Sure, my main interest, inter-tidal movements, was knocked for six with the resultant low counts, but the birds were still out there, somewhere.
Like me, they had limited vision. Although I'm trying to understand nocturnal as well as diurnal movements, and know wader eyesight has evolved to aid feeding, migration,etc. in extremely poor light, they still don't have a great ability to deal with fog, which is essentially water droplets hanging in the air, impeding any clear view regardless of their light-gathering capabilities.
Fog comes in lots of shapes. Birders probably encounter radiation fog in winter more than any other form during cold, calm conditions. It forms as the ground cools further at night thanks to conduction. It usually forms later into the night, lasting into the morning when the warming sun 'burns it off'.
Precisely what had happened night before last. Clear until the wee small hours* when the tide was dropping and birds would have moved to their 'usual' night areas.
(*An old phrase referring to the time that old people have to get up and visit the loo during the night, and can look out to check the weather...).
So, the early morning onset of fog would see birds trying to roost in limited visibility.
Individual waders that hold a feeding territory can retain some normality, similarly species that do not follow the tide. But those that chase the tide out, that might flight from one flat to another, could well have problems reaching their normal roosts.
Safe roosting is vital. A place to rest, a place for information exchange. A lot has to be taken into account when trying to avoid impacts on the carrying capacity of an internationally important estuary.
Waders are never 100% faithful to one roost site. They have different options for neap and spring tides. Similarly for nocturnal, diurnal roosts- many birds might choose to continue to feed on the roost in daylight, many sleep for longer periods at night and need the safest of sites. Within these choices, they might also swap due to disturbance levels (there is a clear difference on the Medway between weekends and weekdays, thanks to the increases in human disturbance levels out on and around the islands). Annual cycles also have an effect, for example, pre-migratory restlessness causing larger communal roosts.
Ideally, all main roosts on an estuary should be found and recorded, with an assessment of quality and of usage frequency. Distances from roosting to feeding areas calculated, and the availability of secondary sites should any become unavailable. Piecemeal development around an estuary can easily impact on carrying capacities.
What happens in fog? What might a birder experience?
Individual waders holding feeding territories often stay close to their patch of mudflat, retreating to the sea wall. More than likely these will be the lone Grey Plovers, Curlews, Redshanks you put up.
Communal roosters may have to try to hole up adjacent to their last feeding area. Birds such as Dunlin and Black-tailed Godwit will be clumped together in small gatherings of perhaps 100, 200 birds, on the saltings, or tucked up close to the seawall (neap tides under fog better for observing this- more saltings available to keep a dry foothold on).
Of course, any observer might cause a disturbance movement. Stumbling upon a lone Grey Plover the bird might rise up, calling, to flight a short distance along the wall. A clump of ten, twenty Grey Plovers (non-territory holding birds) might well do similar.
Flushed birds might drop inland of the seawall with an intent to return as soon as you have passed, or try to circle tightly overhead, calling frequently. Rarely will they attempt to go any distance, unless they consider you a real threat.
Wander enough of the seawall, and you will find many small roosts on what are usually unsuitable saltings. Wander often enough, and you'll find which ones are routine secondary sites.
I have most experience of foggy behaviour for Rainham creek/saltings and Otterham simply because they are the sites closest to home; much less chance of me getting run down walking/cycling that short distance to the seawall than risking foggy rat-run back roads during an early morning rush hour(!)
One obvious candidate for a case study here at certain times of the year is the Black-tailed Godwit. When on the estuary in number, they might not always find fog binds them close to shore; they may have been favouring mid-estuary feeding when the fog descended. But if they find themselves fogbound when in and around Rainham and Otterham Creeks, to my eyes they will adopt a restricted flight routine.
Foggy morning roosts will be Rainham Docks East for Rainham Creek feeders (red), Otterham saltings for Otterham Creek feeders (orange). The former site suffers from high disturbance levels, being within the Countryside Park and close to one of the main car parks. Birds might try to navigate north to RSPB Motney Hill saltings, but more often than not go low over Motney peninsula reed bed and scrub to join their neighbours. There the low-lying Otterham saltings, like those of Motney Hill, go under on higher tides. Birds might then often choose to drop over the sea wall onto the fields of Horsham Marsh.
If the fog persists through the dropping tide the birds then remain in the northern section of Otterham Creek (away from any existing footpaths), lining both edges of the creek.
This is the pattern for covering tides around dawn. High tides at night see the largest numbers of Black-taileds using either Motney, or more often Oakham island mid-estuary. Fog at this time means many fewer Godwits close to the southern shore at dawn.
|Just after sunrise, on the fall, Rainham Docks East, April 2014, |
following an undisturbed roost along the shore.
All vastly over-simplified of course (a further secondary roost on Motney has been left out to avoid confusing flightlines) but hopefully a useful example of how recording site usage around your own local patches can help. It will help clear the fog a bit.
An example of how powers that be may well miss such things when planning future developments if we birders don't clear the foggy thinking- take this published quote from a professional consultancy report for landowners regarding a stretch of seemingly uninviting man-made stretch of north shore seawall. An area which I've found to be used routinely by large numbers of roosting Oystercatchers; on spring tides, when disturbed or under certain weather conditions. I've pasted the quote on my picture of these Oiks- they're the specks along the concrete apron in this shot taken from Horrid Hill on the south shore.
The more we find out, the more there is to know.