Tree swallows are small, migratory songbirds found throughout much of North America. They are easily identified by their iridescent blue-green upperparts, clean white underparts, and slightly forked tail. Tree swallows are closely related to the more familiar barn swallow and belong to the family Hirundinidae.
Tree swallows nest in cavities, readily using nest boxes provided for them along lakes and in open fields. Their nests consist of grasses and feathers lined with fine, soft materials. The female typically lays 4-7 white eggs. She alone incubates the eggs, which hatch in about two weeks.
A common question asked about tree swallows is whether the female sits directly on the eggs to incubate them. The answer is yes, tree swallow females do sit directly on the eggs in the nest. Incubation doesn’t begin until the clutch is complete, at which point the female starts sitting for long periods throughout the day and night.
The Importance of Incubation
Incubation serves a critical function for bird eggs – it provides them with the consistent warmth they need for normal embryonic development. Without proper incubation, the embryos inside the eggs are unlikely to survive. The body heat provided by the adult bird sitting on the nest is usually key.
For most bird species, only the female incubates the eggs. The male may occasionally sit on the eggs to give the female a break, but the bulk of incubation duties fall to the female. This is the case with tree swallows as well. The female alone incubates the clutch, while the male is responsible for defending the territory and bringing food back to his mate.
Sitting on the eggs allows the adult bird to transfer her warmth directly to the developing chicks. The temperature beneath the brooding parent needs to be kept within a narrow range – too hot or cold and the embryos may not develop properly or at all.
Incubation Behavior in Tree Swallows
Tree swallow females begin incubating their clutch as soon as the last egg is laid. They will sit tightly on the eggs, only leaving the nest for brief periods to obtain more nesting material, seek food, or interact with their mate. Even on short breaks, they often cover the eggs with nesting material to help retain warmth.
Incubation lasts for approximately 13-15 days before the eggs are ready to hatch. Throughout this period, the female tries to maximize the amount of time she spends sitting directly on the eggs. She may take shorter but more frequent breaks compared to earlier stages.
Studies monitoring incubation behavior in tree swallows have found:
- Females spend anywhere from 85-95% of daylight hours on the eggs once incubation is underway.
- Overnight incubation sessions tend to be long – females rarely leave the nest for more than a few minutes at a time after dark.
- Incubation attentiveness increases as hatching approaches.
- Nest attentiveness is highest in the early morning and late evening when ambient temperatures are coolest.
This consistent incubation by the brooding female is essential for keeping the eggs at the proper temperature and allowing the embryos to develop.
Incubation Temperature
So exactly what temperature must tree swallow eggs be kept at for successful incubation? Researchers have found that:
- The optimal incubation temperature for tree swallow eggs ranges from 35.5–40.9°C (or 96–106°F).
- Temperatures above 36°C are required for normal embryonic growth.
- Hatching success declines rapidly if average temperatures drop below 36°C.
- Brief fluctuations above or below the ideal range do not have significant effects.
The female swallow regulates these temperatures carefully while incubating. Specialized brood patches on her abdomen, devoid of feathers, allow her to efficiently transfer body heat to the eggs beneath her.
Monitoring devices placed in tree swallow nests confirm that females are remarkably adept at keeping temperatures within the narrow bounds needed. Their diligent incubation behavior, with prolonged sessions on the eggs, is what allows the embryos to develop.
What About the Male?
While female tree swallows perform all direct incubation of the eggs, their mates still play an important supporting role.
Male tree swallows keep busy with the following activities while the female is incubating:
- Bringing food to his mate so she doesn’t have to leave the nest as often.
- Standing guard near the nest to watch for and chase potential predators away.
- Bringing more nesting material to help insulate the eggs.
- Very rarely, briefly incubating the eggs to give the female a break.
Despite the female’s dominance in incubating, the male’s assistance with nest defense and food provisioning is still crucial. It improves the hatching and fledging success rates. Studies removing male swallows have found worse outcomes for the eggs as a result.
Problems That Can Disrupt Incubation
Even with diligent incubation by the parent swallows, there are some factors that can still interfere with the eggs’ development:
- Temperature Extremes – Particularly cold or hot periods can make it harder for the female to maintain incubation temperatures in the optimal range. A heat wave may overheat the eggs. Cold snaps can chill them faster than the female’s body heat can compensate for if she isn’t sitting tightly.
- Insufficient Food – If food becomes scarce, the female may be forced to spend more time away from the nest foraging. This can lead to the eggs cooling whenever left unattended.
- Predators – If predators like snakes, raccoons, or house cats get into the nest, they may prey on the eggs.
- Nest Parasitism – Brown-headed cowbirds may lay eggs in tree swallow nests. A female distracted by incubating cowbird eggs may pay less attention to her own.
- Storms – Severe storms can sometimes blow nests down or drench and chill the eggs.
- Toxic Contaminants – Environmental toxins like pesticides can be passed from the female to the eggs and disrupt embryonic development.
Fortunately, tree swallows have evolved effective incubation behaviors to contend with most normal challenges. But unusual threats from predators, weather, or toxins can overwhelm the brooding female’s efforts and impact hatching success.
Conclusion
In summary, female tree swallows do directly sit on their eggs during the incubation period of approximately two weeks before hatching. The female alone carries out this important task, which is essential for keeping the eggs at the proper temperature for normal growth and development of the chick embryos inside.
Her attentive incubation behavior, with sessions lasting from minutes to hours, provides the warmth and physiological conditions the eggs need to hatch successfully. While she may take occasional short breaks, the female tree swallow spends the vast majority of her time during incubation faithfully brooding her eggs. This dedication is what gives her hatchlings the best chance at survival.