Thursday, 23 June 2011

Where now for Orioles?

With just 2 males and a single female having returned to Lakenheath so far this year, how long can this remnant population persist? There has been a steady decline in numbers since the heydays of the 1980s when 20+ pairs were in the area. Last year breeding was not proven.

It is perhaps worth thinking about why Orioles are so attached to the plantations of hybrid black poplars in the fens. There are numerous varieties of the hybrid black poplar, but only those with early leafing and large leaves are favoured. One variety ‘robusta’ is frequently chosen. The diet of Orioles appears to be mainly large caterpillars and it is assumed the poplars provide plenty of food, perhaps not exploited by other species. However, elsewhere in Europe, Orioles use a variety of trees as well as poplars.  Areas of woodland with swampy glades or clearings are often favoured, and the proximity of water is frequent.  So, the structure and location of the tree may be important but it has also been suggested that Orioles return to breed in the type of tree in which they were hatched.

So why are they declining?  One key theory is that Orioles need to breed in loosely connected concentrated sub-populations that form nuclei from which birds can radiate outwards. The fragmentation and reduction of the fenland poplar plantations may have resulted in an unsustainably small population of Orioles.  In addition, although climate change predictions seem to benefit Orioles in south-east England, subtle changes may be affecting food supply.  Despite the predicted benefits, at the present time we are seeing declines in northern European countries including the Netherlands.  The fortunes of the fenland Orioles may be linked to those across the water.

So what if anything can be done? New poplar plantations have been established at Lakenheath and are close to being of suitable age for Orioles. But is this enough?  It may be that only the planting of a large 500 ha+ poplar woodland will solve the fragmentation problem. One slight conundrum is that hybrid poplar plantations are not a natural part of the countryside and are not used by a wide variety of species.  So, could we diversify the plantations into a more natural wet woodland community but ensure a proportion of the appropriate poplar variety, and maybe hope that Orioles will increasingly use other tree species.  However, a large poplar plantation will take many years to establish, and no small amount of money.  Whatever the solution, time may be the limiting factor.


Paul Tout said...

>>So what if anything can be done?

Is it really worth doing? Orioles are an abundant species across most of Europe. Why use large scale (and expensive) habitat management and plantings to try and aid a species that is at the very edge of its range and enjoys a favourable conservation status? I vividly remember and enjoyed the Bryant & May orioles (and oddly enough have never met with a similar density anywhere else in 'natural' habitats in Europe)but unless there are co-benefits of oriole management (such as creating Danube-style riparian woodland for a putative East Anglian reintroduction of White-tailed Eagle) then I don't see much point.

Grumpy Ecologist said...

I think you've hit the nail on the head Paul, but I guess some will moan when they finally go.
White-tailed Eagles are on their way anyway (if allowed), but that riparian woodland sounds good for Black Storks to me.