Tree Swallow

A new study has changed the way ornithologists think about baby birds’ diets, but it won’t surprise many parents who are used to checking the nutritional value of children’s food.

In recent decades, any aerial insectivores, such as tree sparrows, have undergone steep population declines. And Cornell researchers have, for the first time, demonstrated that the fatty acid composition in the tree swallow’s diet plays a key role in chick health and survival rates, potentially pointing to new ways to protect fragile bird species.

In the study a team of researchers manipulated the ratio of long-chain omega-3 fatty acids, the good fat that is found in fish oil, to short-chain omega-3 fatty acids, the food fat present in flax seed as well as the amount of food.

Chicks given diets rich in long-chain omega-3 polyunsaturated fatty acids (LCPUFA) grew faster, were in a better condition, had more efficient metabolisms and had a stronger immune response compared with chicks on a low LCPUFA diet.

The researchers also found that chicks had higher growth rates and better body conditions when they were fed a small amount of high-quality food than if they were fed a large amount of low-quality food.

In addition, chicks fed a small amount of high-quality food had similar immune responses and metabolic rates to those fed a large amount of low-quality food.

"This study really reforms the way ecologists see the food of wild animals," said senior author David Winkler, professor of ecology and evolutionary biology in the College of Agriculture and Life Sciences.

"From a preoccupation with how much food is available, we need to turn our eyes to what kind of food is available."

In the wild, tree swallow sand other aerial insectivores typically forage on a mixture and aquatic and terrestrial insects. Aquatic insects are much richer in LCPUFA than insects that live on land.

"We found that aquatic insects are likely a far more important food source than previously thought due to their high-quality fats," said lead author Lily Twining, a doctoral student in the field of ecology and evolutionary biology.

"The destruction and degradation of aquatic habitats that produce insects with long-chain omega-3 fatty acids may be having important negative impacts on tree swallows and other declining aerial insectivores.”

The study provides new clues into how nutrition and food webs have an impact on the decline of prominent species and provides further reasons to protect freshwater habitats not only for aquatic animals such as fish, but also for terrestrial animals like songbirds.

Tree swallow populations have declined 36 per cent in the past three decades, a circumstance explained at times by reduced insect populations, ecological fragmentation and the effects of climate change.

Other North American aerial insectivores, including nighthawks and chimney swifts, have also undergone similar major disruptions in populations.

This new study suggests that the mismatch between the fatty acid composition of insects and nutritional needs of aerial insectivores could be a crucial driver of fitness and reproductive potential of these birds.

The researchers also suggest that tree swallows may be timing breeding to coincide with times when the abundance of high-LCPUFA aquatic insects is at its highest.

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