Mixed corvid roosts

 

A couple of days ago I was entertained by what can loosely be described as 'a murmuration' of corvids. Of course that noun is mostly used in reference to starlings, yet on that morning what I witnessed was perfectly summed up by the word murmuration.

Corvid roosts are well known for providing spectacular displays overhead. I was once invited to view the famous Maddingly roost in Cambridgeshire with scientists from that University. On that wonderful day it was breath-taking to watch thousands of rook and jackdaw not only alight en-masse at dawn, but provide unbelievable aerial displays in the evening as they came into the roost site. What a privilege to be in amongst them, and what a noise!

And it was the noise earlier this week which caught my attention as to what on earth was going on. It was a few minutes before dawn, and in that half light a cacophony of rook and jackdaw calls drifted to me from the fields behind the house. I watched from the bedroom window. This mixed flock of around two hundred was moving swiftly and acrobatically over the fields. Some birds alighting on trees, others simply playing with the wind. Repeatedly they grouped together like black snow, twisting and turning before moving out over the landscape only to then return moments later and repeat this acrobatic show. On the side-lines carrion crow joined in and at one point a raven flew through the melee cronking loudly. I've seen these mixed flocks many times before, as I mentioned above, but always at or near to the roost site. This more recent event was across flat fields and over 2 miles from the nearest roost site on Worlebury Hill just to the south. It was all very much part of the normal behaviour of corvids at dawn as they reaffirm social bonds, however in the 20 years I've lived here I've never seen this over these particular fields. Had this flock been disturbed in the roost site and amassed over fields awaiting dawn? I wasn't sure but it was a wonderful fifteen or so minutes as the light improved.

I mentioned what I'd seen on Facebook and my friend Chris Sperring came back with an open invite to come and see his roosting corvids. These are 15 miles north of me (as the crow flies) and he mentioned that he has observed a number of ravens 'herding' jackdaw as they arrive over the woods, herding them to a specific area of the wood and therefore presumably the right part of the wood, in raven terms, to roost in. Raven have increased dramatically in this area and so later he posted a Facebook Live where indeed a dozen or so raven seemed intent on chaperoning the jackdaw to the part of the woodland the ravens wanted the smaller corvid to use that night. Time and time again watching that video, raven rose up from their perch in trees and flew with and around the jackdaw until the latter settled, with the raven then returning to their tree and waiting the next influx of jackdaw. I'll have to visit and see this for myself.

I'd not heard of this ravens providing a guard or a herding behaviour with jackdaw, or rook for that matter, so later in the evening I did a little research. Mentioned within Raven in Winter by Bernd Heinrich is an intriguing mention of Raven in Wales by W. A Cadman. In this observation from 1947 mixed flocks of corvids were observed coming to a roost with the ravens at the periphery of roost activity. This led me to another article by H. G Hurrell, A Raven Roost in Devon from 1955-56.  This was fascinating. A raven pair held a 5 acre wood as territory. While they did, they had the wood to themselves. Eventually they disappeared and a number of raven began using the wood as a winter roost, eventually over 80. However once the number of raven at the roost increased jackdaw then came to the wood and roosted there too. The summary therefore suggests that ravens provided security and support for the smaller jackdaw, both benefitting through collective feeding and roosting behaviour.

All very intriguing and leaving me wanting to know more. Are corvid roosting patterns, and indeed behaviour changing as the number of raven in lowland Britain increases? Certainly in the raven roost north of me, when I lived up that way in the mid 1990's you'd never see a raven, or maybe once a year as it flew by. Now Chris tells me 20, 30, 40 are not uncommon at night, every night.  Do these intelligent birds have a threshold number after which their influence to smaller corvids is triggered. I have read studies of ravens at a roost and there seems to be information intelligence happening, with some observations suggesting as one raven arrives one raven departs to prevent overcrowding, in other words they, the collective ravens at the roost maintain the numbers there at any one time through observation and communication amongst themselves. Society in fact. 

Whilst persecuted in the past, ravens were often describes as solitary pairs, driven to upland and wild areas. In fact throughout history ravens have been gregarious, lowland inhabiting birds, and who knows, the catalyst for extraordinary mixed corvid flocks, and behaviour as seen recently at the roost site? I simply don't know nor have the answers yet, but I have a feeling this question is going to return to me over and over again this winter, and so, I must visit that roost in the Gordano Valley and see for myself.