In North America about half of all of the migrating birds on the continent common 860 miles on their southward journey. The opposite half averages 1960 miles. Form of illogical, on condition that migration is a harmful exercise, the birds that journey 1960 miles have a 23% loss of life fee over the winter whereas these touring the shorter 860 miles have a better loss of life fee at 35%. This can be a huge oversimplification as we’re lumping a lot of birds into solely two classes. Nevertheless it truly is smart. Though travelling farther has its dangers, spending the winter in colder climes farther north is extra hazardous.
A songbird can stay 1-3 days with out meals, relying on the fats shops of the hen and the ambient temperature. Small birds want to search out sufficient meals to get by way of the day and in addition construct up sufficient fats reserves for the approaching evening – all in the midst of the restricted sunlight hours. Lengthy-tailed Tits at 8-9 grams and Willow Tits at 10-12 grams, for instance, want to extend their physique weight by 10 p.c to outlive an 18-19 hour evening. The typical survival fee of Black-capped Chickadees over winter averages 43-95 p.c. The older and extra dominant birds survived the longest.
The period of time Widespread Nightingales spend singing through the evening is immediately associated to their physique mass and the ambient temperature within the night; the heavier the birds are, the extra they sing and the colder the evening the much less they sang. European Robins sing extra once they greet the daybreak with the next physique mass than a decrease one. After a chilly evening, which required them to make use of their fats shops, the birds have been much less apt to sing at dawn. Male Black-capped Chickadees, experimentally fed supplementary meals, sang extra at daybreak than different male chickadees.
However, along with the stress of consuming sufficient to outlive comes the potential of predation. If you’re a small hen needing to realize 10 p.c additional physique mass for a winter evening, you should spend time foraging. However the extra time you spend foraging, the extra time you might be uncovered to the chance of predation. Small birds in winter thus face a tradeoff as a result of they need to reduce fats reserves to scale back predation dangers and maximize them to keep away from hunger.
The physique mass of wintering birds displays a trade-off between predation danger and the chance of not discovering sufficient meals.
So birds which can be on common uncovered to colder temperatures, like people who don’t migrate as far south, may have extra days throughout which the temperature is chilly sufficient to require a number of time foraging to acquire sufficient meals to outlive the evening. These birds danger being eaten themselves. So not flying far sufficient south not solely exposes a hen to the stress of colder temperatures, however to elevated predation as properly. Winter is a tricky time for all birds, particularly small ones.