Gause's Law or the Competitive Exclusion Principle: where two species compete for the same limiting resources, they cannot coexist at constant population values. When one species has even the smallest advantage over the other, they will dominate in the long term.
Odd place to start, but for the Medway, worth understanding the differences between Bar-tailed and Black-tailed Godwit.
Bills. In general, Bar-tailed is shorter, fractionally, with overlap between nominate race and islandica. But Bar-tailed also has slight upcurve, Black-tailed is straighter. Why? Well, we'll tie the legs in as well at this point, Bar-tailed also have shorter legs. So they sit closer to the ground, feed with a shorter bill with a slight sweep. They are designed to feed, ever so slightly, differently. Black-tails feed in deeper water. The creeks of an estuary are perfect. They are happy to stand up to their bellies and probe along the water edge on the more sharply-shelving edges. Bar-tails are happiest on the more open, shallower-shelving flats of open bays and shores. 'BWP' notes studies showing they seek out active lugworm casts. On the flats around the Swale, Bristleworms are taken in high numbers; these are not seen in the Medway in any number. Other studies confirm lugworm, even where not main prey, is main energy content.
Some interesting work has been done on their feeding ecology. Flock-feeding is successful- spread along shore, more shorter-billed males to the shore, more females in the water. Lone birds are poor feeders in comparison.
All this helps explain why on the Medway most Bar-taileds are seen on the eastern basin, especially between Stangate and Queenborough Spits plus, as the winter progresses, Bedlams (though this does not necessarily an influx, merely a change in area usage.
The central flats often turn up a handful of birds, as does the western basin. Hardly ever in large enough a number to suggest a regular flock. Part of the reasoning behind this will be the mudflat profiles, part may well be down to preferred prey species. Whilst needs must, and most waders will be catholic in tastes, when preferred food is available, they will take it. Lugworm is erratic and in small numbers up to Gillingham. Next most taken in studies, a type of bristleworm, is one that, for whatever reason, favours only the deep water channel- so, found in profusion near Stangate/ Queenborough. (The screengrab is from the excellent National Biodiversity Network website.)
Any double-figure count in the eastern basin is noteworthy. Holloway, summarising records for Gillingham to 1984: "a few birds spend the winter on the mud-flats each year and small parties of birds in full summer plumage are occasionally seen in late spring.." Randomly grabbing one of the Country Park reports, 1993: "two records only. Motney Hill on 17th September, two birds and a single bird at Riverside 18th November.." Small parties or lone birds. Based on feeding rates, they're probably not in optimum habitats up-estuary.
Overall, in all probability the status of the Bar-tailed Godwit on the Medway estuary has changed little since the 1990s, in line with English trends (as per the WeBS screengrab).
In recent winters three-figure counts in the western basin have not been hard to come by, though usually just below nationally important numbers (380 plus).As most data comes from the WeBS winter months, the oversummering birds are not always recorded (usually found between Ham Ooze and Queenborough) and the short-staying passage flocks get overlooked; these are usually small, not making three figures, but great to see.
One caveat- as with most species on the estuary; distance will play tricks on you. In flight, in some lights, the Black-tails will not give their white flashes up easily. If you suspect Bar-wits in number away from the regular areas, work 'em hard.