The gregarious House Sparrow is an Opportunist

In spite of being the world's most widely distributed bird - occurring naturally in much of Asia, the Mediterranean and much of Europe, as well as having been introduced (partly by accident and partly intentional) to parts of Africa, the Americas and Australia - populations of the House Sparrow, known by the scientific name of Passer domesticus, are declining rapidly in many areas, including the UK, where numbers have decreased by as much as 60 per cent in cities and by almost half in rural areas. 

Though currently only listed as 'least concern' on the ICUN Red List, the RSPB has this lovely bird classed under the 'Red List Status'.

Description

The fairly compact House Sparrow, a member of the Passeridae family, is typically between 14 and 18 cm (5.5 to 7.1 in) long and has a relatively large, rounded head. Its stout bill has an upper ridge (culmen) length of between 1.1 and 1.5 cm (between 0.42 and 0.59 in); its wing chord measures 6.7 up to 8.9 cm (2 1/2 to 3 1/2 in), and its tail ranges in length between 5.2 and 6.5 cm (2 to just over 2 1/2 in). 

The weight of a House Sparrow can range between 24 and 39.5 g (0.85 to 1.39 oz), depending on the bird's age and sex, as well as the time of year. Females are typically a little smaller than males, but will be larger during the birds' breeding season, while the males tend to be bigger during the winter. 

The plumage of House Sparrows consists mostly of varying shades of brown and grey, and differs between males and females.  Females have brown heads and upper parts, with some dark streaks around the back and wings (mantle). They do not have the black markings - or the grey crown - of the males, but feature a very distinct, pale supercilium (a stripe running from the beak's base over the eye and towards the back of the head). Usually, the underparts of the female are of a pale grey-brown colour.

Male House Sparrows have reddish backs, bold markings and dark grey crowns reaching from their bill's top across to their backs, with the sides of their heads being chestnut brown. They also have black around the bill, on the lores (the spaces between the eyes and the bill) and on the throat. 

There is a fairly small white stripe located between the crown and the lores, and behind the eyes are small white spots (known as postoculars), and black patches above and below them. The male's ear coverts (covering feathers), cheeks and underparts are white or pale-grey, as are the stripes located at the head's base.

The mantle and upper back of the male are warm brown and feature broad, black streaks. The rump, lower back and the upper tail's coverts are typically grey-brown. Much duller - and featuring whitish tips on the ends of many fresh feathers - during the non-breeding season, the black and bright brown markings (including the black patch, badge or bib on the male's throat and chest) are gradually exposed more prominently through preening and general wear. The badge varies in size from one bird to the next, and it is believed that the increase in size is related to the age of a bird.

The plumage of juvenile House Sparrows is generally fairly similar to females, although it tends to be plainer above and of a deeper brown colour below. Although juvenile males often have white postoculars and darker throats, this is not a reliable method of determining the sex of a juvenile.

Sparrows are certainly vocal and can often be heard before they are seen. Most of their vocalisations are essentially variations of their short, but incessant chirping - often transcribed as either chirrup, philip or tschilp - call. This particular note is generally used:

• As a general contact call among resting/ flocking birds
• As a nest-ownership proclamation 
• As an pairing invitation during the breeding season

The latter is typically performed with great speed and emphasis - though not necessarily adhering to a specific rhythm - and repetitively by the male, forming a song-like 'ecstatic call', although it may also be described as a song.

Aggressive males - as well as females establishing dominance over their partner when replacing them to incubate eggs or feed offspring during the breeding season - often also give trilled versions of this call, generally transcribed as 'chur-chur-r-r-it-it-it-it'.

A sound transcribed as 'quer' is given as a nasal alarm call, and a bird in serious distress will produce what can only be described as a shrill 'quee'. Mated pairs also appease one another with an aggression-inhibiting, soft 'quee'.

Young birds, especially those with the misfortune of ending up in captivity, often also produce a 'true song' similar to the warbling of another species of bird, the European Greenfinch.

Habitat 

Well adapted to living among humans, House Sparrows can be found in both urban and sub-urban areas around the world - often even breeding indoors, such as in zoos, warehouses or factories, for instance. Sparrows have even been found breeding 640 m (2,100 ft) - in a coal mine in England - below ground. Others prefer lofty heights - the observation deck of the Empire State building is apparently a favourite haunt for feeding Sparrows during the night.

Although this bird tolerates a broad variety of climates, it does tend to prefer drier conditions, and can survive periods without water by eating berries. In essence, the only habitats in which it is not possible to find Sparrows are tundra and densely forested areas. 

Behaviour, Biology and Diet 

Gregarious throughout the year, especially when feeding, House Sparrows are extremely social birds and can often be seen flocking with other bird species. They roost communally; nests are typically grouped in clumps, and there is nothing a Sparrow enjoys quite as much as sharing a communal dust or water bath. Feeding mostly at ground level, these birds flock in bushes and trees - often in large groups (during the non-breeding season) - where they can often be heard indulging in a little communal singing. 

The main nesting season for House Sparrows ranges from April through into August, although this may occasionally be extended either end, mostly depending on weather conditions. Nest sites vary, although cavities - like eaves/ other crevices found in buildings, holes in rotten trees/ branches or sandy banks - are generally preferred. In warmer regions, Sparrows may also nest in hedges or the branches of - preferably evergreen - trees. 

Wherever the site may be, pairs often use the same site year after year. Many pairs also remain faithful to each other for life, although it rarely takes more than a few days to replace a lost partner. At the nest - as well as at feeding stations - the female, though smaller than the male, is dominant.

Typically domed (although nests built/ taken over inside buildings, etc, may be left uncovered) nest holes are first of all filled with straw and/ or dry grass, then lined with feathers, string, hairs, scraps of paper or any other soft material the bird can find (or steal - sparrows have been known to pluck feathers for their nests straight out of living pigeons). 

The female then lays a clutch of four to five white, greenish-white or bluish-white, spotted (grey or brown) eggs, often over a period of two to three days. While nesting duties are shared by male and female Sparrows, the female is better equipped for incubation (she develops a special brooding patch of bare skin) and therefore tends to spend most of her time - especially during the night - incubating, while the male perches nearby.

After 11 to 14 days, the chicks begin to hatch. They are then brooded for between 6 and 8 days, although they only begin to control their own body temperature from the age of 10 or 11 days. Typically remaining in the nest for anything between 11 and 23 days, the hatchlings - which will open their eyes after four days and grow the first bits of down after around 8 days - are fed by both parents.

Occasionally, hatchlings fall out of the nest. Those that survive such a fall should - if at all possible - be returned to the nest, as hand-rearing them is extremely difficult and rarely successful, even for experienced rehabilitators (to whom they should be passed on). 

Fledglings still unable to fly should only be moved if likely to be at risk from vehicles, cats, etc - their parents will be nearby and will continue to feed them until ready to sustain themselves. As it is, most fledglings will be able to fly by the time they leave the nest, although they will not start feeding themselves - at least partly - until a day or two after leaving the nest. Their parents will continue to feed them for up to 14 days, after which they reach the ability to sustain themselves and gradually disperse further and further away from the nest.

Females will then lay another clutch, commencing the cycle again. The care of fledglings may, as a matter of fact, be left completely left to the male, while the female prepares the nest - and herself - for the next clutch' often laid within days of the previous brood leaving her care. Altogether, a pair may produce three to for clutches in a single year. 

While the majority of House Sparrows tends to move little further than a few kilometres, most regions have limited migration. In coastal areas, for instance, young birds may disperse over comparatively long distances, while Sparrows living in mountain regions tend to move down to lower altitudes for the winter.

Young birds - especially the 'newly independent' ones - tend to gather in huge flocks, seeking areas with an abundance of food. Feeding stations in local gardens and patches of wasteland are some of the preferred areas these 'teenagers' hang out in. Ripening fields of grain are also highly tempting for rural birds, and once the adults have finished nesting, they often join the crowd to feast on the ripening grain. As autumn arrives, the flocks tend to break up, and the Sparrows return to the sites of their nesting colonies.

Adaptable and opportunistic, House Sparrows predominantly feed on seeds of weeds or grain (with a preference for wheat and oats), but will supplement their diet with whatever else is available. This includes anything provided - accidentally or deliberately - by humans (including bread, although they much prefer seeds); flower buds, berries and a variety of fruits like cherries and grapes, for instance. Sparrows in temperate areas are also known to tear up flowers (in particular yellow ones) during the spring.

This does by no means indicate that Sparrows are vegetarian, though. Their diet also includes:

• Ants
• Aphids
• Beetles 
• Caterpillars 
• Crickets
• Crustaceans and mollusks (where available)
• Dipteran flies
• Earthworms 
• Grasshoppers
• Sawflies
• Spiders and more

Even frogs and lizards have been known to compliment the Sparrow's diet on occasion. Basically taking advantage of whatever food happens to be in abundance, Sparrows have also been know to steal prey from robins and other birds.

Until around two weeks after hatching, young Sparrows are typically fed predominantly on insects, but they may also be given the odd spider and small quantities of seeds and grit. The latter may sound a little odd, but grit - which may consist of tiny stones, bits of masonry, egg or snail shells - is important for a Sparrows' ability to digest the typically hard seeds it feeds on.

Considering all this, it seems remarkable that this gorgeous little creature's numbers should be declining. Unfortunately, young Sparrows suffer a comparatively high rate of mortality - on average, only between 20 and 25 per cent of hatchlings will actually survive long enough to experience their first breeding season. 

In addition, House Sparrows are preyed on by cats, corvids, squirrels and birds of prey (merlins, accipiters and others), as well as - sadly - by humans. In particular around the Mediterranean, House Sparrows are still very much part of the menu for human consumption. To add to the tragedy, these little birds are - especially on European roads - also common road kill victims.

Pesticides also take their toll, as do diseases like Salmonella and Escherichia coli, for example. There are also several parasites that, while usually harmless to adult Sparrows, can seriously affect youngsters. In spite of laws governing the removal of Sparrows' nests without government licences and making the killing of Sparrows illegal, the fact that farmers often see them as pests often also still results in persecution in many areas.