No, birds do not have to mate before laying each egg. Female birds can store sperm internally after mating and use it to fertilize multiple eggs over a period of time. This means they do not need to mate right before laying every egg.
There are a few key things to understand about how birds reproduce:
Oviparity
Birds are oviparous animals, meaning they lay eggs that develop and hatch outside the mother’s body. The egg contains all the nutrients and protection an embryo needs to grow.
Seasonal egg laying
Most bird species mate and lay eggs during specific seasons, typically spring and summer in temperate regions. Egg laying is triggered by increasing day length, which stimulates the female’s reproductive hormones.
Sperm storage
Female birds have sperm storage tubules where they can keep viable sperm alive for weeks or months after mating. The sperm is slowly released to fertilize eggs as they pass through the oviduct prior to being laid.
Clutch
A group of eggs laid sequentially is called a clutch. The clutch may contain as few as one or as many as 20 eggs for some species. Clutch sizes vary widely between different types of birds.
Mating and Fertilization
Here is a more detailed look at the bird reproductive process:
Courtship
In most species, birds engage in courtship displays like singing or dancing to attract a mate and demonstrate fitness. Usually the male performs elaborate shows until a female chooses him as a suitable partner.
Cloaca kissing
Birds mate by pressing their cloaca (all-purpose orifice) together in what is whimsically referred to as the “cloacal kiss.” This transfers sperm from the male to the female reproductive tract.
Sperm storage tubules
Females have specialized glands called sperm storage tubules (SSTs) that allow them to store viable sperm for an extended period. Sperm can remain fertile inside the SSTs for a few weeks in some species and over 100 days in others.
Intermittent fertilization
As eggs mature in the female’s oviduct, some of the stored sperm are periodically released from the SSTs to fertilize the yolk. This happens independently for each ovum.
Multiple paternity
Female birds may mate with multiple males within a breeding season and fertilize individual eggs with sperm from different fathers. This is called multiple paternity and increases genetic diversity.
Egg Formation and Laying
Females can continue laying fertile eggs as long as they have sufficient sperm reserves and are ovulating:
Yolk development
Eggs form in the ovary as yolks mature in follicles. It takes several days for a yolk to reach full size before it is ready for release.
Ovulation
When a yolk is mature, it is released from the ovary into the oviduct in a process called ovulation. Hens ovulate about once a day during the breeding season.
Fertilization
As the yolk travels down the oviduct it may be fertilized by sperm stored in the SSTs if the female bird previously mated. The sperm and egg nuclei fuse to create a zygote which begins cell division.
Egg formation
Over the next 24 hours in the oviduct, albumen, membranes, and the shell are added around the yolk to transform it into a whole egg. Pigments are deposited to give colored or speckled shells.
Oviposition
After an egg is fully formed, muscles in the oviduct contract to push the egg outside through the cloaca in a process called oviposition or egg laying.
Time between eggs
The time between ovulation/fertilization and laying ranges from around 24-36 hours depending on the species. Some birds lay eggs nearly daily while others have longer intervals.
Why Mate Only Once?
Given that female birds can store sperm for multiple eggs, what are some reasons they might still mate more than once within a breeding cycle?
Run out of viable sperm
If a female lays many eggs, she may eventually deplete her sperm reserves from a single mating before the end of the breeding season. Remating allows her to replenish sperm in her SSTs.
Ensure fertility
Fertility decreases the longer sperm are stored. Repeated matings improve the chance eggs will get fertilized, especially later in the season.
More genetic diversity
Multiple mates means offspring will have a mix of paternal genes. This genetic variety helps hedge against inheriting undesirable traits or illnesses.
Replace lost mate
If a female’s mate dies or disappears, remating quickly supplies fresh sperm. This ensures she can continue laying fertile eggs.
Trade up mate quality
Females may occasionally “divorce” a current mate to obtain a better one. Trading up increases the chance her offspring inherit superior genes.
When is Multiple Mating Required?
There are some situations where birds absolutely must mate repeatedly to produce fertile eggs:
No sperm storage
Some domestic fowl like turkeys and waterfowl lack sperm storage tubules entirely. So females must mate immediately before each clutch.
Prolific layer species
A few bird species are such prolific layers they would exceed the capacity of their SSTs with a single mating. For example, the domestic chicken mates about once a week to sustain near-daily egg production.
Males without cloacae
In a very few species like the ostriches and ducks, males do not have a true cloaca. So they cannot store sperm themselves and must mate directly before each fertilization.
Artificial insemination
When birds are bred artificially at scale for industry purposes, each female is inseminated right before she starts a new clutch. Natural mating is bypassed.
Conclusion
While mating before every egg would ensure fertility, most female birds do not require this due to their ability to store sperm. One mating session provides enough sperm to fertilize multiple eggs in a clutch or even multiple clutches. However, multiple matings are still common to guarantee productivity and diversity.
Here is a table summarizing the key differences between species that must mate repeatedly versus those able to store sperm:
Trait | Require Multiple Mating | Can Store Sperm |
---|---|---|
Sperm storage tubules | Absent | Present |
Maximum clutch size | Small | Large |
Laying frequency | Low | High |
Example species | Turkeys, ostriches | Chickens, ducks |
In summary, most bird species have evolved special tubules to store sperm so they can lay multiple eggs fertilized by a single mating. However, many still remate frequently to maximize fertility, genetic diversity, and reproductive success across a breeding season.