Survival instinct
BRITAIN has relatively mild conditions given its geographical latitude, largely due to the Gulf Stream. However, birds can still have a hard time during February, which is on average one of the coldest months of the year, often with prolonged periods of very low temperatures. In order to survive, birds need protected roosting places and a good source of food. When watching your garden birds in cold weather, don’t be fooled by ‘fat’-looking birds – they’re probably fluffed up to provide extra feather and air insulation from the cold.
Artificial feeding of wild birds helps many to survive, and more than half of British homeowners put out food. A review of British bird feeding statistics over 40 years revealed that 133 species take advantage of this. It is estimated that the food provided could feed almost 200 million birds, while the population of garden birds is less than 75 million. Squirrels probably take the excess!
A bird can lose 10% of its body weight overnight during cold periods and replenishing such a loss can take a great deal of energy. Regular supplies mean that birds expend less energy in finding vital sustenance. In urban and suburban areas where there is extra feeding birds may find it easier to survive, while in the wider countryside there is more of a challenge to find food.
Estimates of survival can be calculated from ringing returns. Most mortality occurs just after fledging and during the bird’s first winter. Using European Robin as an example, about 40% of young birds survive into adulthood, and adult survival is about the same. A look at European Robin populations since 1995 shows that they are mainly increasing, but that by far the greatest increase has been in urban and suburban habitats where numbers grew by 43%.
A study of suburban Blackbirds gave an adult survival rate of 67%, which was thought to be high due to artificial feeding; it also noted that about 30% of Blackbirds live in similar ‘human habitats’.
In general, the larger the bird, the better the chance of survival. With Blackbirds, about 56% of adults survive, while in Eurasian Wrens only about 26% do. Seabirds are some of our best survivors and 90% of adult Blackheaded Gulls survive.
Do not confuse longevity with average age, though. A Robin will live about two years on average, but the oldest individual recorded in Britain is just over eight years old. With some species, like Northern Fulmar, the typical lifespan is 44 years and individuals may be 50 or more years old as the adult survival rate is more than 98%. Chris Harbard