The reclamation of post-mining localities in the NBBCB has usually resulted in land use similar to the land use in the surrounding fragmented cultivated landscape (Hendrychová and Kabrna 2016; Hendrychová et al. 2020) with a significant predominance of productive habitats over non-productive habitats. Our study has monitored the risk of predation of artificial nests from the point of view of the role of reclaimed post-mining areas in the original landscape not affected by mining, the management of those territories and possible factors influencing the risk of predation. Our results indicate that post-mining sites had been incorporated by surrounding landscape, as similar factors of artificial nest predation appeared in our experiment on both types of localities.
The higher risk of predation of experimental nests on spoil heaps and in the surroundings of spoil heaps was reflected primarily on sites divided horizontally into several patches with few layers of vegetation. This kind of environment on the spoil heaps is interpreted as a combination of mainly artificially established grasslands with woody plants or other non-productive habitats, i.e. an environment with fragmented habitats and with a possible impact of the edge effect (see again Gates and Gysel 1978) and/or an ecotonal effect or a matrix effect (Lidicker 1999) on the predation of the experimental nests. The character of these biotopes corresponds to the biotopes in normal landscapes, where the edge effect on predation has already been detected, i.e. a combination of a matrix in the form of agricultural land with fragments of forest cover (Andrén 1995; Sánchez-Oliver et al. 2014), non-production elements of vegetation such as hedges (e.g. Ludwig et al. 2012), or mown lawns and meadows disrupted by patches of ruderal covers or wetland covers (e.g. Suvorov et al. 2014). An environment of this type may have been more attractive for predators than the homogeneous interior parts of agriculturally reclaimed areas or forest monocultures, due to an increased amount of potential sources of food (e.g. real nests) near to and on the border between different habitats (Gates and Gysel 1978), or the use of these habitats as transport corridors for moving around the environment (Gehring and Swihart 2003; Šálek et al. 2009). Various predators, typical for certain neighbouring sites, may have interacted with each other, thus increasing the risk of predation of the experimental nests in the edge habitats (Lidicker 1999; Svobodová et al. 2012).
A question that remains to be answered is whether environments that increase the risk of predation include near-natural sites within reclaimed spoil heaps. In woody covers, the risk tended to be higher in vertically differentiated growths, which are more typical for succession forest covers. However, this could well also apply to aged or more nearly-natural and more structured forest covers with multiple layers of vegetation. These forest habitats have higher species diversity of local birds, including potential nest predators, such as those appearing in the post-mining forest localities of the NBBCB, as mentioned above: Eurasian jay, great spotted woodpecker, red-backed shrike and carrion crow (Hendrychová et al. 2009). An attractive environment and a greater presence of predators may have led to the higher risk of predation of the artificial nests located there than of nests located in homogeneous forest covers. The opposite effect, a significantly decreasing trend in the risk of predation of experimental nests, resulted from their location on the vertically and horizontally diverse sites associated with near-natural sites, especially with succession localities covered by forest-steppe vegetation with a varied mosaic of patches without vegetation, grass and herbs, and woody plants of various ages and species growing both solitarily and in groups (Prach and Pyšek 2001; Moudrý et al. 2021). The lack of predation pressure may have been caused by the greater complexity of the local habitat, which provides better concealment of the experimental nests from predators (e.g. Martin 1993). In the context described above, the detected lower rate of predation of nests in the near-natural sites than in the technically reclaimed sites could be explained, but it should be noted that the difference was not statistically significant.
The risk of predation of the experimental nests was statistically significantly higher on the territory of the spoil heaps than in the surrounding landscape, and the risk decreased statistically significantly with growing distance from the border of the spoil heap. This means that the post-mining sites were a source of predation pressure in the original landscape. Nest predators may regard the spoil heap as some kind of refuge in the surrounding landscape, with no permanent presence of people and with comparatively favourable site conditions: the landscape is more diversified than the non-mined landscape, which is mostly extensively agricultural. The prey of the predators may also have regarded the spoil heap in the same way. Especially birds, for which, compared to other organisms, the recolonization of post-mining sites is often the most successful in terms of population density, species richness and diversity or spatial distribution (Cristescu et al. 2012; Šálek 2012). Greater amounts of prey can attract larger numbers of predators. These might include species that are not ordinarily nest predators, but that might predate a randomly found and unprotected, poorly concealed or inappropriately located experimental nest, thus increasing the risk of incidental predation of a nest found when looking for some other prey (Hulbert et al. 1996; Arbeiter and Franke 2018). All these influences resulted in an increase in the risk of predation of experimental nests on post-mining sites in comparison with nests located in the surrounding landscape. If, as a result, the same pattern applies to the predation of real nests, the post-mining sites could be a sort of ecological and/or reproduction trap for birds nesting there (Gates and Gysel 1978; Robertson and Hutto 2006).
We mentioned above that the higher risk of predation on spoil heaps was connected with more diversified habitats, e.g. edge habitats and non-productive habitats. By means of these habitats, the predators, especially those that are more mobile, could have moved from the spoil heaps to their surroundings without recognising the border between the post-mining landscape and the original environment (Gehring and Swihart 2003; Šálek et al. 2009). This may have caused the demonstrably detected more probable predation of experimental nests in similar habitats near spoil heaps.
We observed that the prevailing probable predators of experimental nests were birds. However, we believe that this result must be taken with some caution. In the case of almost one fifth (18.4%) of the nests, no probable predator was identified, and the experimental nests and/or plasticine eggs could, in some cases, have been subject to secondary predation and, as a result, to incorrect identification of the predator (Krüger et al. 2018). Our recordings of predation events by camera traps and also some other studies (e.g. Ludwig et al. 2012) have shown that, in the case of artificial ground nests, a greater representation of mammal predators should probably be considered. The structure of the species of the recorded predators corresponds to Central European conditions (Weidinger 2009) and to current knowledge of the dominant position of corvids Corvidae among bird predators in agricultural landscapes and in forest covers (Purger et al. 2004b; Krüger et al. 2018; Bravo et al. 2020).