Migrant departures 2017

I have posted a couple of times in recent years about the months of first reporting migrants.  The obvious sequel to this is to do the same for departures.

However it isn't quite so easy: once a species has been spotted it is definitely here and future sightings (or lack thereof) don't matter.  However with departures it is only really possible to say the bird has gone some time after the last sighting.   For example a species is seen January, but not in February: have they left the area or were they just missed?  If they are subsequently observed in (eg) March it is the latter.

However I thought it worth a try.  I have used the COG classification of migrant species (some of which show a somewhat different pattern of movement here) and classified them according to month in which they are usually last seen here.  I have deleted some species (eg Masked and White-browed Woodswallows) as they are too unusual here to be commented on.

 As at the date of writing (4 April) 9 species have departed early while only 1 has been recorded after the expected date of departure.  My hypothesis is that this reflects the very dry Summer leading to a dearth of insects leading to many species going elsewhere to seek food.