Why female Tree Swallow eats eggshells?

Introduction

Tree Swallows (Tachycineta bicolor) are known for their striking iridescent plumage and agile flight. The hatching behavior of these birds is closely linked to their parental care and social structures. Female Tree Swallows typically lay 3 to 7 eggs in a nest made of grass, feathers, and other materials, usually located in cavities or artificial nesting boxes. The incubation period lasts about 14 to 16 days, during which the female primarily incubates the eggs, keeping them warm and protected from predators.

Once the eggs hatch, the female's role becomes even more critical. The hatchlings are altricial, meaning they are born naked, blind, and helpless, relying entirely on their parents for food and protection. The hatching usually occurs over a relatively short period, which helps synchronize the care required for the young.

Eggshell disposal is an important aspect of bird reproduction that can significantly impact the survival of both the eggs and the fledglings. After hatching, the presence of eggshells around the nest can attract predators, as the shells are indicative of recent nesting activity. Many birds exhibit behavior where the female or both parents will remove or bury the eggshells to help conceal the nest and reduce the risk of predation. Eggshells can carry the scent of the nest and its contents, which may attract scavengers and predators. By disposing of these shells, adult birds can help mask the location of their nest. The act of disposing of eggshells can serve as a form of nest maintenance, indicating the health and productivity of the nesting site. A well-maintained nest can also attract future mates or serve as a sign of good territory to other birds. 

1 Overview of the Tree Swallow Hatching Process

Certainly! Here’s a structured description of the preparation before hatching, the hatching of the first hatchling, and the subsequent actions of the female bird.

- Appearance: The eggs typically have a smooth, glossy surface, often varying in color depending on the species. They may be speckled or solid, providing camouflage against predators.

- Temperature Regulation: The female maintains optimal temperature through careful brooding, frequently adjusting her position to cover the eggs adequately. This careful management is vital for the embryos’ development.

- Monitoring: The female remains vigilant, occasionally leaving the nest for short periods to feed, but she quickly returns to resume her protective role. During this time, she may also engage in behaviors such as turning the eggs to ensure even warmth distribution.

As the hatching day approaches, the embryos inside the eggs are fully developed and ready to emerge. At this stage, the characteristics of the hatchlings are as follows:

- Physical Appearance: The hatchlings are typically featherless, displaying soft, pinkish skin. They are small and fragile, with their size relative to the egg they emerged from.

- Sensory Development: The chicks are born with closed eyes and are unable to see, relying entirely on their other senses, especially touch and smell, to navigate their immediate environment.

- Dependency: Newly hatched chicks are altricial, meaning they are dependent on their parents for warmth, food, and protection. They instinctively open their mouths widely to beg for food as they sense the presence of the mother. 

2 Observation and Care

- Monitoring: The female watches closely as the first chick emerges. She may chirp softly to encourage the hatchling, providing reassurance during this vulnerable time.

- Adjusting the Nest: After the hatchling breaks free, the female may adjust the nest materials to ensure the chick is adequately supported and warm. She will carefully reposition any loose nesting material to create a cozy environment.

Post-Hatching Behavior

- Brooding: The female instinctively broods over the newly hatched chick, providing warmth and protection. Her body acts as a shield against potential threats and helps regulate the chick's temperature.

- Feeding: As the hatchlings become more aware and begin to solicit food, the female will start bringing insects or other food items to the nest, transitioning into her role as a caregiver.

- Observation: The female carefully scans the nest, her sharp eyes catching sight of the eggshells that remain. She understands that these remnants could attract predators or indicate to others that the nest was recently active.

- Picking Up the Eggshell: With a swift and deliberate motion, the female bird uses her beak to grasp a piece of the eggshell. She delicately lifts it from the nest, showing her instinctive drive to maintain a clean and safe environment for her newly hatched chicks.

3 Reasons for Consuming the Eggshell

In addition to the act of disposal, the consumption of eggshells provides several important benefits for the female bird, particularly in terms of nutrition and health.

Recovery of Nutrients

- Calcium and Minerals: Eggshells are primarily composed of calcium carbonate, a vital mineral that plays a crucial role in various physiological processes. By consuming the shells, the female is able to recover this calcium, which is essential for her health and for the development of future eggs.

- Other Minerals: Besides calcium, eggshells contain trace amounts of other minerals such as phosphorus and magnesium. These nutrients are important for bone health and metabolic processes, contributing to her overall well-being.

Replenishing Nutrients Lost

- Nutrient Loss During Egg-Laying: The process of laying eggs depletes the female bird's reserves of calcium and other essential nutrients. Consuming the eggshells allows her to replenish these lost minerals, helping her to recover physically after the demanding process of reproduction.

- Preparation for Future Clutches: By ingesting the eggshells, the female prepares her body for future breeding cycles. Adequate nutrient levels are vital for the production of healthy eggs in subsequent nests, ensuring the continuation of her lineage.

4 Ongoing Care for the Hatchlings and Unhatched Eggs

- Feeding the Hatchlings: As the male brings food, the female feeds the hatchlings, regurgitating the food to provide them with easily digestible nutrition. This process is essential for the chicks' rapid growth and development.

- Monitoring the Nest: The female also keeps a close eye on any remaining unhatched eggs, ensuring they are kept warm and safe until they are ready to hatch. Her attentive nature helps maximize the chances of survival for all potential siblings.

Handling and Disposal of Leftover Eggshells

- Maintaining Cleanliness: The female continues to handle the leftover eggshells, a behavior that reinforces her role in maintaining a clean and safe environment. By removing the shells, she reduces the risk of attracting predators and minimizes potential health risks associated with decaying organic materials.

- Nesting Materials Adjustment: As she disposes of the eggshells, she may also rearrange the nesting materials, ensuring that the chicks are comfortable and secure. This attention to detail in nest maintenance is crucial for the overall health of the hatchlings. 

5 Feeding and Raising the Young

With all the hatchlings now present, the parents initiate a systematic approach to feeding and caring for their young, embarking on a crucial 18 to 22-day feeding cycle.

The Feeding Cycle

- Collaborative Efforts: The male and female work together to meet the dietary needs of their growing chicks. The male continues his role as the primary forager, bringing back a steady supply of food, which includes insects, seeds, and other nutritious offerings.

- Meal Distribution: Upon returning to the nest, the male presents the food to the female, who skillfully feeds each hatchling. The parents ensure that the last hatchling receives its fair share, often prioritizing it to bolster its strength and development.

Growth and Developmental Needs

- Nutritional Requirements: During this critical developmental stage, the hatchlings require a high-protein diet to support their rapid growth. The parents must be resourceful and efficient in their foraging to provide enough sustenance for all the chicks, particularly the last hatchling, which may need extra food to catch up.

- Social Learning: As the chicks grow, they begin to engage in social behaviors with one another, learning how to compete for food, communicate, and establish their place within the family dynamic. The presence of their siblings fosters a healthy environment for growth, as they learn from each other’s behaviors and cues.

- Monitoring Development: The parents keep a close watch over their young, adjusting their feeding strategies based on the individual needs of each chick. This attentive parenting helps ensure that all the hatchlings, including the last one, develop the strength and skills necessary for eventual independence. 

The handling of eggshells by female Tree Swallows is a remarkable behavior that underscores the significance of cleanliness and safety in their nesting practices. After the hatchlings emerge, the female diligently removes and disposes of the leftover eggshells from the nest. This seemingly simple act plays a crucial role in ensuring the health and well-being of her young. This behavior is not unique to Tree Swallows; many other bird species exhibit similar practices. For example, species such as the American Robin and the Eurasian Blue Tit also engage in the removal of eggshells or droppings from their nests. This phenomenon invites readers to explore the fascinating world of avian behavior further, revealing how different species adapt their parenting strategies to enhance the survival of their offspring.