Each year I try to explain the reasons why. And at least I'll now be able to refer people to this post to explain some of them.
April is early doors. We have returning passerines setting up shop, but not too many in transit for further north yet. There's only really Scandinavia higher up, and when they do make for there, makes a lot of sense for birds to come in along the western edge of the mainland continent- why risk a water crossing? This google earth screenshot oversimplifies things but allows an easy answer to the question- if you want a short journey with fewer risks of drift and overshooting, there's not many other routes you'd choose.
And spring migration is not autumn migration. In autumn, passerine migration can be summed up as adults getting out quick, youngsters bumbling about a bit first to try to find good spots to come back to. In the spring, successful returning adults know where they're going; a high level of site-fidelity. Young that have survived the winter have a fairly good idea too, having learnt a lot about their natal area during the weeks leading up to post-juvenile moult and the onset of migratory restlessness. They know where they want to be. The adults have a slight advantage over the young still, they've made the return before, the young haven't.
Read the migration texts and the point is often made that autumn warblers and the like funnel out through the south-east. Never that they really funnel back that way. Kent is, to a fair degree, in the shadow of south coast sites further west for numbers of UK returnees. Funneling up through Iberia and western France, if you come in over Kent, there's a pretty good chance you could find your nocturnal flight lumps you out over the North Sea. If you come in over Dorset, plenty of land to the north. Certain species are genetically pre-disposed not to come in over the Channel from the southeast, but to make landfall from the south- southwest. Welcome to (a small-scale version of) loop migration.
You also get a lot of chat about migrants being held up at this time. The spring passage window is shorter than autumn- the race is on to breed. Adverse weather patterns don't have as much stopping power, birds do strike out in 'sub-optimal' conditions and why numbers of adults on territories continue to rise slowly at present here, even though the weather this year is more like a mild late winter. Take the past week- an obvious local arrival of female Chiffchaffs. Migration carries on. This last week/ ten days in April never really disappoints, just some good years stick out more in the memory than others. The birds' internal mechanisms drive them, there are more on the move right now.
If someone asks me to pick a good day to witness migration, I usually don't look more than two-three days ahead, the forecasts aren't good enough. What I'm looking for isn't just overnight rain, but the edge of the rain band laying over the estuary at around dawn. Best wind is a south-westerly, moving birds along from the main flyway. We've had one good morning like that so far, and it did not disappoint. A bit of drift from the birds arriving to the west of North Kent, then a reluctance to try to push on through the rain. I chose to ring on a local farm and had a fall of Willow Warblers, Chiffchaffs, Goldcrests (mainly later moving females, if ringed found to be carrying plenty of fat), the odd Ring Ouzel and Wheatear around the site, Yellow Wagtails calling overhead throughout the morning. On an easterly, I'd always expect a lot less here. In fact, an clear south-westerly overnight will still produce a fair number of common migrants. (Don't just take my word for it- there's a lovely barometric chart in the Helm avifauna 'The Birds of Norfolk' showing just such a scenario as good for migration there as well.)
By the time I've answered their question in this way, the penny starts to drop and the questioner might then start asking when best for scarces and rares. Not my cup of tea, but I have a go.
Timing starts to have a play. We get hung up on early dates because of our predisposition towards recording 'firsts for the year' and tend to lose sight of the bigger waves coming later in the month. And at the same time we have the start of larger numbers of passerines heading for northern Europe. About now, the last ten days of April, is from when Kent might start to get excited about a good 'rare' popping up.
A caveat is to remember these internal mechanisms are pre-programmed to wind down and switch off close to base. Every year someone asks why the Bluethroats aren't crossing the Channel to breed, what with global warming and all. Well, they're pre-disposed to return to those natal areas. Their migratory drive is switching off by the time they get to Northern France. Crossing the Channel is so unappealing now. It will be the same with a lot more of the species breeding sur la Manche. It takes a lot to get them to stay. And even if a pair are on our side of the Channel, an argument gaining some strength is that the visual cues for a prime breeding site simply aren't there in the habitat we have- the floral sward is vastly impoverished compared to that present on mainland Europe.
When the Scandinavian Bluethroats are on the move their migratory systems aren't switching off this far south- chances are if they land they will leave as soon as possible. So May always has more of a chance of bumping into something, but more than likely a short-stayer.
This is still based around migrant passerines by the way- non-passerines, with their tendencies not to return to natal areas until old enough to breed, may well find a new site on their wanderings which is to their liking; shorter-lived passerines can't risk that sort of thing, they may well only get the one chance to reproduce in their short lives (why the classic 'overshoot' species are more often non-passerines).
So, come May I'll still be hoping for south-westerlies here. Plenty of 'Brit' migrants pouring through, plenty of good days full of 'common' movement. For an easterly influence to knock rares and scarces down around the estuary up here in north-west Kent, there would have to be bad weather and a lot of birds would be suffering, I never want for that. I'm much more content to try looking for a scarce or two around their appointed dates on their 'migratory calendars'. If that's what you like, check out when certain species are most likely to be pulsing up along the French coast and hope for one with an internal compass on the blink.
Me, I'll be watching the gulls.
(For detailed general info you cannot go much wrong with Ian Newton's New Naturalist, 'Bird Migration', or Peter Berthold's 'Bird Migration; a general survey', though you might need a bank loan to get them...)