House sparrows (Passer domesticus) are small brown birds that can be found in most parts of the world. They are very common birds that live alongside humans, often nesting on or near buildings. House sparrows breed frequently and can produce several broods per year. Their mating habits are well-studied and provide insights into their reproductive success as an invasive species.
Nesting
House sparrows build nests in protected areas including buildings, dense bushes, and tree cavities. The male selects the nest site and starts constructing the nest with grasses, feathers, paper, and other materials. The female assists in nest building once the base is established. Nests are often reused or repaired for subsequent broods.
House sparrows prefer nesting sites close to human habitation because the buildings provide shelter, warmth, and nesting sites safe from predators. Cities and farms with many buildings can support very large house sparrow populations. House sparrows aggressively defend their nesting areas from other birds that may compete for nest sites.
Pair Formation
House sparrows form monogamous pairs, though the bond only lasts for one breeding season. Birds begin establishing pairs in early spring, often re-pairing with the same mate from the previous year. Courtship rituals help form the pair bonds.
Male house sparrows compete to establish dominance in areas with suitable nesting habitat. Dominant males occupy the best territories and attract the most female interest. Younger males with lesser territories mate later in the season. Females ultimately select their mates based on the quality of the male’s territory rather than specific courtship displays.
Pair formation starts with the male approaching potential mates and initiating courtship feeding. The male pecks at the ground as if he has found food and calls the female over. If she is receptive, she will allow him to feed her. The pair also engages in ritualized displays of collecting nest material together. Once bonded, the pair defends the territory together.
Mating
Mating occurs frequently once the pair bond has formed. The male house sparrow usually initiates mating by singing a specialized song to the female. If interested, the female assumes a crouching posture with her wings slightly spread.
The male house sparrow mounts the female from behind and holds onto her shoulders with his beak. The actual copulation is very brief, lasting only a few seconds. The male dismounts immediately once finished.
Mating can occur multiple times per day to ensure fertility. Both males and females often mate with multiple partners throughout the breeding season, despite their season-long pair bond. Extra-pair copulations help produce stronger, genetically diverse offspring.
Egg Laying and Incubation
Once established, pairs remain close to the nest site. The female lays a clutch of 3-7 small white eggs. Clutches early in the breeding season tend to contain more eggs. The eggs are laid at a rate of one per day.
Incubation starts after the laying of the penultimate egg. The female incubates the eggs while the male provisions her with food. Incubation lasts 10-14 days. The chicks hatch altricial and mostly naked of feathers.
House sparrows commonly reuse the same nest to lay additional clutches throughout the breeding season. The female lays eggs in the new nest while still feeding the chicks from the previous brood. Multiple clutches allow house sparrows to produce many offspring during the spring and summer months.
Parental Care
Both parents share duties feeding the chicks. For the first week, the female broods the chicks while the male brings food to the nest. The parents regurgitate insects for the newly hatched chicks.
As the chicks grow, the female spends more time away from the nest foraging. The male continues provisioning the chicks. By two weeks of age, the chicks are covered in juvenile plumage and ready to fledge from the nest.
Fledglings are still dependent on the parents for an additional two weeks. The family group remains together until the next clutch hatches. Parents drive off juveniles as they invest energy in new eggs. The cycle then repeats itself.
Territoriality
House sparrows vigorously defend small territories surrounding their nest sites. Both males and females defend the area through vocalizations and aggressive encounters. Intruding sparrows are chased away by residents.
The size of the territory ranges from 0.01-0.4 hectares depending on food availability. House sparrows often forage for food across a larger area outside the territory. More food reduces the defended area required to sustain the breeding pair and offspring.
House sparrows help regulate their populations by adjusting the number of eggs per clutch based on food availability. Larger territories support fewer eggs and offspring. When food is limited, sparrows produce fewer young to reduce competition.
Adaptations for Reproduction
Several key adaptations give house sparrows exceptional reproductive success:
- Ability to breed multiple times per year
- Large clutches with up to 7 eggs
- Short 10-14 day incubation period
- Both parents invest in feeding offspring
- Reuse nests to reduce construction costs
- Females incubate next clutch while fledglings are still dependent
House sparrows maximize their productivity by reducing the time between consecutive clutches. Their adaptations allow each pair to produce 20 or more offspring annually. The high fecundity of house sparrows helps them colonize new areas quickly.
Mating Systems
Most house sparrow populations are predominately monogamous, with more than 90% of offspring produced by mated pairs. However, some promiscuous mating does occur.
Up to 15% of offspring are the result of extra-pair copulations. Males seek additional mates to produce more offspring. Females also engage in extra-pair mating, presumably to ensure greater genetic diversity.
The extra-pair matings are usually brief and often forced. Males intrude into neighboring territories specifically to seek these trysts. Females play an active role selecting extra-pair mates that offer superior genes or nest sites to her existing male.
Polygyny, with one male mated to multiple females, is rare in house sparrows. It may occur if females greatly outnumber males or a male can acquire an excess of resources to attract and provide for multiple mates.
Sperm Competition
The moderate level of promiscuity in house sparrows leads to sperm competition. Males have evolved larger testes relative to their body size to produce more sperm. This increases their chances of outcompeting the sperm from other males that mate with the same female.
Males also mate repeatedly with the female to ensure their sperm has precedence. The brief period of female fertility means males must inseminate frequently for a chance to sire offspring.
In response, females have evolved reproductive tracts that favor the sperm of some males over others. This allows the female to exert subtle choice in fertilization. However, the primary male still sires the majority of offspring in most cases.
Conclusion
In summary, house sparrows are prolific breeders well adapted to human landscapes. They form temporary monogamous bonds each season, though both males and females seek additional mates. Their high reproductive rates, multiple broods, and ability to nest near humans allow house sparrows to thrive worldwide. Understanding the mating systems and adaptations of house sparrows provides insights into how some bird species successfully colonize new regions.