Air is sucked into the body by muscles acting on the bones of the thorax, into the rear air sacs.
When the bird breathes out, this air from the rear air sacs is squeezed through the lungs and on the next intake of breath it is sucked into the front end air sacs.
Next time the bird breathes out, this stale air from the front sacs is squeezed back out of the trachea.
Because of the one-way flow of air through the lungs, the blood supply in bird lungs is very much more efficient at extracting oxygen than that in mammal lungs (it is a counter-current exchanger, for the engineers among you). Oxygen-breathing mountaineers struggling step by step up Everest can watch birds effortlessly flying past at the same altitude.
The air sacs also penetrate many of the bones, making them hollow and much lighter than mammal bones ideal for flight.
Flying, walking and swimming
Birds are good movers. Different species of bird have different ‘styles’ of flight. Smaller, lighter birds like finches and sparrows tend to have short bursts of wing beats, interspersed with ‘ballistic’ glides with wings closed which reduces the air resistance against their light bodies. Larger birds like crows and pigeons can glide but tend to use powered flight when getting about over middle distances. Larger birds often use rising air thermals and glide on outstretched wings, circling to very high altitudes.
Birds have different modes of walking. Pied wagtails tend to walk left-right-left-right as we do. Others like dunnocks hop using both feet simultaneously. Blackbirds do both, walking about on the lawn and then hopping when they pounce on prey. Blue tits cling onto branches and clamber around upside down and sideways whilst seeking food among the buds and twigs. Smaller garden birds tend to be good at perching on small branches with their feet wrapped round for support. Their long claws make this easier.
Many birds are good at swimming above and even below the surface. Swimming is not something you see birds doing in most gardens, although many garden birds like to bathe when water is available. There is evidence that water can restore the shape of battered feathers which may account for this love of bathing.
Because they can fly many birds are able to move very long distances. This is obvious in our winter, when many birds leave and head south for equatorial regions, and birds of the Arctic move into our temperate areas where the cold is less severe. You might wonder why birds fly north to us to breed - it’s because temperate regions have a huge burst of insect and plant food available in spring and summer just right for feeding chicks. Bird ringing and modern tracking devices show some enormous feats of travel – one bar tailed godwit has been observed flying from Alaska to New Zealand non-stop.
In UK gardens some of our familiar birds leave for the south during winter while some are joined by or replaced by others of the same species from further north. Winter changes the feeding patterns of many of our birds. The tits tend to assemble in flocks and move through gardens en masse, so you may have ten of them there for a short period, and none for the rest of the day.
Garden bird ecology
Ecologically, birds fall into three main categories, although most garden birds are pretty much generalists in this respect. Often they will prey on insects during the spring and summer, and nuts, seeds and fruit in autumn and winter.
Carnivores: This brings to mind birds of prey, and the one that turns up most frequently in UK gardens is the sparrowhawk. This will catch and eat smaller birds sing a sneak attack strategy, hopping over hedges and walls without warning. Big ones tend to be female, and they can handle woodpigeons with a bit of luck. Smaller ones tend to be male, and naturally they eat smaller birds.
However, many of the smaller garden birds are also carnivores (or actually insectivores – i.e. they eat whole little creatures rather than bits of big ones). Wren and blackbird are good examples of these, although blackbirds also eat berries in winter. Blue tits slaughter many baby moths to feed their young.
Herbivores: The herbivores are well represented by greenfinch and goldfinch. They eat seeds and nuts primarily and other plant items (like spring buds) at other times of year, House sparrow adults are mainly seed eating, but they use insect food as protein-rich high-energy nutrition for their chicks, demonstrating another blurring of the ecological boundaries.
Scavengers: The various crows tend to be scavengers of dead meat, killed by other things (e.g. roadkill, cat kill, sparrowhawk leftovers) – hence the name “carrion crow”. If you live in some parts of the UK red kites might also be part of your garden birdlife – people put out meat scraps for these determined scavengers.
Residents and visitors
Page written by Roy Smith and compiled by Steve Head