The Barn Swallow (Hirundo rustica) is a highly recognizable and widely distributed migratory songbird found across the Northern Hemisphere during its breeding season. It is the most widespread swallow globally. Barn Swallows prefer nesting near human structures, frequently building their characteristic mud nests inside barns, sheds, and under bridges. Their annual return signals the arrival of spring in many regions.
Average Clutch Size and Variation
Barn Swallows typically lay a clutch of four to five eggs, though the range can vary from three up to seven. Clutch size is significantly determined by the availability of flying insects, which are the bird’s main food source.
Several factors influence the number of eggs laid. Older, more experienced females generally produce larger clutches than those breeding for the first time. Additionally, northern populations tend to have a higher average clutch size than southern counterparts, reflecting differences in resource abundance and breeding season length.
Clutch size also varies throughout the breeding season. The first clutch laid is usually the largest. Subsequent clutches, often called “second broods,” typically contain fewer eggs due to seasonal declines in food resources or the energy expenditure from raising the first brood.
The Annual Breeding Cycle
The reproductive period begins following migration, typically in late spring or early summer. Egg-laying generally occurs from April to July in North America and May to August in Europe. Timing depends on local climate and the return of a sufficient insect population to support the young.
Barn Swallows are generally double-brooded, meaning they will raise two separate clutches during a single breeding season, especially in favorable conditions. The pair often reuses the original mud nest for the second clutch, making repairs between breeding attempts. Occasionally, a pair may attempt to raise a third brood in exceptionally favorable climates, though this is rare.
The interval between the fledging of the first brood and the start of the second clutch is approximately 51 days. This double-brooded strategy allows the parents to maximize their reproductive output within the limited summer period.
Physical Characteristics of Barn Swallow Eggs
Barn Swallow eggs are small and oval-shaped. The base color is usually white or creamy white, sometimes appearing with a slight pinkish tint.
The shell features distinct markings, comprised of speckles and spots in shades of reddish-brown, brown, lavender, or gray. These markings are often denser toward the wider, blunter end of the egg. On average, the eggs measure between 1.2 and 1.5 centimeters in width and weigh approximately two grams.
Incubation and Hatching
Once the clutch is complete, the incubation period begins, lasting for approximately 14 to 16 days. While both the male and female share in the incubation duties, the female generally takes on the majority of the responsibility, sitting on the eggs more often and exclusively incubating overnight. In North American populations, the male may contribute up to 25 percent of the daytime incubation time.
The eggs hatch asynchronously, meaning they do not all hatch at the exact same moment. The young are altricial, meaning they are born nearly naked and helpless, with their eyes closed. After hatching, the nestling period is lengthy, lasting about 18 to 23 days before the young fledge. Both parents feed the nestlings, sometimes making up to 400 feeding trips to the nest per day to deliver compressed insect pellets.