Leks have been described as singles bars for birds, though with all the singing and dancing that can go on there, a Karaoke bar might be the closest human analog. Male birds, such as the Great Bustard, Otis tarda, get together at traditional display grounds (leks) and strut their stuff, providing no material resources for females, and being visited by females solely for the purpose of mating.
After the mating season concludes, some male great bustards in central Spain fly further north while others remain near the lek area. Migrants benefit from cooler and moister environmental conditions, and, in some cases, greater food availability. But migrants flying to a new area consume calories, and more recently, run the risk of flying into power lines, thereby injuring or killing themselves.
Carlos Palacín and three other researchers used radio-tracking technology to follow the behavior of 180 male bustards over the course of 16 years. They knew that some bustards died from collision with power lines, but they didn’t know whether these collisions were affecting migrants and non-migrants (sedentary birds) differently, nor if these collisions were changing the migratory behavior of bustards in the 29 breeding groups they studied. So they tracked their birds by ground and by air and determined whether each bird was migrant or sedentary, how long each bird survived, and when possible, the cause of death. For migrant bustards, the researchers measured when and where they migrated, and whether they remained migrants their entire lives.
Palacín and his colleagues discovered that birds migrated away from the lek primarily in May and June, and returned to the breeding grounds over a much more prolonged time period during the autumn and winter.
About 35% of the birds were sedentary, while 65% migrated an average of 89.9 km, with the longest migration of 261 km. Migrants had much higher mortality rates; for example among 73 birds captured and marked as juveniles, migrants survived an average of 90.6 months (post marking), while sedentary males survived an average of 134.7 months, almost 50% longer! The same pattern follows for 107 birds that were captured and marked as adults. The lesson here is that migration kills.
So why migrate? Well it appears that before humans (and in particular, before power lines), migration was a much more beneficial strategy. The researchers identified three causes of bustard mortality: collision with power lines in 37.6% of the cases, poaching (9.1%) and collision with fences (2.6%). The bustard forensic team was unable to determine mortality in the remaining cases, so these percentages may underestimate human impact. Importantly, the researchers discovered that death from power lines was more than three times greater in migrants than in sedentary birds.
This study clearly demonstrates that human infrastructure can shape the migratory behavior of a population. Over the time period of the study, the percentage of sedentary birds has increased sharply even though food availability actually decreased near the breeding grounds as a result of urbanization.
The decrease in migration may be compounded by a finding that juveniles learn to migrate (or not) from adults during their first three years of life. So if there are more sedentary adults to serve as role models for juvenile behavior, more juveniles will develop into sedentary adults. But sedentary behavior can have several drawbacks. A greater number of sedentary males will increase competition for food and other resources. Also, birds may overheat during particularly hot summers near the breeding grounds. In addition, sedentary birds may have higher inbreeding rates and lower genetic diversity, which in turn can make a local population more susceptible to disease and other environmental changes, ultimately making it more prone to extinction.
Very interesting, Fred, I always wondered why birds migrate – makes sense. I thought they just couldn’t take the cold.
Hi unassuming hiker. Birds migrate for lots of reasons. Like most things alive, they are in the business of optimizing things that are essential to them, like a good thermal regime, lots of good food and sex, and not becoming food for someone else. Migration can help with any some or all of those. The good news for birds is they have wings, and can cover large swaths of territory for relatively small costs. Interestingly Great Bustards are the world heaviest flying birds (they actually might be tied for that honor), so flying for them requires considerable caloric input (and may also explain why they tend to run into things!).
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An unfair–but irresistible–question. Of the unfathomable number of studies that might have prompted a new blog, is this choice in any way related to our president’s proposed immigration initiatives?
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Hi Ralph! The choice was made on the basis of how interesting the article was (to me), and also how easy it would be to present both the article and some data with a relatively small number of words and pictures. The title however – that was no accident!
Fred, all l could think about when reading about the sedentary birds was “they’ll get fat and develop heart disease and osteoarthritis”!
While I’ve only read 3 posts so far, I’ll say this: you share a good deal of info in a succinct and readable way, allowing us to be fascinated by the subject. You manage to make the data interesting and not too “researchy” 🙂
In short, nicely done!
These are truly heavy dudes. But they strut well, and burn off considerable fat on the dance floor. Thanks for your kind words.