The frogs move down the Zelin River and instinctively know when to leave it in order to reach the swamp. After the end of the breeding season, adult frogs return to the high parts of the mountain by land, covering approximately the same distances. Tadpoles hatch from the fertilized eggs, metamorphose into froglets and leave the swamp by the end of May and the beginning of June. This breeding migration is unique for the species and is among the longest known migrations in the world of amphibians. In addition to the common frog, 8 other amphibian species have been registered in the swamp: common newt ("Lissotriton vulgaris"), Balkan crested newt ("Triturus ivanbureschi"), yellow-bellied toad ("Bombina variegata"), common toad ("Bufo bufo" ), European green toad ("Bufotes viridis"), tree frog ("Hyla orientalis"), agile frog ("Rana dalmatina") and marsh frog ("Pelophylax ridibundus"). There is also a single registration of European pond turtle ("Emys orbicularis"). This relatively rich biodiversity in a rather small area confirms the significance of protected area “Muhalnitsa” for the conservation of the local fauna. In the last 10 years there is a marked increase in the swamp size as well as overgrowing with bulrush ("Typha sp."). This is due to a prolonged leakage from the pipes of the local water and sanitation facility and has a pronounced negative effect on the frogs, which need open water in order to breed. This leakage has probably influenced the water regime of the swamp, changing it from a temporary pond into a permanent one (with better conditions for the bulrush). The progressive growth of the bulrush will probably lead to another change, with the entire swamp drying off. This would mean that in the following years the local common frog population will lose its main breeding site, and the unique migration will cease. This threat would also affect all other amphibian and reptile species in the protected area. Another significant threat to the frogs is the mass mortality on the asphalt road running along the north-eastern bank of the swamp. Migrating frogs that come from the river must cross the road to reach the swamp (except the few individuals that pass through the drainage pipe under the road). Vehicle traffic is not intensive, but nevertheless during the breeding season, the road is covered with ran-over dead frogs.