of a poor vole season. Osprey Pandion haliaetus: the Dutch population comprised of three pairs, exclusively nesting in De Biesbosch where each pair fledged 3 chicks; two other pairs were present with nests but did not commence breeding. Kestrel Falco tinnunculus: onset of laying averaged 1 May (range 27 March-20 June, n=470), almost two weeks later than in 2021. Of the last 27 years, 2021 was among the four latest years regarding onset of laying, due to poor vole numbers. The proportion of pairs producing clutches of 7 eggs showed steep ups and downs throughout this period, reflecting outbreaks of voles, but peaks are nowadays reduced in height compared to the 1990s. Mean clutch size in 2021 was 4.81 (N=589), mean number of fledglings/ successful nest 4.0 (N=675). Out of 773 nest boxes with attending pairs, 140 remained without eggs (18%). Almost all pairs nested in nest boxes, i.e. 882 compared to 8 crow’s nests in trees and electricity pylons (stick nests). Voles were the single most important prey species, e.g. 69% of 701 preys recorded in nestboxes and in pellets in several parts of the country. Hobby Falco subbuteo: mean start of laying was 10 June (range 1 June-27 June, N=23). Clutch size was 3x 2, 10x 3 and 2x 4, the number of fledglings/successful nest 3x 1, 14x 2, 25x 3 and 2x 4. Sex ratio in 3 nests was 2 males and 6 females. All pairs used old nests of crows (N=70 nests). The proportion of pairs nesting on old nests in electricity pylons amounted to 72% (N=91 nests), again an increase after decades of steady increase from 0% in the 1970s-1980s to >50% recently. This change in nest site choice partly reflects biased interest of raptorphiles for electricity pylons in farmland but also a habitat shift (away from forests, increasingly nesting in open farmland). Mean nest height in tree nests was 15.6 m (SD=2.8, N=19), compared to 32.2 m in pylons (SD=10.9, N=42). Two ringed adult males were either captured or the ring was deciphered from a distance; both had been ringed as nestling at respectively 12 and 18.6 km distance in respectively 2015 and 2018. Peregrine Falcon Falco peregrinus: most nests were in nestboxes, but breeding on crow’s nests in electricity pylons in farmland and niches in industrial buildings is increasingly recorded. Lay date averaged 14 March (range 28 February-16 April, N=80). Clutch size was 1x 1, 2x 2, 24x 3 and 42x 4, brood size was 11x 1, 24x 2, 32x 3 and 26x 4 (mean 2.78). Secondary sex ratio on 68 nests was 101 males and 96 females. Extensive prey lists from breeding sites in Zeeland, Zuid-Holland and Noord- Brabant showed a preponderance of racing/feral pigeons (45-60% of all prey items), complemented with a wide variety of small avian prey (usually <150 g).