Tree Swallow: Nest and Eggs

A lack of nesting sites limited the number of tree swallows (Tachycineta bicolor). When people noticed that they breed successfully in nesting boxes, tree swallows’ small populations in parts of its range tripled in numbers. Tree swallows breeding in nesting boxes are known to raise larger broods than those nesting in abandoned woodpecker cavities and other natural cavities. This article is intended to aid in identifying tree swallow nests and eggs.

Tree Swallow breeding facts

Breeding Period	As early as mid-April in the south, mid-May in the north. Typically May through September in the South and mid-May through mid-July in the north.
Nest type	Round nest with a central cup that measures 2.5 to 3 inches across and 1.5 to 2 inches deep.
Substrate & Location	Nests inside a cavity or enclosure, either natural or nesting boxes.
Nesting Activities	Male chooses the nesting cavity and adds some nesting material. The female, alone, builds the nest. Mated pairs may delay the initiation of nesting activities.
Egg Description	Oval to long oval. Plain white with no markings.
Egg Length and width	0.8 in x 0.6 in.
Egg-laying	Variable. Some pairs begin to lay eggs days after arrival, while others may delay up to 3 to 4 weeks to initiate breeding activities.
Clutch size	Typically 4 to 6 eggs.
Number of broods	Single brood per year. Birds in the southern portion of the breeding range may try second broods.
Incubation Period	Typically 13 to 16 days. Shorter incubation periods in the south part of the breeding range.

 

Breeding range and habitat

Tree swallows breed in most of North America except for roughly the south half of southern states from South Carolina through southern California. 

The range shown in the image shows the regions where the tree swallow is expected to breed.

Tree swallows feed on flying insects. Therefore, they need open and semi-open spaces to maneuver as they chase after them. Flocks of all sizes zig-zag over various habitat types but are closely associated with bodies of water where flying insects are plentiful.  

Migration and social behavior 

Tree swallows are short to medium-distance migrants. They return to the breeding grounds starting in early March through mid-April. After breeding, swallows return to the wintering grounds in the Southern U.S., Mexico, the Caribbean Basin, and Central America by late July through early September. 

Unlike other migratory birds, tree swallows migrate “slowly” to the breeding grounds. Some individuals arrive early and begin to breed, while others continue returning and initiating a breeding attempt half or late during the breeding season.

Tree swallows migrate in loose flocks. While their diet consists mainly of insects during the breeding season, swallows can supplement their diet with berries and other plant-based food.

Breeding Period

The tree swallows’ breeding period varies among populations in different regions. Populations in the southern portion of the breeding grounds start laying eggs as early as mid-April, whereas populations nesting in the northern reaches of their range starts breeding by mid-May. 

The breeding period is broad in the south and narrow in the north. In the south, tree swallows typically breed between May and September. In the north (the Northern U.S. States and Canada) between the months of mid-May through mid-July.

Tree swallows can have an irregular breeding period in that not all pairs begin to breed upon arrival in the breeding grounds. Some birds take weeks to form a pair, and even after being paired up, it may take them some time to start building a nest and initiate a breeding attempt.

Some birds form a pair early but take weeks to initiate breeding activities.

Tree swallow nest appearance

As with other cavity-nesting birds, tree swallows fill the bottom and side space with coarse nesting material. The nest cup is built with more delicate material and is typically lined with feathers. 

Plenty of bird feathers lining the cup is a key element to identify the nest of tree swallows.

The material under and around the nest cup used depends on the nest’s location. Tree swallows collect the available material generally from just around the nest. 

In grasslands, the prevailing nesting material is dry grasses, whereas in other places may be predominantly made of pine needles due to the presence of pine trees nearby.

The nest cup is consistent in size, measuring approximately 2.5 to 3 inches across and 1.5 to 2 inches deep. The total nest’s depth and length vary according to the space available in the cavity chosen to nest.

Nesting habits

Males arrive at the breeding grounds a few days earlier than females. They pick a nesting cavity and may add some nesting material to claim the nest site. 

Females arrive a few days later and inspect the available nesting sites and choose what they find suitable. The female then takes over the nest building while the male may or may not help.

The male finds and brings bird feathers to line the nest cup. The female also searches for feathers for the nest cup.

Feathers used are mostly contour feathers from waterfowl, gulls, and domestic ducks and chickens.

An interesting aspect about tree swallows is that while they can build a complete nest in 4 to 8 days, they may delay the initiation of nest building for over an extra week or more and then resume nest building.

Some breeding pairs defend a cavity, even when they may start building nests weeks after claiming ownership.

Tree swallows typically attempt only a single brood in the northern and more restricted breeding seasons. In the warmer conditions in the south region of their breeding range, they are more likely to attempt two broods in one season.

Nesting cavity

Tree swallows nest in existing cavities such as those excavated by woodpeckers, usually in dead trees outside densely wooded areas. They also nest in natural cavities formed by broken limbs that are stuck in the water.

Being surrounded by deep enough water is essential to prevent land predators from reaching the nest.

Tree swallows readily use artificial boxes. In parts of the species range, they have significantly increased after bird enthusiasts began to offer them nesting cavities.

Egg appearance

Eggs are oval to long oval. The shell is smooth, plain white, and without a gloss.

Tree swallow eggs are approximately 0.8 inches long and 0.6 inches wide. There are not many variations in color and measurement among eggs from different clutches and separate regions.

Although the tree swallow’s eggs are unmistakable, they are more likely to be found in the habitat and nest boxes where bluebirds also lay eggs.

Egg Laying

Nest building and, therefore, egg-laying can be sporadic at times. Some pairs start building a nest and stop for several days and even weeks. As such, the initiation of egg-laying can experience a similar type of unpredictability.  

Females start laying eggs after the nest is complete. In normal conditions, the female lays an egg every morning to the last egg of the clutch. 

If a cold or rainy spell comes through, females stop laying eggs even after having already laid a few. Studies have shown that females skipped up to 7 days after laying a few eggs before resuming laying the rest of the clutch. Studies also found that the eggs already in the nest did not lose viability after the long periods of being left unattended.

Clutch size

Female tree swallows typically lay clutches of 4 to 6 eggs. 

Incubation of the eggs

The female starts incubating the eggs before she completes the clutch. As with other aspects of tree swallow breeding, females may delay starting incubation due to bad weather.

The female alone does all or most of the incubation.

In some populations, males take only part of the incubation duties, particularly during days of cold air.

Incubation period

The female tree swallow incubates the eggs for 13 to 16 days, with populations in the southern and warmer part of the swallow’s range having shorter incubation periods.

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References:

• Cohen, R.R. and Dymerski, M.L. (1986). Swallows taking insects from pond surfaces. Wilson Bull.. 98(3): 483-484.
• Hundertmark, C. A. (1974a). Breeding range extensions of certain birds in New Mexico. Wilson Bulletin 86:298-300.
• Hussell, D.J.T. (1983). Tree Swallow pairs raise two broods per season. Wilson Bull.. 95(3): 470-471.
• Lombardo, M. P., R. M. Bosman, C. A. Faro, S. G. Houtteman and T. S. Kluisza. (1995). Effect of feathers as nest insulation on incubation behavior and reproductive performance of Tree Swallows (Tachycineta bicolor). Auk 112 (4):973-981.

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