Bird Breeding Season: Eggs | Zoo-Atlanta

Lawrence W.
May 18, 2022

Hello again everyone ! Lauren Wilson here to tell you more about the ins and outs of the Bird Department’s breeding season here at Zoo Atlanta. In my last blog we discussed all things nesting including types of nests, nesting materials and what the bird team does to ensure each species of bird has what it needs. needs to build proper nests. Now we’re going to discuss the whole point of building nests… laying eggs!

The eggs are absolutely amazing! Think about it…once an animal lays an egg, that egg has everything the developing embryo needs to survive from the time it is laid until the time it hatches. And, although many different types of animals lay eggs, birds are unique in that they lay hard-shelled eggs. They are stronger than soft-shelled eggs and can support the developing embryo in harsher conditions like arid environments or nest deployment. Birds also produce eggs of various shapes, sizes and especially colors depending on the species that lays them, the nests they build and the camouflage they need to protect them from predators. So, without getting too much into egg biology, how do we manage eggs here at Zoo Atlanta? The short answer is very conservative (ha ha), but I’ll give you the long answer because, just like nests, a lot of planning and effort goes into egg management.

The first step in egg management is deciding which birds should and should not breed in the bird department. I work with the Bird Management Team, which includes the Senior Bird Keeper and Senior Bird Curator, well before the start of the breeding season to decide which species and which pairs of each species should breed each season. These decisions are based on recommendations from the Species Survival Plan® (SSP), interest in species from other zoos and aquariums, available habitat space, and each bird’s history.

Candling of a Hooded Vulture Egg

The best and easiest situation is when the pair of birds are used to successfully building nests, laying eggs and raising chicks. The Bird Team simply lets the parents do their thing! We will monitor the nest via camera or, if the pair is not subject to disturbance, physically every three or four days to know when the eggs are laid. Knowing when the eggs are laid is important because, depending on the incubation period of the eggs of each species, we will then know when we can expect the chicks to hatch, which will change our husbandry practices. But that’s a topic for my next blog! We will also check the eggs at least once for fertility. We do this by candling the eggs. Candling is a technique where you shine a light into the egg to see the developing embryo. If an egg is fertile, we will first see the blood vessels followed by the embryo at different stages of development. If the egg is infertile, we say it “lights up like a light bulb” because we won’t see anything inside the egg but light passing through it. You can see what I mean at home by shining a flashlight through a chicken egg! If the eggs are infertile, we can remove them from the nest, so that the pair lays more eggs sooner and we have another chance to have a successful clutch of chicks during the season.

Sometimes birds are good at building nests and laying fertile eggs, but not so good at incubating those eggs. Sometimes it is extremely important for us to raise the species and we don’t want to risk, especially with new parents, that they will not successfully incubate their eggs. In these cases we can choose to incubate the eggs ourselves. When we have chosen to artificially incubate eggs, we remove the real eggs from the parents and replace them with fake or, as we call them, dummy eggs. Team Bird makes all of their own dummy eggs, usually by scooping out infertile eggs and filling them with plaster, so they’re more durable. The parents will continue to “incubate” these dummy eggs and be ready to raise the chicks rather than start the laying process over again.

artificial incubation machines sit on a table in the incubation room
Hatching room at Zoo Atlanta

Artificial incubation is advantageous because it allows us to follow developing embryos very closely. We use three incubators here at Zoo Atlanta. They are on a timer to turn the eggs and are kept at the same temperature but are set to three different humidity levels. Every two days we weigh and candle each artificially incubated egg. Why weigh them? All normally developing bird eggs, regardless of species, should lose 14-16% of their weight from the time they are laid until they hatch. This is where the different humidity levels come in. Eggs are porous and basically breathe through their shell. If an egg is expected to lose too much weight, we can place it in an incubator with higher humidity to slow its weight loss. If an egg is expected to lose too little weight, we can place it in an incubator with lower humidity to speed up its weight loss. And we candle them to make sure they’re fertile, growing normally, and to know when they’re about to hatch (again…the subject of my next blog!).

Over the years a handful of individual birds have shown that they will not breed successfully and allowing them to lay eggs can be detrimental. Let me explain. When a bird lays an egg, a certain amount of calcium must be deposited to make the shell hard enough for it to move smoothly through the bird’s reproductive tract and be successfully laid. If this process is faulty in any way and not enough calcium is deposited on the egg, the eggshell will remain soft and the egg itself could get stuck in the egg. inside the bird’s reproductive system. We call this being egg related, and sometimes surgery is needed to remove these eggs to prevent infection. For birds that have this problem chronically, we have used a hormonal implant that physically prevents the birds from producing eggs.

In other cases, individual birds can breed successfully, but we may not want that. Maybe they are housed with their siblings or maybe we don’t have the habitat space to house the offspring. We may want these individuals to breed in the future, so we let them experience nest building, egg laying, and egg incubation; we just don’t let the eggs hatch. As soon as the eggs are laid, we remove them from the parents to prevent them from developing and replace them with dummy eggs. At the end of their incubation period, we remove the dummy eggs and the parents act as if their eggs just haven’t hatched and prepare for another clutch of eggs.

As you can see, eggs are one of my favorite parts of working with birds! They are biologically amazing and by knowing this biology allows us to give every pair of breeding birds the opportunity for successful breeding. I hope you have gained an appreciation for the care and thought that goes into each egg laid by our birds here at Zoo Atlanta!

(Photos: Bird Team)

Lauren Wilson

Associate Curator of Birds