When should an animal embark on its migration?
The answer here lies in “why” it’s migrating in the first place. And it’s worth making a mental note on whether the animal has any choice. If it has choice, what are “determining factors”: Feeling good? Weather? Body Condition? Animal Leadership? Temperature?
Probably some or all.
But here are two examples of two very different animals.
The Monarch Butterfly migrates north from its forests in Mexico to the borders of Canada and beyond. It does this as an unrolling carpet of generations – the great great great grandchild reaches the north and then does a u-turn and flies south by itself to winter in Mexico.
Temperature dictates the migration north as does the length of the day. And exactly the same factors come into play for the decision to head south.
Note the tactic of breeding all the way north – no adult would live long enough to complete the migration north and have the energy to breed. This extraordinary life history is the complex status quo evolution has driven to allow this species of butterfly to maintain its population in Mexico.
Without a migration, the food plants in Mexico would support a fraction of the population – And in North America, the winters would be too severe.
But, if the weather is unseasonably warm early in the year, the butterflies may stir, but their food plants may not.
The Whooper Swan winters in the UK and breeds in Iceland. In any one year these birds migrate as families.
The southbound journey has a big weather element in it. The adults want to lead their new signets away from the impending Arctic winter and bring them to the balmy conditions of the UK. But they are big birds and they carry a lot of weight on their wings. They face a 800km non stop journey over the North Atlantic – and one of the most tempestuous sea crossings anywhere.
Their options are:
- Wait until it gets too cold where they are and go for broke
- Be restless in a wide range of time and wait for a good southerly air stream
- Fly high and find a good southerly jet stream and be blown down.
- Fly very low (1m above sea level) and benefit from the gusts from waves and the lift it offers.
- Wait for a clear day regardless of wind.
And what is the answer?
You’ve got it, the best of all. In a broad range of time in autumn they wait for a clear day and fly low to the sea and hope to make the crossing in one go. They have been known to wait for southerly air streams. If the visibility plummets, they put down on the sea.