Saturday, 30 August 2014

Corvid Capers

This photo was taken a few weeks ago, above Stuntney, when I heard a commotion of Rook and Jackdaw. I went out hoping to have been alerted to a passing raptor to find 200 (there's just shy of 150 in this pic) or more corvids swirling low and then upwards very quickly to quite some height over the A142 and then descending in flat glide out of the top of the kettle.  This reminded very much of watching swarms of migrating Honey Buzzards in Israel, moving from thermal to thermal while expending virtually no energy.

I didn't see any raptor presence to have casued the commotion and having taken a few shots didn't think much about it. Next day my neighbour, a good old Fen Boy who has spent his life working the land around Stuntney, excitedly asked me " You see the 'daws welling yesserdee ?".  He recounted that "welling" is traditionally a precursor to unsettled weather and he'd not seen it for some years. 

We certainly got a change in the weather and the afternoon brought strong winds and some localised flash flooding. This lone Swift powered southwards against the thunderous firmament.

Swift Exit

This made me think of  a possible connection between the "welling" and weather change. I hypothesised for a while and came up with this.  On a sunny day with a slow moving cold front (unusual in summer) there may be more likelihood of strong updrafts as there may be a greater density differential between the rising air warm thermals (created by features such as roads, dark bare earth etc) and cooler surrounding air.  Cold fronts apparently move faster than warm fronts and tend to bring greater changes in weather so the conditions may be unusual, time limited and followed by poor weather.  Which leaves me to ponder the function of welling for the Rooks and Jackdaws and why they should take to doing it in such large numbers.