I'm now well into week four of the 'Uberpatch Meteorological Autumn' project and will wait until the end of September before going through the highs and lows that have been experienced. I have been doing a lot of sky-watching, or vis-migging if you prefer that description, and this has opened up avenues of pleasure, frustration and bewilderment, especially in trying to understand what weather conditions are best to witness a decent movement and where I'm best to position myself to do so. It isn't straight forward!I have a number of 'go-to' places to practice this dark art and all can produce the goods. In rough order of prominence they are: Denbies Hillside; Colley Hill; Box Hill; Epsom Downs; my Banstead garden; Canons Farm; and Priest Hill. Apart from my garden all have great viewing conditions (open sky, long horizons) although Box and Colley Hill lack decent northward views. This post is a distillation of what I think I know about autumn bird movements along the North Downs between Denbies Hillside (in the west) and Colley Hill (some four/five miles to the east).
The annotated photograph above was taken looking north-eastwards along the North Downs scarp slope. As can be seen, the hills here are breached by 'gaps', the Mole Gap being the widest (approx half-a-mile) and created by the River Mole that flows through it (seeing how narrow and minor the river is today one can only wonder as to its previous state considering that it has carved through an awful lot of chalk). The Buckland Gap in comparison is much narrower, ravine-like almost. Ranmore, Box and Colley Hills are of similar elevation (224m) and are wooded, although Colley Hill has a far more open top. Viewing northwards from their summits is difficult, particularly on Box Hill, so there is a need to position yourself down the slope to open up the sky when looking back northwards (and, to a certain extent, westwards and eastwards). The view south is excellent.
This is the view from the lower slope at Denbies Hillside, looking eastwards across the Mole Gap to Box Hill. I position myself here because it opens up the sky whichever way I look. Any extra height cuts off at least one line of view. If you were to turn left you would be looking up the western side of the Mole Gap. Behind you the North Downs carry on westwards. If you turn right you will be looking across to the Greensand Ridge (5 miles distant), where Surrey's premier migration watchpoint, Leith Hill, is situated.
This photograph was taken from Box Hill looking back westwards (to where the Denbies Hillside image was taken). The Mole Gap is hidden by the wooded area (top right). The distant hills to the left are the Greensand Ridge.
Migrating birds use differing fly lines at both Denbies and Box Hill, some species even swapping direction on the very same day! What also confuses the picture is that there are several narrow and steep valleys behind both sites which can - and do - act as flyways. It is therefore almost impossible to be able to see all that is moving. The picture above is of the 'zig-zag' area behind Box Hill. This is just one of several 'copycat' valleys that are wide enough to funnel birds. I have seen large numbers of thrushes passing on days when I had earlier been watching them doing so in similar large numbers along the scarp slope. The Mole Gap acts as a fly line, and on one particular September day hirundines started to arrive from the south and fly up it (northwards and then veered off eastwards towards Box Hill via the back valleys) before then switching to an easterly line along the scarp slope, ignoring the gap totally. On another occasion, again September, birds came out of the Mole Gap from the north and carried on southwards in a straight line towards the Greensand Ridge. You really do need a team of observers to cover this area properly! Some days see birds hugging the western side of the Gap, on others the eastern side - if you are positioned on the 'other' side of the gap accurate counting is compromised. I don't often have 'broad front' movements at Denbies, movements here tend to be on narrow flight lines. This is not the case at Box or Colley.
Above is Colley Hill, looking westwards along the downs towards Box (out of sight) and Denbies (in the distance). If birds are coming out of - or heading up - the Mole Gap they will be missed here. At Colley it is usually a case of watching birds arrive from the east (thrushes and finches) or the west (hirundines), with a few birds, particularly pipits, appearing above your head on their way south. Just to confuse matters, just 800m north of here is a wide open area of farmland and grassland (Mogador) where I have had excellent passage heading westwards. I am of the opinion that much of this movement is following the A217 road - which from the air is an excellent marker - and will be missed at Colley. Whether or not these birds cut in and carry along the scarp slope at Box Hill is unknown, although I have had mornings when birds arrive from the north over Mogador and join the downs to the west of Colley, maybe making use of the Buckland Gap to do so. These would then be largely missed by anybody birding on the open hill top.
Box Hill autumn thrush fly lines: Yellow (scarp slope, steady westwards parallel to hill), Purple (following line of River Mole although many carry on on a straight westward bearing), Orange (Mole Gap, birds that have taken the back valleys and then reorientate - can also head due south towards Greensand Ridge).
Box Hill: a bit more detail for October thrush movements - birds arriving from the north (top) or east (right).
Box Hill: the hidden valleys. All these are proven fly lines. You'd need a team of five minimum to stake it out.
Colley Hill: only a couple of miles to the east of Box but at times seems as if it is divorced from it when it comes to sharing bird movements. Three proven flight lines.
Sometimes it all comes good!
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