What does 'delicate' mean?

The hazards of migration range from storms to starvation,
but they are outweighed by the advantages to be found in the
temporary superabundance of food in the summer home. The
process of evolution ensures that a species migrates only if it
pays it to do so. Birds of the same species may be migratory
in one area, but sedentary elsewhere. Most song thrushes
migrate from northern Scotland; but in the south of England,
the balance of advantage against disadvantage is so delicate
that while some migrate to Spain and Portugal, the majority
normally stay in England over winter. Moreover,
England’s winters have been getting warmer since the late
1980’s and if the trend continues it is likely that our song
thrushes will become increasingly sedentary.