For 20 years now we have been recording the first arrival of migrant birds to our farm in Wales. The table below lists the dates for some of the species, those we have recorded every year.
I have also included the date of the first oak leaf bursting for each year I have a record, and a calculated mean for the 20 years of data.
The oak leaf burst data doesn't reflect how long it took for all the trees to come into leaf, some years they all come almost together, in another year it may have been several weeks between first and last. In 2017 the trees came into leaf very early, but were scorched by snow and hard frosts in the last days of April. Many still were without a full canopy in the middle of June that year.
The 11th April date for the first Cuckoo is a strange outlier, a male flew over and called twice only. Another wasn't heard until the 26th, a much less remarkable date, but it only has a marginal effect on the mean. The 11th April is I think the earliest date I have heard a cuckoo anywhere in the UK.