Carrying around a few extra pounds might not seem like a big deal to most. While it's important for humans and other mammals to maintain a healthy body weight, it may be even more important for birds. Flying becomes more difficult if the birds are heavy: they are less manoeuvrable and unable to accelerate as quickly, leaving them vulnerable to predators. Conversely, being too light could leave them vulnerable to starvation if food becomes scarce. But how birds sense and then adjust their weight so that they can fly but not starve remains a mystery. To unravel this question, Brett Hodinka and Tony Williams of Simon Fraser University, Canada, teamed up with Lewis Halsey of the University of Roehampton, UK, to see if zebra finches (Taeniopygia guttata) lose weight by behaving differently when they suddenly find themselves heavier than before.
First, Hodinka had to design and fit tiny ‘backpacks’ for the finches that would add ∼10% more weight to the birds. After loading the birds down with their packs, the researchers weighed them and recorded their behaviours with a video camera for an hour every other day over the course of 2 weeks. By day 2, the birds had already lost 33% of the weight of their backpack. They continued to lose weight steadily until day 6 when they started to gain some weight back – though they still weighed less than the birds without packs. Surprisingly, even though the finches were losing weight, they ate the same amount of food as birds without the weighted packs, and even reduced their activity levels. But maybe the finches would gain weight if the team provided them with a tasty diet that was loaded with fat.
After creating a high-fat, calorie-dense diet by mixing up eggs and sunflower oil with breadcrumbs and cornmeal, the team allowed another group of birds to eat as much of this new diet as they wanted before fitting them with their own weighted packs. During the 14 days that they wore their backpacks, the birds still lost weight, even though they were eating more calories. If the birds were still losing weight without exercising and being on a high-fat diet, some other mechanism must be at work.
The team thought that the bird's weight loss could be due to the stress they experienced while wearing their backpacks. If the birds were stressed out, that would also explain why they weren't more active. So, the researchers took blood samples from the finches and measured the concentration of corticosterone, a hormone released when the birds are stressed. The backpack-laden finches didn't have higher levels of the stress hormone in their blood. They also had normal levels of haematocrit and triglycerides in their blood, suggesting that the birds weren't burning more energy. So, how do the finches change their weight without diet and exercise? One possibility is that the finches may not be absorbing as many calories from their food as it passes through their digestive tract, but Hodinka and colleagues are quick to point out that there is still much to be done before they uncover exactly how the birds are losing the weight.
With the ongoing obesity epidemic plaguing the western world, many people are flocking to the latest fitness fad or trending diet. Eating a diet high in fat not only causes obesity in humans, it also has the same effect in other mammals. But for some reason, birds seem to be immune to the obesity-inducing effects of fatty foods. Regardless of how birds can shed excess weight without exercising or dieting, there will certainly be some jealous mammals waiting for the answer.