I mentioned to a Kenyan friend that we'd had good Swallow passage over the last week and asked if he'd seen his first yet. I always used to see my first in Zambia around the second week of September. His reply was very interesting:
"In Kenya the situation with Swallows is more complex than just arriving in August. The birds appear in some numbers (particularly around Lake Victoria) by the end of June, they are all adults, but the increase in numbers at the end of June is contrary to an almost absence in early June. Certainly a very few winter on the lake, but this increase supports immigration to the area. From beginning of August they are widespread all over central and western Kenya, mainly near waterbodies and rivers. Maybe there are early breeders in the southern part of the range, which having bred at the beginning of the year and move south quite quickly. Northern birds have multiple broods to maintain numbers, but birds in tropics/sub-tropics do not share this tendency. I also wonder what happens to first brood offspring of Swallows in UK after they leave the nest, and the parents stay on to raise another couple of broods. I don’t think they are coming back to their winter quarters, as the first birds in late June are adults. Still a lot of intrigue in the commonest species!"
Roll on satellite trackers small enough for passerines...