The Vistula is one of the last large rivers in Europe, which has preserved the characteristics of a natural lowland river over a large area. The braided (meandering) nature of the riverbed means that in the middle section of the Vistula we can observe habitats which, as a result of the regulation of watercourses, are very difficult to find in Western Europe. These are mainly islands and sandbanks in the riverbed and steep banks. The uniqueness of this environment results from the currently rarely recorded species diversity of avifauna (more than 160 species found), but above all from the presence of bird species characteristic for the little-altered valleys of large lowland rivers. Charadriiformes, including several species of gulls, terns and plovers inhabiting mainly islands and sandbanks in the riverbed, form the core of the bird community of the middle Vistula riverbed. This place is of exceptional importance for common gulls Larus canus and little terns Sternula albifrons, maintaining 75-90% of their breeding populations in the country. It is also crucial for common terns Sterna hirundo, Mediterranean gulls Ichthyaetus melanocephalus, ringed plovers Charadrius hiaticula and oystercatchers Haematopus ostralegus, being a breeding ground for at least 25-35% of the national population. Not much less important is for little ringed plovers Charadrius dubius and black-headed gulls Chroicocephalus ridibundus, maintaining annually about 10-15% of the national breeding population of each of these species. The natural consequence of concentrating a large part of breeding population in one site is that the fate of nesting birds (breeding success, adult survival, etc.) decisively affect the status and abundance of the species throughout the country. This is the case with the above-mentioned species, most of which have been showing a downward population trends in the Vistula valley in recent decades.
The strategic, long-term goal is to stop population declines of key species of the Middle Vistula riverbed bird community: primarily the common gull - a species currently considered to be one of the most endangered species in Poland (decrease by over 70% in 2007-2019) and little tern, common tern and black-headed tern, which, similarly to the common gull, although to a lesser extent, record a decline in numbers, as well as the Mediterranean gulls, which joined the group of species declining only in recent years.
However, even immediate and extremely effective protective measures will not stop the trend immediately. This requires, above all, the effective implementation of short-term goals, which we will present below, and the time needed for the young that survive to mature sexually (2-4 years), then return to their natal sites on the Vistula River and successfully breed each season.
The short-term, intra-seasonal goals are:
(a) reducing the mortality of adults on breeding grounds (no more than a few individuals per season in the area covered by protective measures),
(b) increasing the breeding success, and in the case of common gulls also,
(c) increasing the genetic diversity of birds within the colony (reducing the frequency of unhatchability to 10-15% of eggs laid).
We plan to take active protective measures to cover clutches/broods and breeding grounds of: common gulls, Mediterranean gulls, black-headed gulls, little terns, common terns and still very rare in Poland, oystercatchers. Indirectly, effective protection will also be provided to the following species using the same breeding sites: ringed plovers, little ring plovers and lapwings Vanellus vanellus, which are currently, like common gulls, one of the most endangered species in the country.
During the last two decades, the type of threats and the level of pressure that Charadriiformes nesting in the Vistula riverbed had to face changed completely. While until 1999 the greatest threat was a mass occurrence of blackflies (blood-sucking flies from the Simuliidae family) and large river floods, locally also cattle grazing, already in the years 2000-2004 an absolutely key threat to both clutches/broods and adults was the rapidly increasing pressure of American minks Neovision vision and red foxes Vulpes vulpes. As a result, the breeding success of gulls and terns nesting on the Vistula islands was more and more often zero or close to zero, and the mortality rate of adults was extremely high.
Between 2005 and 2015, the pressure of American minks and foxes continued to increase, we also recorded the presence of raccoon dogs Nyctereutes procyonoides and more recently raccoons Procyon lotor. The abandonment of intensive cattle grazing after 2004 resulted in the dynamic disappearance of optimal breeding habitats for terns and common gulls. In the last five years (2016-2020) in areas which were not under active protection, the key threat to clutches/broods and adult gulls and terns was still mammalian predation. Long periods of hydrological drought further accelerated the succession of vegetation, activated mass appearances of blackflies, but above all, they facilitated and definitely increased the human penetration of islands, as well as of feral dogs and cats. In the case of the common gull, we are dealing with yet another important phenomenon that has a negative impact on reproduction, related both to the life history of this species and to the rapidly decreasing population number on Vistula River breeding grounds. It is the low, intra-population genetic diversity (Bukaciński et al. 2000). It leads to reduced hatchability and low immunity of hatchlings.
Due to the fact that the strong pressure of predatory mammals led to zero breeding success and high mortality of adults, and the dynamic succession of vegetation led to an increasing deficit of good breeding sites, the further presence of several species of Charadriiformes in the Vistula riverbed was strongly endangered. With this in mind, in the years 2008-2010, as part of the implementation of the GEF and EcoFund projects, we searched for effective methods to protect ground-nesting birds against the above-mentioned threats. Financial support allowed us to develop several proprietary protective measures, which we successfully implemented in the years 2011-2015, carrying out together with OTOP the project entitled: Active protection of endangered avifauna species of the islands in the area of OSOP Middle Vistula Valley, granted by EU and NFOŚiGW.
The key to the success of any conservation program is to conduct protection activities comprehensively and continuously. Therefore, in this project, we propose a continuation of active protective measures for endangered species of Charadriiformes of the Vistula in the years 2021-2024.
(a)reducing the number of American minks through trapping and euthanasia and of red foxes, raccoon dogs and raccoons by shooting,
(b) neutralizing the pressure of terrestrial predators, people and livestock on the breeding grounds by electric fencing (for colonies) and through a temporary procedure involving the use of incubators and dummy eggs (for single nests and during the mass occurrence of black-flies),
(c) restoring breeding habitats by grazing; and in the case of Mew Gulls also,
(d) ncreasing genetic diversity within colonies by exchange of clutches between nests from distant colonies. A detailed description and schedule of these treatments are presented in the best practice manual (Bukaciński 2015), which is to be a guide in the creation of conservation plans for reserves and the implementation of protective tasks in the SPA PLB 140004 area.
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