– Like many bird species, whooping cranes tend to mate for life. A new study led by researchers from the University of Georgia and of the Senckenberg Biodiversity and Climate Research Centre has found that whooping cranes can form these lifelong attachments well before they are ready to start breeding. The findings, just published in Animal Behaviour, are an important addition to the body of knowledge about these endangered birds.
Whooping cranes are a migratory species native to North America. Hunting and habitat loss drove them nearly to extinction by the early 1940s, and all whooping cranes alive today can be traced to fifteen or fewer birds that were alive then. Intensive conservation efforts have brought them back from the brink, but only four populations, with about 500 birds total, currently exist, according to the International Union for the Conservation of Nature. Three of them were founded by birds bred and reared at several captive breeding centers in the U.S. and Canada.
The new study focused on one of these reintroduced populations, which migrates between its breeding grounds in Wisconsin and wintering grounds in the southeastern U.S. This population is monitored intensively. Researchers keep detailed records of individuals, including age, sex, and all sightings. Birds are banded and tracked using VHF transmitters, and that information is entered into a database that stores the location of birds over the course of their lifetimes. A nesting database includes daily observations of the locations, start dates and hatching—or failure—dates of every nest during the breeding season.
Claire S. Teitelbaum, a doctoral student at the University of Georgia Odum School of Ecology and previously at Senckenberg, led the study. She and her coauthors, Sarah J. Converse of the U.S. Geological Survey, Washington Cooperative Fish and Wildlife Research Unit at the University of Washington in Seattle, and Thomas Mueller of Senckenberg and Goethe University in Frankfurt (Main), Germany, realized they could use that wealth of data to explore unanswered questions about long term monogamy in whooping cranes. Specifically, they were interested in whether this behavior is driven by its benefits for individual birds or by the high costs of bird “divorce.”
Previous studies have shown that pairs of birds grow more compatible behaviorally and hormonally the longer they’re together, which could increase their success in breeding as well as in finding and acquiring resources. At the same time, when pairs divorce the costs are high, including lower breeding success and reduced survival.
Teitelbaum and her colleagues looked at location and nesting information about each bird in the Wisconsin population starting four years before its first breeding attempt. Because the data were so comprehensive, they were able to calculate how much time each bird spent with each other bird, and when those that eventually became pairs first started associating with one another.
They found that 90 percent of birds that became breeding partners began spending most of their time together between nine and twelve months before they first attempted breeding. A majority—62 percent—began associating at least a year before, and 28 percent began associating more than two years before. And 60 percent of these pairs began their associations before one or both of the partners had reached breeding age.
“Not only did pairs start spending time together earlier than we’d thought, they spent much more time with their partner than with any other bird,” said Teitelbaum. Birds that formed pairs were observed together 77 percent of the time during the year before their first nesting. By contrast, they spent 21 percent of their time with their most favored non-partner associate.
Knowing when cranes begin to form pairs could indicate which benefits and costs are at work. This is important to understand because these pair associations will affect breeding outcomes for many years.
“The primary barrier to this reintroduced population of whooping cranes becoming self-sustaining is lack of success in breeding. Birds aren’t very successful at hatching and raising their chicks,” Converse said.
“We don’t know exactly why the birds are failing,” said Mueller. “The more we understand about breeding behavior, the more likely we are to identify the cause of breeding failure, and we hope, come up with a solution.”
The study was supported by the Robert Bosch Foundation and data was provided by the Whooping Crane Eastern Partnership.