Do Birds Lay Eggs All Year? [ANSWERED! + Some FAQs]


So I’ve been seeing some birds start to lay eggs recently and I was wondering to myself: ‘do birds lay eggs all year?’ As there were limited answers found online, I took the liberty to do my own data analysis I did on Nestwatch data to come up with a convincing answer. Here’s what I found:

Birds lay eggs all year round, with a 86% majority of their eggs laid during spring; in April, May, and June. Different species can begin egg-laying either early or late spring. Birds lay eggs the least during September. The time of year birds lay eggs depends on factors like fitness, day length, and latitude.

It’s definitely fascinating to discover when birds actually begin to lay their eggs and when the activity peaks! Read on more as I dive deeper into the analysis I’ve done and discuss some factors that affect egg-laying! Let’s go!

Do Birds Lay Eggs All Year? (Data Has An Answer)

If you’re a rather new birder such as myself, you would not have known that birds could lay eggs all year. I mean, I knew that there was a breeding season of some sort and birds perform mating rituals and such in the past, but I never knew when birds lay their eggs, until now. Here’s how I did the analysis:

I used Tableau to analyze raw nesting attempt data available from the NestWatch.org website which contained 281 bird species from the year 2000 – 2020 from the US and Canada. (Thank you participants!) You should definitely check out NestWatch as it’s a free and great resource to learn about nesting!

I took the first lay date of each observation and summarized them into months. Here’s what I found:

Birds lay eggs all year round, with a 86% majority of their eggs laid during spring; in April, May, and June. Different species can begin egg-laying either early or late spring. Birds lay eggs the least during September. The time of year birds lay eggs depends on factors like fitness, day length, and latitude.

Interactive Bird Egg-Laying Timing Tool

Check out the interactive tool below to discover more about different egg-laying timings for different birds!

Birds actually lay their eggs throughout the entire year! Of course, among egg-laying events I analyzed that have occurred for the past 20 years, there seems to be a peak egg-laying period from around March to July, during the spring season.

Which Month Do Birds Lay Eggs The Most In A Year?

I was then curious as to find out which month had the highest number of bird egg-laying events, so I referred to the tool I created above and came up with the following conclusive answer:

Birds lay their eggs the most during the month of May with a majority of 43.2% out of all 12 months in a year. Other months that have relatively high egg-laying frequencies are April (20.1%) and June (23.7%) in descending order.

Now, this makes the most sense! The months with the most egg-laying events: April, May, and June, coinciding with the breeding period during springtime in the Northern Hemisphere. This period is when birds begin to find a mate, mate with them, find a nest, and lay their first eggs.

Here’s a table summarizing the percentage of egg-laying events by month:

MonthPercentage of Egg-laying Events In A Year (%)
January<0.1
February0.3
March5.7
April20.1
May43.2
June23.7
July6.8
August0.2
September<0.1
October<0.1
November<0.1
December<0.1

Which Month Do Birds Lay Eggs The Least In A Year?

Birds lay eggs the least during the month of October, taking up less than 0.1% of all the egg-laying events in months throughout the year.

This period coincides with the migration period during Autumn, where birds are less like to be laying eggs.

Birds Lay Eggs At Different Timings During Spring

Now we know that birds lay eggs the most during the Spring, I got really curious to find out which timings birds lay their eggs during spring. While there are many birds that choose to nest during the spring, birds actually lay their eggs at different timings during spring. Some choose to nest early, while some choose to near later in spring! Let’s find out more about that:

Early-nesters

A barn owl

Early-nesters refer to birds that start building their nest to lay their first eggs, with a majority before the month of May. Examples of early-nesters include the American Robin, Barn Owl, Eastern Screech-Owl.

Let’s look at an example with an American Robin!

American Robin egg-laying pattern over 12 months

When I keyed in American Robin to the search of the tool, I found that they lay most of their eggs in the month of May (42.3%). I have also found that they tend to be more of an early nester, given that they also lay a relatively high percentage of their eggs in April (32.1%), before the peak of the nesting season.

Late-nesters

Late-nesters refer to birds that arrive back later in the North after winter to start building their nest to lay their first eggs. Examples of such late-nester birds include the House Wren, Barn Swallow, Common Yellowthroat.

Let’s look at the example of a Barn Swallow!

This time when I searched up Barn Swallows in the tool above, I noticed that although its peak egg-laying month is in May at 42.2%, with most of its other high-percentage months after May: June (31.9%) and July (18.1%). This means that the Barn Swallow is most likely a late-nester, as expected!

Examples of Early and Late Nester Bird Species

I summarized the abovementioned early and late nester species with links to their relevant eBird profiles for you in a table for easy reference!

Highest Egg-Laying Months Highest Egg-Laying Month Percentage (%)
Early-Nesters
American RobinApril, May42.3
Barn OwlFebruary, March41.7
Eastern Screech-OwlMarch, April52.2
Late-Nesters
House WrenMay, June, July38.6
Barn SwallowMay, June, July42.2
Common YellowthroatMay, June57.3

Birds That Lay Eggs Often All Year Round

Now that we’ve discovered a little more about the eggs being laid during and around springtime, what about the other periods of the year. Do birds lay their eggs often during other months? There are some birds that lay their eggs often all year round!

An example is the Great Horned Owl. Here’s how it’s graph looks like!

As seen in the image above, most Great Horned Owls lay their eggs in February (40.7%), which is before the nesting season. What’s interesting to find out is that it also lays its eggs in January (8.5%) and in December (3.4%), which are basically months that are not the typical spring season when most birds will lay their eggs!

How Birds Know When To Lay Eggs

Now you may wonder, with so much variation among birds and when they begin their journey to lay their first egg, what exactly are the factors that affect when birds lay their eggs? Here are the 3 factors:

1. Fitness

Wait a minute, fitness?! I don’t mean how well you can bench-press in the gym or whether you’re able to complete a full marathon! Fitness is actually an evolutionary biology term that describes the reproductive success of a bird, and its ability to pass it down to its kids (in layman’s terms).

If a pair of birds decide that it’s best for them to mate early and begin the process of nesting early, there may be benefits in doing so for some bird species, as their eggs will hatch earlier and fledge earlier, providing them a higher survival advantage after.

In addition, if birds want to lay another clutch at another time of the year other than spring, they would have to face other environmental pressures such as higher predation and competition for food in their niche.

2. Day length

Scientists have recently found that day length affects the secretion of certain hormones in a bird’s body, which affect the size of their sexual organs, which help in reproduction. They made a controlled experiment where they exposed male starlings to varying levels of photoperiods (periods exposed to light) from the winter solstice. They found that their sexual organ growth was directly proportional to the photoperiod! [1]

Male starlings sexual organ growth is directly proportional to the photoperiod it was exposed to (right) source: https://www.sciencedirect.com/science/article/pii/S0091302214000764#f0010

Thus, with more chances of reproductive success from increased testicular growth rate, birds mate and lay eggs more often as the day lengthens after winter.

3. Latitude

Latitude also affects when birds mate and lay eggs. Birds at high latitudes have a shorter time frame in which they can breed during the springtime, compared to their counterparts that live at lower latitudes. An example are sandpipers. Their young have even adapted to develop faster in such conditions too!

Final Thoughts

Now you know that birds lay eggs all year round, you know that birds lay eggs mostly in springtime, mostly during May to July, and you know what factors affect when birds lay their eggs. I hope that you have found this article useful and sufficient to satisfy your curiosity as much as it did mine! I had fun writing this article. Anyway, thank you so much for reading the end, take care. Happy birding!

References

  1. A Dawson. (2015). Annual gonadal cycles in birds: Modeling the effects of photoperiod on seasonal changes in GnRH-1 secretion, Frontiers in Neuroendocrinology, Volume 37, Pages 52-64, ISSN 0091-3022. https://doi.org/10.1016/j.yfrne.2014.08.004. (https://www.sciencedirect.com/science/article/pii/S0091302214000764)

Justin

Justin is a hobbyist birder that hopes to share his birding knowledge with the world. His favorite bird is the Large-tailed Nightjar and he really loves potato chips!

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