The right-hand column (green) carries a scoring system best suited to resident birds which rarely have a need to carry out long journeys. This only scores the trachial pit (the throat), as these birds never really routinely deposit fat for flight and have no urgent need for balance at low scores. In fact, it has proved more useful to score the lower amounts at a different scale, as deposits are much more subtle. The green lines point to the illustration (where available) that best matches the fat deposit.
Ask a ringer if they've seen fat scores of eight- they'll name nocturnal migrant species, making long jumps. Ask them if they've ever seen a Blue Tit with that same high fat score. They won't have. Blue Tits, where they do migrate, do so diurnally.
There are wholescale physiological differences in nocturnal and diurnal migrants, for example, many of the former have adaptations to their blood that allow high altitude flights. Intriguingly, residents tend to have any stores of fat at a cost of slightly lower pectoral muscle scores than migrants- they really won't be looking to leave.
Diurnal species often have large percentages of resident birds within parts (usually the southern parts) of their range. There is a spectrum of movements, and the fun of deciding whether that coastal Tit is a continental or a local is never, ever as easy as we try to make it. Of course, we really want them to be continentals. But even on a day when a bird ringed on the continent turns up, congeners on the coast can just as likely be dispersing residents.
From late June, migration to wintering areas starts. Mainly by nocturnal means. It goes over the heads of most birders. Why else have the greatest gathering of birders in the UK (the Birdfair) in mid-August? Not really missing much of the migration spectacle taking place at the same time, because we don't see it.
Come early October, viz mig sets in. The diurnals are coming. Shorter distance migrants, less faithful to one particular wintering area. They move about more during the winter period, in response to weather changes. Some only move in the first instance if there's problems with food supplies.
Us birders love viz mig, but we are often only looking at a part of migration.
So, why is nocturnal migration seen as a better option than diurnal?
Diurnals don't have that star map.
Diurnals have to share feeding and migration during the same (daylight) periods.
Diurnals suffer a greater range of extremes (e.g. higher daytime temperatures) that really have knock-ons for optimal condition.
Diurnals use more energy for similar distances (those extremes take their toll).
Diurnals take longer to cover similar distances.
Diurnals have to put up with way more predators.
Just for a moment, think of those diurnal species as pioneering 'residents' now doing their utmost to take advantage of breeding habitats at the extremes, now retreating. Those nocturnals, they're premier league migrants.
This is, of course, an over-simplification, just a way of starting to make people think about what they are watching fly past. Because a lot of the joy of viz mig comes from wondering just what exactly we are seeing.