Journal Club: Wise old birds teach migration route to young whooping cranes
SUMMARY: A study of captive-bred whooping cranes found that young birds learn their migration routes over many years, and migrating alongside older birds improves the migratory efficiency of younger birds.
This young whooping crane is on its first autumnal migration, guided by an Operation Migration ultralight. The tan markings will fade by its next migration in spring.
Image: Joseph Duff/Operation Migration USA Inc.
Ever since people first realised that birds migrate long distances on a seasonal basis, they've wondered how they do it: how can birds reliably find their way year after year, between their summer breeding areas and their wintering grounds? Is this "instinct" or learning? And if it is learning, how did the first birds learn to migrate and where to go?
Birds are intelligent and highly social animals, and we now know that many of their behaviours result from a combination of both genetically inherited innate programs -- "instinct" -- and learning. But how can we as scientists untangle the contributions from innate genetic programs versus social learning and experience?
To examine this "nature versus nurture" puzzle, an international team of scientists was assembled by ecologist Thomas Müller, who studies animal migration at the University of Maryland in College Park, and at the Biodiversity and Climate Research Centre (BiK-F) in Frankfurt, Germany. According to their findings, which were just published in the journal, Science, young captive-bred whooping cranes perform better when migrating in the presence of older, more experienced birds, indicating that "instinct" is not enough for efficient and accurate migration. Further, the researchers also found that these birds refine their knowledge of their migration routes over many years.
The whooping crane, Grus americana, is a large and long-lived migratory bird in North America. Due to hunting and habitat loss, this formerly widespread species declined until just 16 individuals remained in the wild in 1941. Currently, this species is slowly recovering: the US Fish and Wildlife Service estimates that as of 2011, there were 437 birds in the wild and more than 165 in captivity.
These 16 surviving wild birds represented just a tiny relict of the historical population, which was more numerous and had a much larger range. These birds and their offspring followed just one migratory route between their breeding area in northwestern Canada and their wintering site along the Texas coast.
But it's very risky for an entire population of birds to be limited to just one small wintering and breeding location, especially when they are critically endangered -- just one hurricane or a disease could wipe out them all out. For this reason, several separate populations were established. Most of these populations are nonmigratory. However, one particular population, which originated solely from captive-bred individuals, were taught a migratory route as fledglings after being conditioned to follow costumed pilots in an ultralight aircraft, as dramatised in the 1996 film, Fly Away Home.
Whooping cranes following an ultralight on their migratory journey.
Image: Joseph Duff/Operation Migration USA Inc.
This experimental population of whooping cranes, known as the Eastern Migratory Population (EMP), learnt their 2000 kilometre (1,250 mile) route by following an ultralight southward from Necedah National Wildlife Refuge in Wisconsin to Chassahowitzka National Wildlife Refuge in Florida. This autumnal flight was the birds' only training flight; they returned to Necedah National Wildlife Refuge on their own the following spring.
Birds in the EMP all carry identification bands and many also carry tracking devices so they can be intensively monitored on their journeys. These records, collected by Operation Migration, which has played a leading role in the reintroduction of endangered whooping cranes into eastern North America, are a treasure-trove of detailed information for each journey undertaken by all of the EMP birds over the years.
"The data allowed us to track migration over the course of individual animal's lifetime, and see how it changed over time", said Dr Müller. For this study, his team analysed eight years of EMP migration data that were collected between 2002–2009.
In addition to detailed information about flight group composition and their precise locations, two additional factors made this data set especially attractive for teasing apart the genetic and socially learned aspects of migration. First, all the EMP birds are captive-bred with known pedigrees. Second, all individuals had the same initial learning experience (ultralight training on their first southward autumnal migration -- all data came from the birds' subsequent migrations, beginning the following spring.)
To conduct this study, Dr Müller's team extracted and analysed four variables from the Operation Migration databases:
- the age of all individuals in each group on each flight (a measure of experiential learning)
- the age of the oldest individual(s) in a migrating group of cranes (the upper limit on experiential learning within each flight group)
- flight group size (a measure for potential group navigation)
- genetic relatedness in each migrating group (this captures the genetic relationships for the birds' innate navigation programs)
Using these data, the authors identified every stop-over for each flight group (coloured dots, figure 1A) and each individual crane's summer and winter ranges (the black crosses and exes in figure 1A). They also determined how far each bird strayed from a straight-line route. Since birds expend more energy when they stray from their established route and end up in unfamiliar stop-overs that are often not very safe for roosting and foraging, straying from a straight-line route was a useful proxy measure for each individual's migratory efficiency (figure 1A & B; larger view):
Figure 1. [doi:10.1126/science.1237139]
The authors then examined the effects of a number of variables -- age, sex, flight group size, and genetic relatedness -- on the birds' migratory performance (figure 2; larger view):
Figure 2. Migratory performance of whooping cranes measured as deviation of locations from straight-line path versus age of oldest individual(s) in each flight group with more than one individual. [doi:10.1126/science.1237139]
As you can see in the above figure, the presence of older, more experienced birds clearly enhances the accuracy of migration. The team found that the age of the oldest individual(s) in a flight group decreased the average deviation from a straight-line route by 4.2 km per year of age, thereby improving the migratory performance of young birds by 5.5 percent for each year of age. Thus, seven years of experience translates into a 38 percent improvement in migratory performance.
Further, on average, one-year-olds that traveled with older birds strayed by 63.9 km from straight line paths, which was 33 percent less than for one-year-olds that traveled in same-age flight groups (see figure 3 at right; larger view).
Essentially, "[i]f there was one older bird in the group, they would fly more accurately," said Dr Müller.
Based on these findings, the authors predicted that flight groups where the oldest individual(s) had a migratory age of one year would stray from the straight line path by roughly 76.1 km, whereas flight groups where the oldest individual(s) was eight years old were predicted to deviate from the straight-line path by only 46.8 km.
Neither genetic relatedness nor gender had any effect on the whooping cranes' tendency to follow the shortest migratory route. But the researchers were surprised to find that the migrating groups' size also made no difference.
"Many biologists would have expected to find a strong effect of group size, with input from more birds' brains leading to improved navigation, but we didn't see that effect", said co-author William Fagan, a biology professor at the University of Maryland, in a statement.
The study shows that migration training for captive-born whooping cranes is working, according to Dr Müller. However, their analyses showed that the birds were still refining their knowledge and improving their migratory performance for at least five years.
But social learning may play an important role in other complex behaviours as well.
"We need to take into consideration that these birds may also reproduce more successfully as they age," said Dr Müller.
Indeed, these findings may also apply to reproduction since reintroduced whooping cranes are having trouble breeding in the wild, according to co-author Sarah Converse of the U.S. Geological Survey.
"These birds' behaviours have evolved over millennia," said Dr Converse.
"Managers here are trying to restore a culture -- that is, the knowledge that these birds accumulate over time. We need to give these birds the time and the opportunity to get the breeding right. We might need to be a little bit patient."
Mueller T., O'Hara R.B., Converse S.J., Urbanek R.P. & Fagan W.F. (2013). Social Learning of Migratory Performance, Science, 341 (6149) 999-1002. doi:10.1126/science.1237139
Thomas Müller; emails [15, 16 & 27 February 2013; 15, 16, 17, 18 & 26 August 2013]
University of Maryland press release.
Science podcast [mp3]
And of course, Bob O'Hara, second author on this paper with whom I communicated frequently about this study, especially as I commented on and helped rewrite this paper.
Alerstam T. (2006). Conflicting Evidence About Long-Distance Animal Navigation, Science, 313 (5788) 791-794. doi:10.1126/science.1129048
.. .. .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. ..