" In terms of meteorology, Spring begins on 1st March. The meteorological Spring is based on statistical temperature cycles, and that, plus extending daylight, is also what governs nesting birds."
1st March is an entirely arbitrary date decided on by the Met Office to keep their records tidy, and to assist in the exchange of information!
Most older people have always been taught that the first day of Spring is 21 March , the Vernal Equinox (OK, so this year it's 20 March,, but you get the principle!). I can assure you that the Met Office decision does not govern nesting birds! I repeat, while temperature, and the consequent availability of food, can modify behaviour, it does not control it. That applies to most life, both plant and animal; you may see an Essex girl on the beach in March, but May is far more usual!
Some plants require a minimum of 8 hours daylight to live. The insects which live on them will not appear until the plant growth appears, and the birds which feed their young on those insects will not breed until those insects are available, it doesn't matter what the temperature is. While swallows normally follow insect hatching northwards, in the event of a cold wet and late spring they will still appear, albeit very late, and die or fail to breed, in their tens of thousands.
I cannot add much about wrens, they're not a species I know well, but many of the robins about at the moment are the Scandinavian sub-species, brighter than our own, though this is not obvious in the field. Robins are not migratory as such, but do move south for the winter. These Northern Robins are getting 8 hours of daylight at the moment, triggering their behaviour. If they were at home they would only be getting 6 hours, and they will settle down again once they go home.
Because of the similarity of day length and temperature, rooks are one of those species which will frequently show breeding behaviour in the autumn. It normally peters out after a few days, but Jourdain (1865 - 1940) actually recorded autumn breeding, though this is the only documented case I know of.