Our Changing World: Can birds adapt their nest building for a warming world?

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It's spring, a busy time for many bird species across Aotearoa - calling, flirting, mating, and gathering material to build their nests. Across hours, days - or for some, weeks - they will carefully construct a safe space in which to lay their eggs, to secure their future generations.

Some will be carefully woven constructions - cup, dome or pendant shape. With grass, reeds, twigs and flowers intertwined and secured perhaps with spider web. Some will have a soft lining - feathers maybe, or animal fur. And others will have elements of deception to deter predators - a fake bowl, or secret entrance.

Nest building is such a complex behaviour that for a long-time scientists presumed that it had to be innate - encoded in the genome and hardwired into small bird brains. But now evidence indicates a certain level of adaptability and flexibility in some species, suggesting an element of learning is taking place.

Getting a birds-eye view on nest design variation

To investigate whether certain bird species might be able to adapt to rapidly changing conditions, Dr Iliana Medina Guzmán of the University of Melbourne is keen to learn more about variation in nest design.

Differences between some bird species is clear - they have different sized bodies, different beak shapes and sizes, occupy different ecological niches and have different material to work with.

But what about birds that are very closely related? And what about within species - what kind of nest differences show up between individual birds? And can these birds flexibly adapt when conditions change?

To study this Iliana and her team are focusing on nests built by the superb fairy wren found in the south-east of Australia. Working with a banded population of the birds they've been collecting abandoned nests throughout their long breeding season (August to February), after the chicks have fledged, or nests have failed due to predation.

With three field seasons completed, they are able to assign nests to specific female wrens across multiple years. The data is yet to be analysed but they can already see a trend in nest walls being built thinner as the season goes on, perhaps in response to temperature increases.

But finding the nests is time intensive and will only allow the team to understand variation in this one species. To investigate the levels of variation in nest design across multiple species Iliana and her collaborators decided to make use of an extremely helpful resource for their purposes - museum nest collections.

The collections

In the basement of Tūhura Otago Museum natural sciences curator Kane Fleury has pulled out some native bird nests from their dry store collection.

Mohua, titipounamu, pipit, mātata or fernbird - they've all been collected around Otago, some over 100 years ago, some from places where those birds are no longer found.

Some contain samples of small, speckled eggs. Collection practices have changed now, explains Kane, with many native species endangered their conservation takes precedent. But the small collection of 120 bird nests housed here, both native and introduced, allows study into how these birds lived.

Not just the nest construction - whether a woven small cup of grass in the case of the pipit, or a spherical design build in a nook somewhere for the titipounamu - but also the environment these birds lived in. Bits of moa nests found in caves around Otago have enabled researchers to investigate the vegetation they interacted with.

Museum nest collections like this exist around the world. But one of the most extensive, and famous (including items collected by Charles Darwin and Alfred Russel Wallace) can be found in the Natural History Museum in Tring, in the UK.

Here over 4000 nests from around the world are carefully stored, along with the description of who collected them, when and where from, and what species they belong to.

It was this collection that Iliana's research colleagues visited to help them answer their questions around nest variation.

Evolution-honed nest design

At Tring the team looked at nest measurements of 49 tanagers species, and then compared these results with climatic conditions of where the nests were found. They found that nests made in places with high rain fall had thinner walls. And the converse - those with thicker walls tended to be from drier environments.

This had already been reported for a few bird species, with the thought being that thinner walls would allow nests to dry much faster, so puddles don't form inside.

They also found that nests had deeper cups if they were built in locations with high wind speeds - helping both the keep the eggs secure and prevent them from cooling down too much.

They also team investigated over 700 nests from 55 species, noting the design and taking measurements of the nest height, depth and thickness to look at variation.

When they looked, some nests from the same bird species were very uniform - the width or height varied less than 2 percent, but other species showed extreme variation - for example building both open-cup and enclosed-dome nests.

Closely related species tracked in the amount of nest building variation, which makes the researchers think there's a genetic component to variation. Evolution has built in high or low levels of variability in different bird families.

General behavioural flexibility - like being able to adapt and say eat different things when your tree of choice is no longer available, can help make species less prone to extinction.

So far Iliana and her collaborators haven't seen any correlation between this general behavioural flexibility and building flexibility, though she hasn't ruled out a link yet because their sample size is still relatively small.

Birds who do have this building flexibility will likely be better able to cope in our rapidly changing climate. The others will be forced to respond in other ways - like moving elsewhere or breeding at a different time.