Swallows are long-distance migratory birds whose annual cycle culminates in a distinct breeding season across North America and Europe. These aerial insectivores return each spring to begin nest construction and raising their young, a timeline governed by seasonal cues and the availability of food. Understanding the seasonal timing of their breeding cycle offers insight into the lives of these common neighbors.
The Annual Timeline of Nesting
The nesting timeline is dictated by the swallows’ spring migration, which depends on latitude and climate. In North America, Barn Swallows may reach southern states as early as February or March, while peak arrival in northern states and Canadian provinces typically occurs in May. In Europe, swallows generally arrive from their wintering grounds starting in March, with the majority appearing in April.
Once they arrive, breeding activity is sensitive to local environmental conditions, primarily temperature and the abundance of flying insects. The main window for nest construction and egg-laying spans from late April or May through July across most of the temperate breeding range. Warmer climates can shift this schedule earlier. The availability of food resources determines when the birds begin feeding a brood of nestlings.
Distinct Nesting Habits of Common Swallows
The structure and placement of a swallow’s nest vary significantly among species, reflecting diversity in habitat preferences. Common swallows generally fall into three distinct construction types, tied to available materials and the need for predator protection.
Cavity Nesters
Many swallow species are obligate cavity nesters, relying on pre-existing voids. Tree Swallows, for instance, utilize natural tree hollows, abandoned woodpecker cavities, or man-made nest boxes. The female alone builds a soft cup nest within the cavity, often lining it with feathers she collects to insulate the eggs and young.
Mud-Cup Builders
Other common species are mud-cup builders, most notably the Barn Swallow. Both partners construct a sturdy, half-cup nest using pellets of mud mixed with grass and reinforced with a soft lining of feathers. This structure is attached to vertical surfaces inside or beneath human-made structures, such as barn rafters or bridge undersides.
Colonial Mud-Gourd Builders
A third group, including the Cliff Swallow, are colonial mud-gourd builders. They create densely packed colonies on vertical cliff faces or artificial structures like highway overpasses. These nests are more enclosed than the simple cup of a Barn Swallow, often taking on a gourd shape with a small entrance tunnel.
The Cycle from Egg to Fledgling
Once the nest is complete, the female begins laying eggs, typically depositing one egg per day until the clutch is full. The average clutch size for common species like the Barn Swallow ranges from three to seven eggs, with four to five being most common. Clutch size often decreases as the breeding season progresses.
The incubation period lasts approximately 12 to 17 days, with the female performing most of the sitting. The nestling period—the time the young birds spend in the nest—lasts between 15 and 27 days, depending on the species and environmental factors. For species like the Barn Swallow, it is common for a pair to raise a second brood during the same summer season if the first attempt was successful and completed early.
Protecting Active Nests
Swallows and their nests are offered legal protection across North America under the Migratory Bird Treaty Act. This legislation prohibits the intentional killing, injuring, or destroying of the bird, its eggs, or its nest without a permit. Once a swallow pair has begun laying eggs or if young are present, the nest is considered active and must be left undisturbed.
If a nest is built in an inconvenient location, the time to act is before the birds arrive in spring or during the early stages of construction when the nest is incomplete and no adult is present. Deterring swallows can be accomplished by installing exclusion methods, such as netting or specialized deterrents, before the breeding season begins. Once eggs or young are present, the structure must remain in place until the young have fledged and the nest has become inactive.