Does size really matter?

An interesting email exchange with John Furse prompted this blog post about the size of our Stonechat territories. As the Stonechat population at Dersingham Bog has increased, we’ve often speculated as to what seems to dictate the size of the territories themselves and the choice of boundaries.

With only a handful of Stonechat pairs until a couple of years ago, the territories were either very large or widely dispersed but as the reserve has hosted ever more Stonechats, the territory boundaries seem very much defined by the topography of the Bog rather than just the pure size of each territory. We’ve never been able to predict how big a Stonechat territory will be just by looking at acreage occupied – it’s always been defined by the layout of the habitat.

It’s a feature of Dersingham Bog that the habitat and topography can change in a relatively small area. Essentially, the reserve is almost on two levels, with the plateau extending the full length of the reserve and

Plateau, main track and mire

bordered on its northern side by the wetter mire, with the main footpath running through the middle. Dividing all that are various ridges or ‘noses’ – headlands extending out onto the mire which result in some valleys being closely bordered, giving a very sheltered and enclosed feel to some relatively small areas.

Two of those territories are long and narrow as a result of the topography rather than being squeezed in between other pairs. They are bounded on three sides by the plateau, so the territory has stretched out in a long rectangle onto the mire rather than spreading sideways as the plateau pairs do.

Three other territories have always been the largest, spanning areas which start up on the plateau and extend down the slopes and out onto the mire. But these territories have gradually been nibbled away this year with new pairs encroaching on the edges as it’s presumably harder to defend territories on separate levels.

Our oldest and most territorial male has always defended a very large territory. He has previously occupied an entire heath on the plateau, the slopes leading down to the area around the dragonfly pond, as well as a prominent ridge extending out into the mire and a large chunk of the mire itself. In the last year, two younger males have established territories on his boundaries, with one taking over the heath on the plateau and the other taking half of the mire area.

By contrast to other areas, this last part of the territory has very little in the way of defined features dividing it and the boundary line seems to be a cleared area halfway along the ridge. Both males have taken to patrolling back and forth either side of the clearing, displaying and posturing at one another like a couple of gunslingers!

Another male has also lost two portions of his territory this year. He once defended an extensive heather plateau, a gully, an area of mire and a low heather valley. It was always a difficult one to monitor to find nests as there are very few spots where you can see all of the territory from one place, so his territory defence must have been doubly difficult to maintain. This year, two new pairs have moved into the edges – one pair taking over the higher plateau, with another pair carving out part of the mire, leaving the original pair with roughly a third of the area they once occupied.

In another part of the reserve, what was one territory now holds three pairs and that area is unusual in that it’s large and flat, with no topographical features dividing up the ground. Feeding areas overlap but by and large, the birds stick to their boundaries.

The topography also defines how close territories may be to one another. Territories may be back to back either side of a ridge and technically within 50 metres of each other, but to all intents and purposes, they are hidden from neighbouring pairs so presumably the males don’t have to spend all their time on territory defence.

Some feeding areas on the boundaries do overlap, with pairs using the dividing ridges themselves, which the males tend to patrol as their boundary lines. As more pairs squeeze onto the reserve and existing territories are carved up into smaller chunks, presumably these shared feeding areas will increase as pairs seek enough food for themselves and their young.

So lots of food for thought, not least about how much of an impact does this territory carve up have on food availability for the pairs? Will we see an impact on productivity if it continues, especially if we get too bad weather events like last week during a breeding season?

It’s tempting to think that the overall boundaries of the Stonechat territories can’t be pushed out any further as the habitat shouldn’t be suitable. Currently, the only places on the reserve which are empty of Stonechait pairs are relatively small gaps right out on the mire, or up on one plateau which has very sparse heather cover at the moment as it was the last part of the reserve to be cleared of pines. However, the Stonechats have surprised us before with adopting areas we would have thought unsuitable so we’ve stopped making predictions!

Dersingham Bog has perhaps an unusual layout, the plateau being the line of an old sea cliff, with the mire occupying what was once the seashore. It can’t be a unique layout for a heathland reserve but it’s certainly unusual. So has the topography contributed towards helping the establishment of such a density of Stonechat territories? It would be interesting to discover how territories are divided up in reserves which share a similar topography to Dersingham Bog.

And our original and frequently asked question remains. Just what is the carrying capacity of Dersingham Bog? How many Stonechat territories can the reserve sustainably support?

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