The Red Kite Milvus milvus breeds alomost exclusively in Europe, its population is in decline. In the past, it was regarded as a rare migrant in Slovenia. In the 1990s it was classified as a possible breeder, extremely rare winter guest with more observations during migration. Between 1977 and January 2017 117 records of 121 individuals were gathered. Most (113) involved single birds on migration. Exceptionally, birds were present for a longer period - a single record of a summering bird is known. Since 1980 the number of birds per year increased from 1 to 1.1, 3 and 9 after 2009. Possible reasons are increased awareness and population increases to the north and northeast of Slovenia. Red Kites were observed throughout the year with the lowest numbers during summer and winter, which surprising, because the species is increasingly common during winter in central Europe. Migration of Red Kites in Slovenia begins in February with a peak in early April and May and between August and mid-November with a peak in early in October. The migration period coincides with arrivals and departures from breeding grounds. Red Kites have been observed all across the country, with more observations at well-watched sites. Birds were observed between sea level and 1740 m a.s.l. with only a handful of observations above 1000 m a.s.l.
Between 1984 and 2017, 1,388 Black Kites were recorded, mostly in lowlands with 70% of observations made at Dravsko polje. They were observed from sea level to around 1,600 m a.s.l. with an average elevation of 271 m a.s.l. The species was present in Slovenia from mid-March to early December with indistinct spring and autumn migrations. The highest number of observations was recorded in May. The Black Kite was observed in 71 out of 238 10x10 km grid squares in Slovenia (29.8%), with more observations around known breeding sites and at sites with higher observer effort. Both the number of observations and the number of probable and confirmed breeding pairs increased. In 2011–2018, 10 breeding pairs were found at 7 sites (3–7 per year). Additionally, 11 probable breeding pairs at 9 sites (0–6 pairs per year) were found. The breeding population in 2011–2018 is estimated at 10–21 pairs with an average breeding density of 0.3–0.9 breeding pairs per 100 km2. The highest density was recorded at Dravsko polje with 0.6–2.2 breeding pairs per 100 km2. If possible breeding (breeding attempts) were also taken into consideration, the estimate would be up to five breeding pairs higher. The species was recorded at known breeding sites in most years after the breeding was confirmed. Black Kites were observed closer to larger water bodies and to rubbish tips than expected by chance. More Black Kites were recorded in areas with a lower percentage of forest and arable land and a higher percentage of meadows, settlements and wetlands.
Spring migration of birds, particularly waterbirds and raptors, was monitored from dawn till dusk from an observation point located on the levee on the northern side of the Medvedce reservoir (Dravsko polje, NE Slovenia) in three separate years (1-17 Mar 2009; 25 Mar-10 Apr 2011; 28 Feb-12 Apr 2012). In all three years, 42,045 individuals of 89 species (66 waterbirds, 23 raptors) migrated over the site. 38,238 of these were waterbirds. The most numerous species was Lapwing Vanellus vanellus (11,813 individuals), followed by Blackheaded Gull Chroicocephalus ridibundus (10,515 individuals). In total, 3807 migratory storks, raptors and Cranes Grus grus were counted. Harriers (2303 individuals) were by far the most numerous group. The prevailing direction of migration was SW-NE. Half of all migrating birds were observed in the altitudinal belt between 10 and 100 m above ground level. Harriers were mostly observed up to 100 m above ground level, whereas storks, other raptors and Cranes were observed most often in the altitudinal belt between 100 and 500 m above ground level. The highest numbers of waterbirds were observed between 10 and 100 m above ground level. In the first five hours after sunrise, 61.3% of all waterbirds, 44.0% of all storks and raptors and only 20.5% of Cranes were observed. Although other parts of Dravsko polje have never been studied in such detail, it is possible that a significant portion of migrating birds from a larger area gathers at this site. This study confirms the importance of the area for migrating and staging birds, especially for Great Egret Ardea alba, Marsh Circus aeruginosus and Hen Harriers C. cyaneus, Osprey Pandion haliaetus, Crane, Ruff Calidris pugnax and Black-headed Gull that all reached at least 0.1% of their respective biogeographic populations. Internationally significant are particularly the numbers of migrating Marsh (1079 individuals in 2012) and Hen Harriers (408 individuals in 2012), as there are only few sites where these species migrate in such high numbers in spring.
Between the end of February 2007 and mid-February in 2008, 73 systematic surveys of waterbirds and raptors were carried out during 10-day periods at Cerknica polje (Southern Slovenia). The main objectives of our research were: to ascertain the abundance and temporal dynamics of the species occurring here, to specify their status, to make a comparison with the past period of systematic data gathering in the 1991-1992 period, to present estimates of the breeding, migrating and wintering populations, as well as to make, on these bases, a nature-conservancy evaluation of the area. Other ornithological data, acquired till the end of 2010, were gathered as well. Between the beginning of May and the beginning of September 2007, the water in the area virtually ran dry (it was retained only in the channels of the streams), while the surface itself was partially or fully icebound from mid-November 2007 till the beginning of February 2008. By the end of 2010, a total of 129 waterbird and raptor species were recorded at Cerknica polje, while in the 2007-2008 period 83 were registered. Dynamics of the species occurrence with more than nine observations made in the area is presented in a greater detail in the species overview. The breeding status was held by 27 bird species, while further seven were forage guests that bred in the vicinity of the study area. Most of the species (118) had the status of passage visitor, whereas those with the status of summer visitor (34) and winter visitor (40) were fewer. Among the recorded species, 34 were accidental and 21 rare visitors. There were 16 winter and seven summer residents, while year-round residents were six. In all 10-day periods of the research period, four species were recorded: Mallard Anas plathyrhynchos, Grey Heron Ardea cinerea, Buzzard Buteo buteo and Kestrel Falco tinnunculus. In more than 90% of 10-day periods, the Great Egret Ardea alba was recorded as well. In the 2007-2008 period, two species were eudominant (Mallard 27.2%, Buzzard 10.1%), whereas another two were dominant (Garganey Anas querquedula 7.4%, Coot Fulica atra 6.4%). Most individuals were registered at the end of March and in early April (up to 1,978 ind.), whereas the greatest numbers of species (48) were recorded in mid-April. Between May and August, the numbers of individuals and species were low owing to the dried up lake. The smallest area of occurrence was occupied by the Cormorant Phalacrocorax carbo and a group of ducks of the genera Aythya, Bucephala and Mergellus. Gulls, egrets, herons, waders, harriers Circus sp. and the Red-footed Kestrel Falco vespertinus occurred in the greater part of the research area. 10 breeders fulfilled the criteria of the species of the greatest conservation importance, two of which (Ferruginous Duck Aythya nyroca and Curlew Numenius arquata) are species of global conservation concern, whereas seven are of conservation concern on a European scale. Among non-breeders, 14 waterbird species and raptors occurred in significant numbers (> 0.1% biogeographical population), five of which occurred at least occasionally with more than 1% of their biographical population. Two species (Red-necked Grebe Podiceps grisegena and Snipe Gallinago gallinago) breed in Slovenia only at Cerknica polje, while further six species have here at least 40% of their Slovenian breeding population
Agriculture has a great impact on biodiversity in Europe. Populations of farmland birds are declining due to both intensification and abandonment of agriculture. The purpose of this study was to determine the effects of overgrowing on the diversity of birds at Cerknica Polje. Six vegetation types were identified on eight survey plots (15.2-31.6 ha each): Short grass - regularly mown wet meadows; Tall herbs - stands of Common Reed Phragmites australis and Reed Canary Grass Phalaris arundinacea; Sparse bushes - stands of low woody plants; Dense bushes - dense stands of bushes; Meadows with trees - mown meadows with scattered trees; Bushes with trees - hedges, trees and dense stands of bushes. In total, 34 species were registered. The heterogeneity of vegetation types correlated positively with the heterogeneity of bird species. The highest density of breeding territories and species was established in Bushes with trees, the lowest in Short grass. No species were registered in all vegetation types, and only Tree Pipit was recorded in five out of six types. Twelve species were registered in only one vegetation type. Cerknica Polje is an important breeding locality for species with the highest breeding density at the national level (e.g. Corncrake Crex crex, Curlew Numenius arquata, Skylark Alauda arvensis, Whinchat Saxicola rubetra, Sedge Warbler Acrocephalus schoenobaenus). These species breed in specific habitats with different stages of succession. A managing plan for the area should therefore combine abandoned plots in different stages of succession and large regularly mown areas. Abandonment of agriculture at Cerknica Polje has led to the emergence of areas with bushes in different succession stages. Efforts should be directed at preserving large complexes of mown wet meadows.
Migration poses a high risk to birds. Crossing of large bodies of water is especially demanding for land birds. One of the dangers faced by migrants are opportunistic predators like gulls. Most gulls Laridae are generalist predators with omnivorous diets. Attacking on migrating birds was investigated during ground observations of bird migration at Ada Island (S Montenegro) between 17 Mar and 10 Apr 2015. We recorded 20 attacks on 22 individuals of six species and on one unidentified passerine. In four instances gulls attacked birds of prey, in two Hoopoe Upupa epops and in 16 passerines. Except for two cases when small flocks of two birds were attacked, gulls attacked individuals migrating singly. Considering our data around one percent of passerines migrating during the day across the southern Adriatic Sea can expect to be attacked by gulls. This percentage can be as high as 9.5% for passerines migrating singly.