Birds warn their eggs of warm weather

There are around 5000 different species of songbird around the world and, together, they create a playlist that can rival Sony. But, most songbirds don’t just sing - they will also make calls and coos to communicate. And in recent years it’s come to light that these calls can be very important very early in a bird’s life; in fact,  before it’s even hatched in some cases, as Eva Higginbotham has been hearing..

Eva - One evening, recently, I went for a walk in Cambridge, on a small path next to a stream shaded by tall trees covered in leaves, and surrounded by grasses, bushes, and, of course, nettles…And along with the green I could feel under my feet, I could hear what sounded like an incredible array of birds singing and calling to each other, likely male birds, singing to defend their territory and try and seduce the ladies. That’s how we traditionally think about how and why songbirds communicate. But, a few years ago, a scientist studying zebra finches, small somewhat stripey, australian songbirds, discovered something.

Mylene - So they have different sorts of calls, but they mostly talk to each other to sort of coordinate parental care.

Eva - that’s Mylene Mariette from Deakin University, and she was studying how mum and dad zebra finches chat to each other to figure out how long each of their egg-incubation shifts were going to be. This is what that conversation can sound like

BIRDS SWAPPING

Eva- Squeaky! But then, Mylene noticed something unusual…

Mylene - And I also noticed that when a parent is sitting by itself in the nest, it produces a call that is really different to the other calls. And because he was alone with the egg, I wondered whether he was talking to the egg. So it's really fast and really high pitched, which is surprising for this species because it tends to have very nasal calls, but that one, yeah, it's really, really quite high pitched. It sounds like crickets, really.

Eva - Mylene set about trying to understand, first of all, what this unusual call could mean

Mylene - So I did a lot of recordings in the nest and it quickly became apparent that this was only happening when it was hot and because the weather is so variable in Jalong, I could very clearly see a link between that call and the temperature outside. So it's only when the parents were too hot that they were producing that call.

Eva - So, Mylene snuck out the real eggs and replaced them with fake ones. Then, she incubated the eggs she’d stolen and either played back recordings of heat calls, or just the normal parental chatter that the birds made whatever the weather. Just before hatching, she returned the real eggs to the nest to see what would happen.

Mylene -  We found that having heard those heat calls before hatching then changed the development of the chicks after hatching. And so the ones that were when exposed to heat call tended to reduce their growth in the heat. Whereas the ones that were exposed to another contact call from the parents tended to get bigger when it was warmer.

Eva - At hatching, the two groups weighed the same, but within 24 hours there was a difference in size, and this just kept getting bigger until, by the end of the nesting period, there was a 25% difference in body weight between the two groups. This opposite growth strategy seemed puzzling at first, but then, being small in hot weather can be advantageous as it allows animals, us included, to dissipate the heat better. Also, growing itself requires quite a lot of energy and so creates quite a bit of heat. Mylene realised that the heat calls were telling the embryos - don’t grow too quickly when you hatch! But how?

Mylene - So we don't know the details yet, but obviously we're really interested in finding out. So looking into neurobiology, there are actually some connections in the brain that go directly from the auditory centre to the centre that controls emotion and hormone production. And we know from adults, for example, that it is those connections that trigger spontaneous response to sound.

Eva - The question was, did this growth difference actually translate into a more successful bird?

Mylene - After they grew up we just let them breed because we wanted to know whether that strategy of reducing the growth in the heat could be beneficial. And we found that the birds that had reduced their growth in the heat were producing more babies. And we also found that those individuals, when they were adults, were preferring hotter nests to breed in, so it does seem to have changed them in the long term.

Eva - And, Mylene realised that the heat call had a benefit for the parents too

Mylene - When the parents are calling, it's actually a special form of panting. So when the birds are too hot, they pant like dogs do. And that's when the heat call is produced.

Eva - Experiments showed that producing the heat call cools the birds more than just their regular panting, and this also provides a potential mechanism for this behaviour to have evolved

Mylene - So parents would produce those calls to cool down in the nest when it's really hot and during a heat wave. And just because of the benefits that it brings to the offspring, then that can be selected for, and it becomes more and more common. And so eventually the parents turn the call into a signal because they also benefit from their offspring being higher quality in hot weather.

Eva - And for the embryos, listening to what’s going on outside plays an important role in hatching too…

Mylene - They all use sound to know when is the best time to hatch so they can synchronise hatching between the embryos in the clutch, or a predator arriving and coming to eat the egg. So sound is used during development a lot more than we thought initially when we started this project. But there have also been some studies in seagulls, where they found that embryos that have been exposed to alarm calls that their parents do when there are predators around also change the development of the chicks. And so it's likely that it's quite common, but we just haven't looked at it basically.

Eva - That zebra finches, and perhaps other birds, do this, might suggest that they are going to be a little more adept at surviving climate change than some other species. Here in Cambridge though, I’m still likely listening to lonely bachelors looking for a mate to start a nest with...