With so many birds worldwide, it would be easy to be overrun with them if their populations exploded. What keeps bird populations constant, and how do they deal with this?
Most bird populations are roughly constant. Birds may produce many eggs each season, but only a few of the chicks that hatch become adults, just enough to replace the adults that die in the same period and maintain the population. Larger birds typically have fewer young but can dedicate more time and resources to them.
The many species of birds we see in the countryside have all been in existence for thousands, in some cases possibly millions, of years, over numerous generations. While individual birds are mortal, it is also certain that in the majority of bird species, the actual populations vary little from year to year.
This ensures the continued existence of these species and guarantees that these natural populations are maintained.
How long Do Birds Live?
The way that ornithologists recognize how long birds live is partly done by bird ringing, which has been around for over a hundred years. Bird-ringing has given us a good idea of the record ages reached by the longest-lived individuals.
The world age record for a wild bird is held by one of a group of albatrosses that were ringed in the sub-Antarctic in the 1930s, and was still alive in the 1980s.
For European birds, an oystercatcher at 36 years holds the record, and individual reports of Arctic terns, guillemots, black-headed gulls, herring gulls, and ospreys have shown birds living for more than 30 years.
For familiar land birds, only swifts, blackbirds, and starlings are known to have exceeded 20 years, and of the smaller species, only a few, such as reed warblers and robins, have even reached ten years, although hundreds of thousands of them have been ringed.
The tiniest species, such as goldcrests, treecreepers, and wrens, all have records of less than seven years.
Calculations based on ringing results show that longevity increases proportionately with size. A larger bird species with a life span twice as long as a smaller one has a body weight of around 32 times that of the smaller one. In terms of species, this is roughly the difference between a tawny owl and a blue tit.
However long individual birds live, it is also generally true that the total number of one species living at any one time is constant. For this to happen, births and deaths must balance out exactly.
A pair of robins, for example, present at the beginning of one breeding season raises five young in each of two broods; from a statistical point of view, this means the death of ten of those birds before the next breeding season.
If only nine died in the average family, that would leave three birds for every two that had been present a year before, and there would be so many robins that they wouldn’t be able to find a nest.
In reality, young robins suffer about 90% mortality, while robins over one year old have a 50% survival rate. For smaller birds, especially those vulnerable to cold, the chance of death is much greater.
A mating pair of goldcrests may rear two broods of ten young, while a pair of kingfishers may have three broods of six young. For these populations to remain stable, each species must suffer about 90% overall mortality.
Although their numbers are relatively stable over the medium term, this can change drastically over the short term. In a severe winter, they may suffer enormous losses, but their large numbers of offspring give them a good chance to increase rapidly following a drastic fall in numbers.
Blue tits can lay up to ten eggs, and the large clutch hatches at the perfect time when there are ample caterpillars to feed on.
For larger species, longevity makes a slower and more relaxed breeding rate possible. A pair of guillemots lay only one egg each year.
Many species in this size range do not start to breed until they are several years old, and their numbers may sustain fairly heavy mortality during the years before they are able to start breeding.
Guillemots can return to breed for 20 years but only lay one egg. If the pair is able to lay 20 eggs, only two need to survive to replace the pair.
Looking After The Young
Individual birds do not have a good start in life. The major problem is the early learning period, when the young birds must learn how to feed themselves. Some longer-lived species devote much of their time to the newly fledged youngster, in which they have invested so much effort in breeding.
Sandwich terns continue teaching their young how to find food throughout the long migration to winter quarters in Africa. However, most small bird species invest their effort in producing a large number of young but cannot look after them for more than a week or two after fledging before they are ready for the next brood.
Even closely related birds have different strategies regarding breeding ages, clutch sizes, and longevity. The barn owl produces about six or seven young, quite large for an owl, feeding them on the large supply of small mammals that appear periodically in open habitats. When the young fledge, they move away from the parental territory quickly and must fend for themselves straight away.
The tawny owl, a bird of the same size as the barn owl and feeding mainly on small mammals, has a smaller brood of one or two young. The parents allow their offspring to stay in the territory and care for them for up to ten weeks, ensuring they are more likely to survive to breed.
It seems to me that both of these strategies are correct. Both maintain the population, even if one does have higher mortality rates.
How Does Migration Help Populations Survive?
One might expect that the long migratory flights made by many species of small birds can cause many to lose their life. While there are risks, migration is successful because it avoids the cold winter.
The whinchat and stonechat are closely related, but stonechats are virtually all British residents, while whinchats winter in West Africa. Stonechats fledge more than twice as many young as whinchats, but both species have survived on these rates of productivity for thousands of years.
This can only be because a greater proportion of the young whinchats survive the winter due to their migration, while the stonechat faces all the hazards of a British winter.