Tree Swallows typically take between 12-16 days for their eggs to hatch. The incubation period can vary based on factors like weather, availability of food, and the health of the parents. Let’s take a closer look at the Tree Swallow’s breeding and nesting behaviors to better understand their incubation timeline.
Overview of Tree Swallows
Tree Swallows (Tachycineta bicolor) are small songbirds that are found throughout North America. They are aerial insectivores, meaning they catch insects while flying. Tree Swallows nest in tree cavities or nest boxes and form colonies, with multiple pairs nesting in close proximity.
Tree Swallows arrive at their breeding grounds in spring, anywhere from late March to May depending on latitude. Shortly after arriving, males will begin defending nesting territories while females build nests. Nests are constructed of grasses, twigs, feathers and sometimes lined with feathers. Tree Swallows are socially monogamous each breeding season.
Egg Laying and Clutch Size
Once paired, the female Tree Swallow will lay a clutch of 4-7 white eggs. She lays one egg per day, usually in the early morning. Clutch size is variable and influenced by factors like the age of the female (older females lay larger clutches), food availability, and weather conditions. Harsher weather and less food sources often result in smaller clutch sizes.
Here are some typical clutch sizes for Tree Swallows:
- 4-5 eggs – young, first-time nesting females
- 5-6 eggs – average sized clutch
- 6-7 eggs – older, experienced females in prime habitat
The female alone will incubate the eggs, keeping them warm until they hatch. She develops a brood patch – a bare patch of skin on her belly that allows her to transfer warmth directly to the eggs. The male plays no role in incubating the eggs but will continue to guard and defend the nesting territory.
Incubation Period
Once the last egg is laid, full incubation begins. The female Tree Swallow will stay on the nest constantly during this period, only leaving briefly to search for food. On colder days she may stay on the nest for longer stretches to keep the eggs warm.
The incubation period for Tree Swallows is approximately:
- 12-14 days – Common early season timeframe
- 13-15 days – Average
- 14-16 days – Late clutches and colder weather
Warmer ambient temperatures tend to shorten the incubation timeline, while colder temperatures lengthen it. The health and attentiveness of the parents also play a role. More attentive females who are able to balance foraging with frequent egg-sitting will have shorter incubation periods.
Hatching
Tree Swallow eggs typically hatch over a 1-2 day period, with the hatching sequence following the laying sequence. The eggs that were laid first are the first to hatch. Newly hatched chicks are altricial, meaning naked, blind, and completely dependent on their parents for food and warmth.
Just before hatching, the egg tooth on the chick’s beak will break through the shell. It continues pecking and cracking the shell until it makes its way out. The female may sometimes help peck off shell fragments to assist the chicks. The chicks are unable to regulate their body temperature at this stage, so the female must brood them frequently to keep them warm and alive.
Within a day or two of hatching, the chicks’ eyes will open. At around 5-7 days old they will develop feather quills. Eventually juvenile plumage emerges, followed by a complete molt into adult plumage later in the summer after fledging from the nest.
Why the Incubation Period Varies
As we’ve seen, the Tree Swallow’s incubation period typically ranges from 12-16 days. But what causes this variability? Here are some of the major factors influencing incubation length:
- Ambient temperature – Warmer weather shortens incubation. Colder temperatures lengthen it.
- Weather events – Severe storms, cold snaps, or heat waves may temporarily halt incubation.
- Age and health of parents – Younger, less experienced parents tend to have longer incubation periods.
- Food availability – Lack of food causes parents to forage more, leaving eggs unattended.
- Time of season – Late clutches tend to have longer incubation as temperatures drop.
- Nest site – Nests in warmer, more sheltered sites have shorter incubation periods.
By being aware of these factors, we can better understand why incubation duration is not the exact same each time. The parents do their best to successfully hatch their clutch based on a variety of changing environmental conditions.
Incubation Behavior
To better understand the incubation process, let’s take a closer look at the female Tree Swallow’s behaviors during this time:
- She sits tightly on the eggs, only leaving for very brief foraging trips.
- She frequently shifts her position to evenly distribute heat.
- She may roll the eggs carefully with her beak to reposition embryos.
- She keeps the eggs covered as much as possible.
- She plumps up her feathers to trap insulating air against the eggs.
- She raises her body temperature to provide warmth to developing embryos.
The attentive female parent ensures optimal conditions for normal embryonic development. Only when conditions are severely compromised do the eggs fail to hatch after full incubation.
Caring for Hatchlings
Once hatched, the Tree Swallow chicks are totally dependent on their parents to provide food and warmth. Both parents cooperate in caring for the hatchlings:
- The female broods the chicks frequently as they cannot yet thermoregulate.
- Both parents make many small foraging trips per day to gather insects to feed the chicks.
- They return to the nest with beaks full of bugs to drop into the gaping mouths of the chicks.
- The chicks are brooded after feedings to keep them warm and satisfied.
- Parents keep the nest clean by removing chick fecal sacs.
- As chicks grow, parents increase the quantity of food provided.
- Chicks fledge from the nest at around 18-22 days old.
The efforts of both Tree Swallow parents are vital to protecting and providing for the chicks in their crucial first few weeks of life outside the egg.
Conclusion
In summary, Tree Swallows take between 12-16 days for their eggs hatch after full incubation gets underway. Warmer temperatures, plentiful food, and attentive parents promote hatching on the shorter end of that timeframe, while cold weather, lack of food, and disturbed nests delay hatching. Understanding the typical incubation period and influential factors allows us to better appreciate the breeding biology of these aerial insectivores.