Imagine cramming an entire life's growth into a single month; that is exactly the challenge faced by some newly hatched chicks. ‘Tree swallows nearly double in mass every other day when they are between 5 and 10 days old’, says Cornelia Twining from Cornell University, USA. And this rapid mass gain places an immense burden on the parents, which must provide their offspring with nutritious food to satisfy their voracious appetites. Twining adds that highly unsaturated omega-3 fatty acids (long-chain omega-3 fatty acids), such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), are vital for the youngster's health and growth. However, only a few birds, such as chickens, are capable of producing these key fatty acids from shorter components; other species must obtain them through their diet, and aquatic insects, such as mayflies and midges, which dine on long-chain omega-3-rich algae, are the best source. As tree swallows benefit from a mix of terrestrial and aquatic insects in their diet, Twining and co-authors Tom Brenna, David Winkler, Peter Lawrence and Alex Flecker wondered whether the youngsters may be able to generate the essential long-chain omega-3 fatty acids from shorter components (alpha-linolenic acid – ALA) or whether they depend solely on the aquatic insects in their diet for their supply of long-chain omega-3 fatty acids.
‘Tree swallows were a great species to work with … because they are very tolerant of researchers studying them during their breeding season’, says Twining, who recalls that the parents were content to keep feeding their young even when the researchers slipped the youngsters additional meals of olive oil laced with a labelled version of the ALA component, in order to trace whether the chicks were able to convert it into the longer chain fatty acids. ‘Getting them to gulp down from a syringe takes a little practice, but is not a problem, so long as they are gaping for food’, says Twining, although the youngsters’ table manners were less than impeccable. ‘Chicks are pretty messy eaters’, she recalls, which made it difficult to be sure exactly how much of the special olive oil the chicks had consumed.
Forty-eight hours later, Twining extracted fatty acids from the chicks and analysed them with Lawrence and Brenna. Impressively, the chicks could convert the short-chain component into the essential long-chain omega-3 fatty acids, but could they satisfy all of their long-chain omega-3 demands from the short-chain components in their diet alone?
Twining needed to know how much ALA various insects could provide the chicks, so she trapped insects ranging from aquatic mayflies and dragonflies to terrestrial beetles, flies, moths and butterflies to find out how much of the short fatty acid they contained. However, after she measured the amount of ALA in the insects’ bodies with Lawrence and Brenna, it was clear that the chicks were not able to produce sufficient long-chain omega-3 fatty acids from the short-chain fatty acid components supplied by their diet.
‘We suggest that omega-3 HUFA [long-chain omega-3 fatty acids] are likely to be “ecologically essential” nutrients for tree swallow chicks’, says Twining, who is now keen to find out whether other wild species are capable of producing the essential long-chain omega-3 fatty acids from components in their diet. ‘I hope to understand what their physiological nutritional needs are and how likely they are to suffer nutritional mismatches if they cannot get what they need in their diet’, she says.