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The study aimed to determine the distribution, numbers, and forms of human activity on the banks and in the riverbed of the middle part of the Vistula River. This section of the river is the most valuable for nature and is located between Puławy and the mouth of the Pilica River (km 373-548 of the navigation route). Censuses were conducted in May-June 2014 and 2015, the breeding season of the key species of the riverbed avifauna. The presence and different forms of human activity were recorded, including fishing, camping, water tourism, hiking, motorized tourism, and grazing animals. The frequent presence of people on the Vistula River was much greater in June than in May. In both months, the most popular form of spending time by humans in the riverbed was fishing. While in areas less valuable for birds, fishing strongly dominated, on parts of the river with key breeding habitats for birds, more aggressive forms of recreation, camping and motorized tourism (quads, motorcycles, off-road vehicles), were reported as often or even more frequently. The increasing popularity of these forms of recreation is a very serious threat to the fauna. They not only disturb the breeding birds but also physically destroy breeding habitats and nests with eggs and increase the mortality of chicks of gulls, terns, and other waders.

The inventory of birds was conducted in the years 2005-2010 on the Vistula River section between Dęblin (km 388 of the river) and Podwierzbie (km 435 of the river). The study area includes a southern section of the European Ecological Natura 2000 Site in Poland PLB140004 "Middle Vistula River Valley” (IBA, PL083). On most areas the Vistula flows here within unregulated or relatively little modified riverbed, having features of a natural, lowland, braided river. Sandy islands and braid bars within a main channel, steep banks and old riparian afforestation create the unique breeding habitats of the Vistula River Valley. Especially the river channel habitats provide suitable breeding sites for many rare bird species, constituting for some of them the key-breeding sites. There are, however, fragments of several kilometres, where people transformed the Vistula River in a more noticeable way (Table 1). These are, among others: an urban section within Dęblin boundaries (km 388 to km 393 of the river), a fragment adjacent to Kozienice Power Plant (km 421 to km 426) and the area, where since 2007 gravel for industry has been mined from the river bottom (km 426 to km 431). The aim of this inventory was the comparison of richness and abundance of breeding bird species linked directly to the river channel in fragments mentioned above. It will allow to estimate soberly how very the intensity of human utilization of the river affects the distribution of avifauna of the Vistula, determining the richness and abundance of valuable and/or endangered species breeding in a given area. In the years 2007-2009 within a whole study area 32 breeding species characteristic for the riverbed and its surroundings were recorded. Among them 6 are threatened in Poland: mew gull (Larus canus), little tern (Sternula albifrons), ringed plover (Charadrius hiaticula), Eurasian oystercatcher (Haematopus ostralegus), corncrake (Crex crex) and hoopoe (Upupa epops) (Table 2a, b). The comparison of bird richness revealed significant differences between adjoining fragments of the river (Table 3). Decidedly the most valuable was the section between Wróble and Kozienice Power Plant (km 416-421). The Vistula River flows here within a riverbed only a little changed by man. It has an appearance typical for a large, lowland braided river with meandering channels, network of small side channels, steep banks, sandy bars and different kind of islands. Such differentiated environment is reflected in richness of birds. The association of 27-30 species reached here the total density of 337-397 pairs/km along the river, unparalleled in other habitats (Table 3). The urban section within Dęblin boundaries (km 388-393) and a 5-kilometer-long area of gravel mining (km 426-431) were much less attractive for birds. Straight, much narrower river channel, small number of islands in a main channel and a degraded valley in the vicinity of river banks caused, that only six to ten species nested there, of the total density ranged between 2.0-2.7 pairs/km on the urban fragment and 3.8-6.6 pairs/km within the gravel pit area (Table 3). However, these values are quite high in comparison to those recorded for the fragment adjoining Kozienice Power Plant (km 421-426). The river channel is here clearly contracted and the bottom is deepened. The river flows fast within one channel. Numerous, concrete, submerged groynes, revetments on both sides of the river and a bank made of concrete, protecting a power station are very characteristic for this place. Such a man-made environment almost excludes the possibility of breeding of birds within a riverbed. Only single pairs of lapwings (Vanellus vanellus), common sandpipers (Actitis hypoleucos), little ringed plovers (Charadrius dubius) and goosanders (Mergus merganser) occurred here, sporadically also common terns (Sterna hirundo) (Table 2a,b). The total density was very low and did not exceed 1.5 pairs/km along the river. The yearly surveys of birds in the years previous to and after beginning of a gravel and sand mining from the river bottom allowed to estimate if the development had changed somehow the status of riparian bird populations. During three years of an activity a negative impact of the development on the richness and abundance of bird species was not recorded, both in direct vicinity and on the fragment below (Table 4). Not the small harmfulness of the development finally influences the status and changes of avifauna in these sections, but the place where it is situated. It should be kept in mind that the sand mining takes place only within 2 km of the section of Vistula that is regulated to a large degree, and the section above is almost unavailable for riparian birds (Kozienice Power Station). One can assume with high probability bordering on certainty that when the gravel excavation took place on the fragment between km 393 and km 421 or below km 431 of the river, the negative impact of the development on breeding habitats and avifauna of the river channel would be much more noticeable and worse still – it would be irreversible. Both fragments mentioned above are now unique places, where the bird association characteristic for the unregulated channel of lowland river can be found – the environment, which irrevocably declines due to human activities.

We conducted our study in the Common Tern colony (STH) located on an island in the middle Vistula River course, at the height of the city of Dęblin (km 393–394 of the waterway), in 2017. Our goal was to investigate some aspects of the biology and reproductive ecology of this species. Due to the fact that STH breeds both in single-species as well as in two- or multi-species colonies, in associations with Little Terns (Sternula albifrons), Black-Headed Gulls (Chroicocephalus ridibundus) (LAR) and/or Mew Gulls (Larus canus), we wanted to investigate whether the neighborhood of other species (in this case LAR) affected hatching success and chick survival in STH. Our results clearly show that the presence of breeding terns in the neighborhood of the LAR colony was not accidental and/or caused by the lack of space on the island and/or the possibility of nesting elsewhere. The height of nesting site, type of nesting habitat, clutch size, mean egg volume and mean egg mass of these STH pairs did not differ significantly from those that formed a single species colony, on the same island but several hundred meters away. However, STH nests in the neighborhood of the LAR colony were established much earlier and both the hatching success and chick survival of STH during the early-chick stage were twice as high. Thus, we can conclude that the LAR colony could provide an effective protection against predation of crows, magpies and gulls, dangers which accounted for the vast majority of STH nest failures in the year of our study.

Over the past 34 years, threats to Charadriiform birds inhabiting islands in the Vistula riverbed, mainly gulls, terns and plovers, have changed considerably. While in 1985-1994 the main risks for their breeding were found to be flood waters, predation by the hooded crow Corvus corone cornix and the Eurasian magpie Pica pica, as well as, locally, uncontrolled livestock grazing, in 2005-2014 they were primarily predation pressure from the American mink Neovision vision and the red fox Vulpes vulpes and massive outbreaks of black flies Simuliidae. These threats led to a reproductive outcome of nearly zero and increased the mortality of adults in their breeding areas. As a result, there was a sharp decline in numbers of the majority of gull, tern and plover species nesting there. In 2015-2018, apart from mammalian predation, key threats included the pressure of feral cats and dogs and the growing presence of people on the islands. In a situation that may lead to the extinction of first the mew gull, Larus canus, and later other species of birds, in 2005-2006, we began the active protection of endangered species on the middle Vistula River islands. This effort included (a) reducing the number of American minks and red foxes, (b) neutralising the presence of predators and livestock at the nesting sites with electric fences and the use of incubators and dummy eggs and (c) restoration of the nesting habitat, as well as, in the case of mew gulls, (d) increasing the genetic diversity within colonies. The most important demand is the conducting of the active protective measures in a comprehensive way and continuously for at least five years (until 2023), optimally throughout the decade (2019--2028). In addition, it will be also necessary to carry out wildlife monitoring in this area. The other important task for the coming years will be to oppose plans for extensive engineering in the middle section of the river, which will irreversibly destroy this unique ecosystem.

The study was conducted in the years 1994-2006 in the middle of the Vistula River between Wróble and Kochów (416th – 418th km of the waterway). The presence of islands and steep banks makes it a key place for nesting avifauna, including a number of endangered species. The aim of this paper is to present the impact of hydro-technical facilities (a diversion weir) on habitat changes, and consequently on the abundance and distribution of birds that inhabit this part of the river. A cross-divider in the riverbed (weir) connecting the bank of the river with one of the islands increased the flow rate and pushed the current to the middle of the river, directing it towards the islands. The result of these changes was a sudden erosion of the sandy, the high slope at the top of the island II, a partition of this island into two parts, and a slow erosion of the island located near the left bank (island IV, Fig. 1B). Lack of fixed, steady flow along the right bank resulted also in a considerably faster succession of vegetation on the island I and slow shallowing of the channel between the island and the bank. As a result, the area of sandy habitats, most valuable to nesting birds, decreased disproportionately faster than the area of the whole islands (Table 1). In the years 2001-2002, when the island I was completely united with the right bank and the islands 0 and I were connected to each other, a width of the riverbed at this point decreased from 870 m to 710 m, i.e. by more than 20 % (Fig. 1).

The inventory of birds was conducted in the years 2005-2010 on the Vistula River section between Dęblin (388 km of the river) and Podwierzbie (435 km of the river). The study area includes a southern section of the European Ecological Natura 2000 Site in Poland PLB140004 „Middle Vistula River Valley” (IBA, PL083). In most areas the Vistula flows here within unregulated or relatively little modified riverbed, having features of natural, lowland, braided river. Sandy islands and braid bars within the main channel, steep banks, and old riparian afforestation create the unique breeding habitats of the Vistula River Valley. Especially the river channel habitats provide suitable breeding sites for many rare bird species, constituting some of them the key-breeding sites. There are, however, fragments of several kilometers, where people transformed the Vistula River in a more visible way (Table 1). These are, among others: an urban section within Dęblin boundaries (km 388-393 of the river), a fragment adjacent to Kozienice Power Plant (km 421-426), and the area, where since 2007 gravel for the industry has been mining from the river bottom (km 426-431). The aim of this inventory was the comparison of richness and abundance of breeding bird species associated directly with the river channel on fragments mentioned above. It will allow us to estimate soberly how very the intensity of human utilization of the river affects the distribution of avifauna of the Vistula, determining the richness and abundance of valuable and/or endangered species breeding in a given area.