Ok, so maybe CO2 is not the primary driver of swan migration

Swan's return from Siberia [6.5 hours into November 1] is latest since 1981 - Nature, Environment - The Independent

Julia Newth, wildlife research officer at WWT, said: "A mild autumn and weeks of westerly winds have not provided the best conditions for migration. So we were not surprised the plummeting temperatures and easterly winds of the past few days have encouraged the swans to complete their 1,800-mile migration from arctic Russia. It is not unheard of for the swans to arrive this late. Over the past 20 years, Bewick's have arrived at Slimbridge between 13 and 29 October, but the last time the swans arrived in November was in 1981."

Should we panic and revamp the economy every time some swans are a week "late"?

The swans were due at Slimbridge on October 21, although they have been late before. In 1969 they did not return until November 6, and in 1981 and 1982 until November 3.
...
The British Trust for Ornithology attempted to calm twitchers’ panic and blamed the late arrival on the availability of good feeding in the Netherlands.