Imagine you’re a bird. You work so hard to successfully raise young each breeding season. Now think about where your nest will be located, what stage of development your chicks will hatch at, and the potential threats around you. Does it make sense for all your eggs to hatch at the same time? Or are there advantages to them hatching in a staggered order, with a new chick emerging every other day?
PHOTO: Asynchronous hatching is seen in this Snowy Owl nest where a new chick generally hatches every other day and results in varied ages and sizes among nestlings.
In reality, the physiological and behavioral adaptations associated with nesting are not decisions that individual birds make each year. They have evolved over thousands of years to support the breeding process and are genetically predetermined. And when it comes to nesting, every advantage is needed. Nesting increases the chance of predation, is energetically taxing, and induces stress. And even in the best of circumstance, a bit of luck is still required.
While many variants to incubation and hatching exist, for the purpose of this simple overview, we’ll address two general categories: delayed incubation which results in synchronous hatching, and incubation that begins with the first egg and results in asynchronous hatching.
PHOTO: These Long-eared Owl chicks are two of five in a clutch we banded in 2018. As chicks who hatched asynchronously, the differences in age and size are obvious.
First, it’s important to note that all eggs are laid asynchronously. It is not physically possible for birds to lay more than one egg at a time as the process of egg creation generally takes at least 24 hours.
Owls are an altricial species: they are born immobile, blind, and unable to keep themselves warm. They are entirely dependent on the care of their parents. Other altricial species include: passerines, hawks, herons, and woodpeckers, among others. Most owls species hatch their eggs asynchronously which has several potential benefits that maximize the chances of survival.
First, many species rely on relatively unpredictable food sources: voles, lemmings, and other small mammals. Asynchronous hatching may give the first chicks to hatch a better chance of survival should food become scarce. It also lessens the energy (food) requirements of the nest at any given time because of the varying stages of development in the nest. Additionally, because chicks remain helpless for several weeks, there is an increased chance of a predator finding the nest and taking the entire clutch. By starting incubation with the first egg (leading to asynchronous hatching) they are essentially getting a chick out as fast as possible, which reduces the chance of that individual getting eaten should a predator raid the nest.
PHOTO: Snowy Owl chicks also hatch asynchronously with obvious variations in age and size.
Precocial species - such as ducks, geese, and pheasants - hatch at a more advanced stage than altricial species. These chicks are born with their eyes open and can walk, run, or swim, and feed themselves almost immediately. Potential benefit of synchronous hatching for precocial species is a reduction in overall incubation time and, once hatched, all of the chicks can move to safety together, right alongside their parents.
PHOTO: As a precocial species, ducklings hatch synchronously, or nearly so. You can see that they are all relatively the same size and age and will be ready to follow their parents to feeding areas and water shortly.
For eggs to hatch synchronously, incubation must be delayed. This means that the eggs laid first may lay dormant for many days before the female initiates incubation. During this time, hormones inside the eggs keep them viable while waiting for the warmth needed to start development.
While most owl species hatch their eggs asynchronously, or not at the same time, Northern Pygmy Owls appear to be an exception. Our direct observations have shown synchronous hatching, development, and fledging, which is different from almost all other owls except the Ferruginous Pygmy Owl, at least in North America. Northern Pygmy Owls seem to delay the onset of incubation until the entire clutch is laid and hence, hatch at the same time (or nearly so).
PHOTO: a Northern Pygmy Owl nest, part of our photographic documentation of incubation, development, and fledge. Note that all chicks are at the same stage of development.
This unique adaptation among owl species has a several potential causes. First, Pygmy Owls eat a lot of birds. This might give them a relatively stable food supply compared to other owl species that are dependent on unpredictable food sources, like voles or lemmings. They also nest in cavities with very small openings which makes them less susceptible to predators. Since this affords them an extra degree of protection, they may not have the same pressure to quickly fledge chicks. Delaying incubation may simply be a way to conserve energy - incubating eggs all at once takes less time and energy. Lastly, Pygmy Owl chicks leave the nest in a unique manner. Not only do they leave the nest cavity at nearly the same time, they often fly directly from the nest. Most owl species have an extended period where they have left the nest but cannot fly, often called branching. Because Pygmy Owls can fly right from the cavity, this also reduces the threat of a predator taking the entire clutch.
While this is a simple overview, keep these strategies to incubation and hatching in mind as you observe different birds. How and why did their chicks hatch in the manner they did? How do you think this particular strategy increases their ability to fledge young? It's a great way to expand your understanding and knowledge of the bird life around us! :)