Friday, 29 October 2010
Of crakes and cranes
The success of Lakenheath Fen is remarkable. A review of this year’s figures shows two pairs of Cranes rearing one chick, five Bittern nests, 120 pairs of Bearded Tits and 13 pairs of Marsh Harrier (as well as Garganey, Golden Oriole, Spotted Crake, Nightingale and Hobby of course). Just one species, Fen Ragwort, has been re-introduced to the site since it was created (unless you include Reed). But it is not faring well, and this prompted much discussion of the pro’s and con’s of such action.
Re-introductions always seem to be controversial. Take the return of Cranes to Somerset as an example. Now that the small East Anglian population has broken out of its original Broads homeland, and increased its productivity, it is clearly 'on the move'. One key criterion for determining if a species should be re-introduced, or not, is whether it is likely to return 'under its own steam' and Cranes are highly likely to get to Somerset under the current expansion (wandering birds already have). There can be no ecological reason for re-introducing them to the south-west.
On the other hand, it is unlikely that Corncrakes will have returned naturally to the Nene Washes. Although it is great to hear multiple ‘crekking’ crakes at the Washes, the ongoing re-introduction scheme has to resolve many difficulties, not least that Corncrakes are long-distance migrants and are somewhat difficult to observe. Captive-bred Corncrake chicks have been released at the Nene Washes since 2002, with an estimated 21 ‘crekking’ males in 2010. Birds are now breeding in the wild and returning. However, the population is not considered to be self-sustaining as yet due to the low number of returning wild-bred males. The return rate of the ringed, released males is estimated to be around 20%, similar to the return rate estimated for wild Scottish chicks in their first year. Released birds have now been re-captured up to four years after their release – the longest recorded re-capture period for a UK Corncrake. So, adult winter mortality does not seem to be a problem. One potential problem is low breeding success on the Washes, possibly due to habitat issues, and this is currently being investigated.
In deciding whether the ‘driver’ for site management is the habitat or the associated species, few would disagree that overall, the habitat approach is the best. However, understanding the requirements of key species is essential, and ‘tweaking’ where necessary can enhance their fortunes. The example of the recovery of Bitterns in recent years is clear. At the Nene Washes, there are several key species, so which way do you ‘tweak’? Black-tailed Godwits must be a priority, as the site holds the majority of the UK breeding population. However, what might be seen as ideal wet grassland habitat management for the Washes and its waders, may not deliver enough ideal habitat for the Corncrake, which brings us back to a second key criterion for re-introduction schemes – is there enough suitable habitat for the species?