Sunday, 1 November 2015
Startling starling revelations.
I looked back through my old photos and found that I had taken them in mid-November, so perhaps I am just too early? The RSPB website suggests that "Autumn roosts usually begin to form in November, though this varies from site to site and some can begin as early as September."
My first reaction to that is "Why?". Why do roosts start on different dates? I read about one near Edinburgh that always starts in September and peaks in February, so could latitude be a factor in the starting date? As always in biology, that answer is just too simple to be true. For a start, our local starlings form flocks all through the summer as each successive brood leaves the nests. Those birds must roost somewhere, but their numbers are small compared the massive winter roosts found around the country.
So, where do the birds that roost here in winter come from? British starlings might join us from the big cities or from a more scattered rural population, but continental starlings outnumber them by the million so that a single murmuration might hold 100,000 birds. A study in the 1960s found no evidence of continental starlings in the big urban roosts (ie no continental ringed birds) so the assumption must be that our starlings lie the cities while the European ones like more natural roost sites, nearer to more natural food sources. That's not right though, because there are big urban starling roosts in industrial Europe too.
I remember being told that you could guess at the origin of a starling from its songs and calls. They love to imitate other birds, people and even machines. If you hear a starling imitating a curlew, then it must be a rural bird and if it imitates a golden oriole or a bee-eater, it's probably not from around here! In Brampton we now have starlings that imitate buzzards and we used to have a song-thrush that imitated the reversing buzzer of the big tippers at the quarry.
Scientists first learned the scale of the autumn influx of starlings when the War Office used radar to monitor air traffic over the North Sea and English Channel during WW2. Soon after sunset their screens would fill with clouds of "Angels" which they found hard to explain until they spotted the birds in searchlight beams and as they flew across the face of the moon.
The mass arrival of migrant continental starlings is not likely to happen on the same date each year because of differences in weather and available food across the region. As the big freeze moves down from the north and east, the birds will move ahead of it until they arrive at the coast. Some might come early from Sweden while German birds might not leave until later. This goes towards explaining the gradual build up of roosts through the winter, but another factor might be that smaller roosts like ours at Paxton join a bigger roost. Certainly our main roost is always abandoned by January and that is far too early a time to be heading home, even for British starlings.
How do they choose where to roost? The RSPB website tells us;
"They roost in places that are sheltered from harsh weather and predators, such as woodlands, but reedbeds, cliffs, buildings and industrial structures are also used. During the day, however, they form daytime roosts at exposed places such as treetops, where the birds have good all-round visibility."
A French study showed that starlings are pretty flexible about roost sites and will move from one type to another. Reedbeds are very popular, but so are man made structures like bridges and piers. They noted that, when starlings chose a tree-roost, they preferred big trees that had a lot of small branches to perch on. They also suggested that this flexibility over where to roost could be a factor in the way starlings have managed to flourish across the globe, far from their native range.
Most research on starlings has been conducted by agencies that want to find out of how to get rid of them because, where they have been introduced in America, New Zealand, South Africa and Australia, their populations have soared. Over here in the UK there's a different story. Its quite hard to unpick the numbers but I understand that our starling population has fallen by over 80% since 1979. In the last ten years we have lost over 30% of them, meaning they are now on the critical list of UK birds most at risk, not because they are rare, but because of the alarming and rapid decline in numbers. One of the ways the population is assessed and monitored is through the garden bird surveys that take place every year, once in winter and once in spring.
The decline in our (UK) breeding population is believed to be due to the loss of permanent pasture, increased use of farm chemicals and a shortage of food and nesting sites in many parts of the UK but surely, for a migratory bird like the starling, the decline in wintering birds must also have causes beyond our shores? It turns out that the European breeding population as a whole has declined by about 30% over 10 years, just like the British one.
Given this decline in the overall population, it is not surprising that many traditional winter roosts have disappeared altogether. I just hope ours is not one of them!
For many years we have had a roost containing maybe a couple of thousand starlings at Paxton Pits, usually at the Hayling Pit by the village. They always seem to move on during the Christmas break and I have no idea where they go. Three years ago, in mid November, I spent several evenings trying to photograph them. It's not as easy as it sounds. The weather has to be right, you have to be there just at the right time and you need to be in the right spot to get a clear, unobstructed view. Even then, they sometimes perform and give you a good aerial ballet and sometimes they just fly in over the houses and drop swiftly into the reeds. When I watched them, they all went into the reeds as far away from the trees as they could be, right on the point of the reed-bed. I assume this is the safest place from predators.
If conditions are perfect, with a clear sky and no wind, the birds may swing back and forth over the roost for perhaps 15 minutes. Starlings from miles around can see and hear their colleagues and they join in from all directions. This murmuration attracts attention from other birds too, particularly sparrow hawks, but sometimes there might be a merlin or even a peregrine around. If there is, the resulting aerobatics can be spectacular as the flock splits into tight balls of birds that hold formation through the tightest turns and loops before the flocks merge again. You can just get a glimpse of a sparrowhawk in the video.
If all this makes you want to grab your camera and head out at dawn to capture the birds leaving again, think twice and don't bother. The birds leave a few at a time as they wake up. Some of them are still half asleep and groggy so aerial collisions are quite common in the mornings. They almost never happen at dusk.
The Hayling Pit is a popular carp fishery and the anglers have probably had the best chance to see the starlings over the years. One of the fishermen has commented to me that the starlings abandoned the Hayling Lake because the rangers cut down the willows in the reed-bed. My first reaction was defensive; "Of course that can't be right." But can it? Could he be right?
We cut down the willows in the reed-bed almost every year and this has been going on for over 12 years. Our management objective was (and remains) simply to keep the reed-bed from becoming an extension of the woodland behind it (the Hayling Carr). To attract birds like bitterns and water rails, you need quite extensive reed-beds. Ours is tiny by national standards but it does attract the target species from time to time, and anyway, it is the best reed-bed we have. As you can see from the photos and video, the starlings roosted in the reeds, despite the fact that we had cut all the willows out the year before. But that fisherman knows what he saw; we cut the willows and the starlings left!
One explanation could be that it was December, when the starlings always leave. Another could be that they moved to Cloudy Pit which is where they roosted last winter. If so, did they move because of disturbance or for some other reason? My guess is that the reedbed in Cloudy Pit has spread rapidly over recent years and could now be more attractive to birds than the one in the Hayling. The reedbed in Washout Pit is also growing, but an army of willow is marching in behind it, so it may not be a reed bed for much longer. Where will they roost this year? WILL they roost this year?
In the scheme of things it doesn't actually matter where the starlings choose to roost as long as we get to enjoy them. This winter, if you see a murmuration on the reserve, or near by, please let me know where it is. This story is like that gathering of birds; it has started with a simple question asked by an angler and it could grow into something much bigger.