The patient courtship of cranes

The mating ritual of whooping cranes is a dramatic affair. Couples dance together, leaping, flapping their wings and calling loudly in a dazzling display of pair bonding. But many months before they settle into what are typically lifelong breeding pairs, these cranes begin quietly spending time with their future partner.

Researchers studying a group of 89 whooping cranes reported in the journal Animal Behavior that 62% of breeding pairs began associating with one another at least a year before they began breeding. And 28% were spending time together more than two years before breeding.

“It’s not necessarily surprising that some birds to associate prior to breeding,” said lead author Claire Teitelbaum. “But that it was so widespread in the population and occurred on similar timescales in different pairs was striking. We were definitely surprised by the consistentcy and strength of our findings. ”

There are advantages to long-term monogamy under certain circumstances: for example, if the costs of “divorce” are too high, or if the benefits of remaining together are substantial. Birds that remain in long-term pair bonds enjoy better breeding success and may improve their social status in the flock. That some monogamous species associate with partners before breeding has been long established, but Teitelbaum’s study was the first to discover that pairs form years in advance. I asked her whether this behavior might be more widespread than we realize.

“Since no one has really investigated pre-breeding associations in depth to date, I have to say that yes, it is likely more common than we think,” said Teitelbaum. “On the other hand, it does seem like it would be more likely in long-lived species like whooping cranes, where delaying breeding with the benefit of increased success is more likely to be beneficial than in short-lived species.”

The researchers also found that 60% of the cranes paired off before at least one of the partners had reached sexual maturity. These early couplings indicate a likelihood that there are advantages to forming pair bonds that go beyond breeding success or the cost of divorce. These may include better protection from predators and access to resources in addition to elevated social status. But the cranes could also simply be getting to know one another. The researchers pointed to multiple studies that show breeding success increases as a result of improved compatibility and coordination in pairs that were more familiar with one another.

Teitelbaum said that as longer-term data on breeding success becomes available, clearly identifying the benefits of prebreeding associations could be a key avenue of research.  “In other species, it would be interesting to examine the social status, territories, or other factors that are related to being in a pair, even if that pair is not yet breeding,” she said.

Whooping cranes remain an endangered species. In addition to around 100 birds in the reintroduced eastern migratory population, there are only about 430 birds in the Aransas-Wood Buffalo population. There are also a few captive breeding programs, but these are delicate affairs. The birds are raised by volunteers who wear crane costumes and feed the chicks with puppets to prevent them from imprinting on humans. Despite these artifices, the reintroduced cranes have had difficulty fledging chicks in the wild for reasons that are unclear. Funding for the largest and oldest program at the Patuxent Wildlife Research Center dried up in September 2017, and its 75 birds were moved to smaller sites.

If it turns out that the cranes’ months-long prebreeding associations influence breeding success, this could have implications for how these conservation and reintroduction efforts are managed. “At the extreme, this could entail encouraging certain pairs to bond to facilitate cross/outbreeding in the wild, rather than having to artificially do so in captivity,” said Teitelbaum. “We are definitely quite a few steps away from being able to reach that conclusion, but at the very least we can say that considering year-round social interactions is important for maintaining or promoting a population’s ‘natural’ social structure.”

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