Songbird Eggs May Survive Short Periods of Freezing Weather – Here is Why

There are many systems in place that ensure the survival of bird eggs survival during short snaps of freezing weather. These systems include the mother bird’s ability to detect egg temperature with her brood patch, the ability for older embryos to produce heat, and the insulative nature of the nest.

Several factors are working in favor for your backyard bird eggs. The first, and most important factor is the mother bird. She above all is equipped to sense when her babies need a warm touch, and when they don’t.

The brood patch is an essential component for regulating the temperature of bird eggs.

Feathers are meant to retain body temperature. So when it comes to transferring heat to eggs or nestlings, feathers can actually cause a problem. This is where the brood patch comes into play.

A brood patch is a featherless region, usually on the belly of an incubating female bird that allows her to transfer her body heat to her eggs with skin contact. Furthermore, brood patches have blood vessels at the skin’s surface enabling even more heat to be transferred.

The brood patch is an evolutionary development. Most songbirds will automatically lose feathers around this region when egg laying finishes. However, other birds such as geese and ducks will pluck out the area to make their brood patch and use the feathers for nest lining (Ehrlich, Dobkin, & Wheye, 1988).

Often, we worry about the mother bird leaving her nest. Don’t the eggs need to be kept warm? Will the eggs be ok?

But the brood patch actually has receptors called thermoreceptors that can very sensitively detect egg temperature. This allows the mother bird to know when the egg is too cold, too hot, needs turned, or is just right. She knows how attentive she needs to be thanks to her brood patch (Ehrlich, Dobkin, & Wheye, 1988).

When these short bursts of freezing temperatures roll through in early spring, trust the power of body heat, and the mother bird’s ability to know when to keep her eggs warm.

Below is an illustration of a brood patch. They aren’t something you’ll see when female birds are perched or in flight because their other feathers are good at covering it up.

Researchers have gotten photographs of the brood patch by holding a female bird in their hands, and spreading the feathers apart, exposing the bald spot.

The bald spot will be a deep red color. So in this illustration, it might look a bit like wound, when really it’s just pinkish skin.

Learn more about how to care for mother birds when they lay eggs.

A Challenge: Before Incubation Starts

Most songbirds lay one egg each day, but they will not start incubating their eggs until the last one is laid.

When temperatures dip below freezing, this can present a problem if some eggs have been laid, but incubation has not started. Sometimes the mother bird will stay on her eggs at night, but sometimes she wont.

Unfortunately when she doesn’t stay the night, there is potential that these bird eggs will not be viable and will not hatch.

The upside, however, is that some of the eggs laid later that week, when temperatures have hopefully risen, might still have a chance of hatching.

For open nesters like robins, cardinals and blue jays, there isn’t anything anyone can do to protect the eggs. And being that native US birds are federally protected, people should not attempt to intervene.

However, for birds that nest in nest boxes like bluebirds, chickadees and tree swallows, some bird landlords have tried to wrap the boxes up with thick blankets or towels (keeping the entry hole open).

First, we are not saying one way or another whether we recommend this methods since we are talking about active nests. However, if this is attempted, people must be very very gentle, and very careful.

Furthermore, the mother bird may return to the box very early in the morning, and she may be surprised by the bundle on the box. So remove it before sunrise.

The nest is natural insulation that retains heat.

Different birds’ nests are made of different materials. For instance, bluebirds will use pine needles or straw or dried grass. Chickadees have a mossy base, with peeled wood and a top layer of fur. House wrens typically use sticks and feathers.

Because the composition of a bird’s nest uses organic material such as leaves, moss, straw and twigs, which all have insulative properties, a bird’s nest naturally buffers drastic temperature changes such as very hot or cold spells.

Here is a little experiment to try: On a cooler fall day, after you’ve raked up the leaves in your yard and put them into a paper lawn bag, stick your gloved hand in the bag, just above the piled leaves. You should notice a significant temperature difference – a warmer temperature difference than the outside air (as long as it’s a cool fall day).

Thicker nests certainly do a better job at insulating the eggs compared to thinner nests. The insulation works both ways, acting as a barrier against extreme cold and extreme heat. Interestingly, thicker nests also help the incubating bird conserve more energy (Skowron & Kern, 1980).

Birds will also position their nests in order to take advantage of some environmental component such as more sun during the morning or minimizing sun exposure in the middle of the day.

What all of this means is that if you observe your birds being away from the nest for a long period of time during a very cold spring day, the properties of the nest and may slow the cooling process of the eggs, giving mother birds just enough time to have a break or forage for food.

Eggs are more sensitive to extreme heat than extreme cold.

Incubation temperatures range between 34°C – 38°C, or 93.2°F – 100.4°F. Another way to look at it is that eggs should be incubated at about the standard human body temperature (98.6°F), which is just a few degrees lower than a bird’s natural body temperature.

Despite having a brood patch to sense temperature and using the nest to insulate, the window for incubation temperature is very tight.

What happens if it isn’t maintained perfectly?

For cavity nesting birds that you might be hosting, when it comes to heat, the nest box and nest should hopefully be set up in such a way to minimize heat penetration. White or light-colored wooden boxes with thick walls and good air vents will minimize this. The mother bird will also spend less time incubating the eggs or nestlings when it’s very hot.

But what about extreme cold?

The good news is that short periods of exposure to cold weather should not harm the embryos. According to a Stanford published essay, embryos inside the egg are more sensitive to extreme heat than colder temperatures. The essay goes on to explain that when an incubating bird has left the nest, the eggs will eventually cool, but this should not harm the embryos inside.

Table 1. Consequences to egg development at different sustained temperatures.

Something important to understand about the listed incubation temperatures is that these zones represent situations of prolonged exposure – exposure of several hours and several days.

If you’ve noticed momma bird absent for half an hour or even up to an hour during a cold snap, don’t panic just yet. Eggs, especially in a nest box, with nesting material shouldn’t be harmed.

One thing that can happen during sudden freezing weather at spring is that the lower temperatures can delay hatching. Don’t be surprised if your eggs hatch a few days later than you expected.

Is it OK for momma birds to leave their nests so often?

It is very normal for mother birds to leave their nests regularly, especially at mid-day, even during incubation and cold weather. The eggs and nestlings should be OK during these off-nest trips.

A mother bird will alter her attentiveness to the nest based on many factors:

• Detected egg temperature
• Weather
• Age of the egg
• Her needs
• Threats
• Time of day
• Nest box insulative qualities

Usually, a mother bird will make midday trips. Don’t be surprised if you see her leaving the nest every hour and being gone for about 5-15 minutes, even up to 30-60 minutes. During these absences, the nestlings and eggs shouldn’t come to any harm.

On hotter days, she may leave the nest for even longer, and this favors the health of the eggs, since eggs are more sensitive to hotter temperatures than to colder temperatures.

As the embryos inside mature, they will come to a point where they can produce heat as well. Embryonic heat production, paired with the insulative qualities of the nest help ensure the egg’s viability on colder days when the mother is absent.

Key Takeaways:

• The mother bird has ways to tell how warm she needs to keep her eggs, and when she needs to turn her eggs.
• The organic material that composes the nest also provides a lot of insulation.
• Short bursts of freezing weather (a day or two below freezing) may not harm the embryos, especially with the mother bird incubating the eggs.
• Long periods of below freezing weather (several days) may harm bird eggs.
• Cold snaps during early spring may prolong the incubation period. Therefore, eggs may hatch later than you would expect.

References

Akresh, M. E., Ardia, D. R., & King, D. I. (2017). Effect of nest characteristics on thermal properties, clutch size, and reproductive performance for an open-cup nesting songbird. Avian Biology Research, 10(2), 107-118.

Brinsea Incubation Specialists. (2010). How Incubation Affects Egg Incubation or What if the Power Goes Out. Retrieved 2021, from https://brinsea.co.uk/latest/wp-content/uploads/2017/04/What-if-the-power-goes-off-2010.pdf

Britannica. (n.d.). Birds. Retrieved April 23, 2021, from https://www.britannica.com/science/thermoreception/Birds

Ehrlich, P., Dobkin, D., & Wheye, D. (1988). Brood patches – Standford. Retrieved April 23, 2021, from https://web.stanford.edu/group/stanfordbirds/text/essays/Brood_Patches.html#:~:text=When%20just%20one%20parent%20incubates,water%20loss%20from%20the%20egg

Haftorn, S. (1988). Incubating female passerines do not let the egg temperature fall below the’physiological zero temperature’during their absences from the nest. Ornis Scandinavica, 97-110.

Skowron, C., & Kern, M. (1980). The insulation in nests of selected North American songbirds. The Auk, 97(4), 816-824.