Why do only some migrate? (1 Viewer)

Since starting this hobby 3 years ago I have increasingly become more interested in bird migration. I read recently that only some Fieldfares move south to the U.K. in winter. Questions.... Why do only some migrate? and is this common with most migratory birds. Hope this is not a daft question but I am quite a novice.

Robert

Any population of any animal in which all individuals behave exactly the same leaves itself open to some unpredictable disaster or unfavourable environmental change and thus go extinct. Most of the species that survive to the present day have done so in part by having a proportion of their population trying different (perhaps suboptimal) strategies, and thus that part of the population is preadapted to new circumstances when these happen along.

An alternative factor
Migrating has great dangers, as does attempting to overwinter. The cost benefit analysis will be different for each individual. It is possible that birds have evolved a "rule of thumb" eg "if you are a strong flier migrate, if not stay put", or if you are a large individual (ie more likely to survive the cold nights of a northern winter) stay close to home, if you are a small individual then the lesser risk will be migrating to warmer climes. One very simple rule some species use is "if plenty of food, stay put, if food runs out then move", this we see in erruptive species like Crossbill and waxwing.

Many thanks for sharing your views on this subject. It would appear that there are several factors to consider with bird migration.

Thanks

Robert