White-tailed eagle does its bit to keep archipelago’s feral mink population in check

Minks avoid swimming between islands when eagles are about

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By Pyry Lapintie in Turku
     
      The white-tailed eagle (Haliaeetus albicilla) - also known as the sea eagle - that preys on feral mink (Neovison vison) among other victims, may actually have a reducing effect on the minks’ moving from one island to the next in the outer archipelago in the southwest of Finland.
      Master of Arts Pälvi Salo, who defended her doctoral thesis on the preying habits of predators at the University of Turku on Saturday, says that in her view it is not unthinkable to suggest that the mink population is kept in check in part because of the hunting habits of the sea eagle - even if this has not yet been scientifically explored.
      “In many studies it has been established that top-level predators, in other words large birds of prey and large predatory mammals, have an effect on the populations of smaller predators, which, in turn, in a way eases the pressure on their prey.”
     
In the research that Salo conducted, twenty wild mink individuals were equipped with radio collars, after which their movements in the archipelago were tracked.
      Especially the female minks were observed to swim shorter distances whenever the likelihood of sea eagles being on the hunt was higher.
      According to Salo, the favouring of top-level predators might thus provide a partial solution to the outer archipelago’s alien predator problem.
     
In Salo’s view there is not much information available on the movements of minks in the archipelago.
      Minks face their greatest risk of falling prey to larger predators while swimming, for all that they are excellent swimmers.
      The mink, which originates from North America and has spread around the globe as a result of fur farming, is very nimble on land, and can take cover even in small holes in a flash.
     
In Finland, the wild mink or American mink spread to the archipelago at the turn of the 1980s.
      Before that time, otters were found in the archipelago, but they have long since disappeared.
      The mink is a cause of great devastation to the bird colonies nesting in the archipelago, as well as to the area’s vole and frog populations, even if its diet mostly consists of fish.
      Where minks were exterminated, razorbills (Alca torda) and black guillemots (Cepphus grylle) returned to nest in areas that had previously been completely devoid of them.
      In addition to birds, voles and frogs also benefited from the culling of the minks.
     
In tests the prey populations of birds have been discovered to grow 1.6 times larger in comparison with areas where these land predators were not eliminated.
      In the Turku archipelago it was noted that in the areas cleared of mink, the frog populations were at their best tenfold compared to areas with mink.
      In addition to the attrition being wrought by sea eagles, the mink population is also being reduced by the Eurasian eagle owl (Bubo bubo).
      In the archipelago, however, the eagle owl population is significantly smaller than that of the sea eagle.
     
According to Salo, the effect of alien predators such as mink on original prey populations is greatest wherever the predators are still relative newcomers.
      The local species do not identify the mink as a predator and therefore fail to react to it accordingly, ending up as lunch.
      Alien predators introduced to remote islands in the middle of oceans have often even caused the total extinction of several indigenous bird and mammal species.
     
Helsingin Sanomat / First published in print 18.9.2009


Previously in HS International Edition:
  Leaf-blower becomes precision-guided weapon for mink catchers in Porkkalanniemi (6.5.2008)

See also:
  White-tailed eagle and osprey fighting a majestic battle over desirable Nauvo property (8.4.2009)

Links:
  White-tailed Eagle (Wikipedia)
  American Mink (Wikipedia)

PYRY LAPINTIE / Helsingin Sanomat
pyry.lapintie@hs.fi