March 2015

Research Recap March 20, 2015

Which Came First: The Cavity or the Cup?

Songbirds range widely in colors and calls but they all have one thing in common: nests. Whether earthen burrows or tree holes, intricate woven structures or messy stick piles, all songbirds lay their eggs in supportive substrate to boost youngster survival. Songbirds typically nest in either enclosed holes known as ‘cavities’ (like a woodpecker) or open cup-like structures (like a robin). But why prefer one nest to the other, and which came first anyway, the cavity or the cup?

To start answering these age-old questions, Cornell Lab doctoral students Sahas Barve and Nick Mason teamed up to examine evolutionary history of nesting in old world flycatchers (Muscicapidae). These small passerine birds aren’t our familiar U.S. flycatchers. Old world flycatchers are restricted to Europe, Africa and Asia and are visually more akin to thrushes like the American Robin, though they do share some behaviors with North American flycatchers (such as hunting insects on the wing).

Most importantly for investigating nesting, the old world flycatcher family includes species that use both cavity and open cup nesting. Barve and Mason used this diversity to trace back through the bird family tree and link nesting type with life history traits. Results were recently published in Ibis.

The ancestral nest for old world flycatchers

Using DNA data from old world flycatchers, the team reconstructed the group’s family tree depicting the evolution of species through time. After looking at nesting type, Barve and Mason found that the ancestor at the trunk of the tree, which gave rise to the old world flycatchers nearly 21 million years ago, had been an open cup nester. Eureka!

But why did some bird species switch to cavity nesting through time, and how do cavity nests help species survive? Such big questions require big data, so the team compiled life history and occurrence data from museum specimens and eBird checklists across Europe, Africa and Asia. They then compared species with open nests and cavity nests to find associations with body size, clutch size and migration. Here’s what they found:

Bigger species win cavities

Photos by Bryan Hix via BirdShare, Lkanth22 via Wikimedia Commons

In old world flycatchers, most cavity nesting birds worldwide are cavity “adopters,” meaning that they find or steal empty holes created by other birds (like woodpeckers). Since there aren’t enough cavities for everyone, birds compete for cavities, often through harassing or attacking other individuals.

Barve and Mason found that within old world flycatchers, larger-bodied species (like the Northern Wheatear, photo above right) tended to nest in cavities, whereas smaller species (like the Black-and-orange Flycatcher, photo below right) often used open cup nests. A bigger body gives birds a competitive advantage, enabling them to claim victory over smaller species and more often secure the use of cavities.

Nesting and Migratory Habits of Old World Flycatchers Ordered By Size

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Cavity nesters lay more eggs

The team also found that for old world flycatchers, species nesting in cavities produced more eggs than open nesting species. Since cavities contain more stable microclimates than open cup nests, which are exposed to the elements, birds face a lower risk of losing eggs to unexpected weather events. “If you’re nesting in a cavity, even if there’s a crazy spring snow storm, then you’re still going to be safe from it,” said Barve, “But if you’re an open nester, and you’ve already made your nest, your eggs are going to be covered in snow.”

With the added assurance that their eggs will be protected from extreme weather, cavity nesters can invest more energy into laying more eggs. Bigger clutches can also help old world flycatchers boost their odds of producing chicks in case they aren’t able to beat out the competition and secure a cavity the following year.

Maximum Clutch Sizes of Old World Flycatchers

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Cavity nests support migration

By comparing nest type and the evolution of migration in old world flycatchers, Barve and Mason found that species using cavity nests tended more often to evolve into migrating species than those using open cup nests (use the button ‘State Changes To Migratory Only’ in the visualization below to see this). Barve explained, “Cavity nests probably provide a really good microclimate for raising chicks, especially in really hot or cold places, because cavities have a more stable temperature inside than outside.” Added Mason, “It’s a similar nesting environment regardless of where you are.”

This stable nest environment may have enabled some old world flycatchers to move north and nest earlier in the season when conditions were still cold outside. Such migratory behavior could have benefitted adult survival without compromising the health of their chicks.

Theorized Character State Changes Between Speciations

Hover over a group arc or character state change arc to explore. Toggle with buttons below.

This chart is a circular flow chart that shows how many times species changed their nesting state (cavity or open), migratory state, or both between speciations. Change grows from the initial state towards the final, changed state.

Code based off original work by Nikola Sander, Guy J. Abel & Ramon Bauer. Licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License

Conclusions: from cup to cavity

Over millions of years, many old world flycatcher species switched from an ancestral state of laying eggs in open cup nests to nesting in cavities. These nesting behaviors started with a few experimental individuals and eventually evolved into a new nesting strategy for an entire species. Even today we may be seeing similar steps towards evolving strategies in birds today. Although this study was specific to old world flycatchers, citizen scientist observations reported through NestWatch are helping us understand nesting in North American birds.

Create a cavity!

Want to play your hand in evolution? Many cavity-nesting species readily settle in nest boxes, and setting up nest boxes in your backyard can give the advantage to a less dominant species in the bird ranks. Some birds are even adapting their nesting behavior to use nest boxes, such as Lucy’s warbler in the Sonoran desert. So make a nest box today (nest box plans here) to support more birds around your home, and join NestWatch to help scientists learn more about how nest site choice affects reproduction over the long-term!

This Month’s Articles

Which Came First: The Cavity or the Cup?

Research Recap

No Nest Required

Factoid March 12, 2015

Top 5 Interesting Nests in North America

Factoid March 6, 2015

One Comment

  1. these pictures are really beautiful and impressive. Now I learn more about the birds’ all kinds of nests.

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