Some mums have a long way to go before they can lay eggs, from the Arctic terns that voyage north from Antarctica, to the enigmatic swallows that migrate to Europe from Africa each year. But what impact do these extreme exertions and the diets that these birds consume have on the quality of the eggs that they lay upon their arrival? ‘Physiological challenges during one part of the annual cycle can carry over and affect performance at a subsequent phase’, say Megan Skrip and colleagues from the University of Rhode Island, USA. However, investigating the direct impact of these factors in migrating species would be an almost impossible task, so Skrip and Scott McWilliams turned to a more sedentary and well-understood species – the zebra finch – to discover how diet and migration affect how females allocate nutrients to their eggs.
Although zebra finches are not migratory, Skrip explains that they simulated the effects of a large-scale migration by encouraging the birds to take regular exercise flights back and forth between perches over a period of 6 weeks in the lab: ‘Repeated takeoffs and short flights require three times the energy of sustained flapping flight’, she says. And by spiking the drinking water of some of the birds with lutein, catechin and anthocyanin antioxidants, the team was able to test whether the antioxidants provide any protection from the effects of endurance flight and whether the benefits were also passed on to the eggs.
Monitoring the antioxidant levels in the birds’ blood – before and after their simulated migration and after they produced a clutch of eggs – the team could clearly see that the birds suffered oxidative damage as a result of their exertions. Even the birds that were supplied with antioxidants in their drinking water were not fully protected from the effects of the exercise; when the team searched for an increase in antioxidant levels in the blood of these birds, they were surprised to see that the protective antioxidant levels did not increase. However, the team suspects that if the birds had consumed the antioxidants as part of their diet, instead of in their water, they may have experienced some benefits: ‘It's possible that the absence of a natural food matrix resulted in lower absorption of dietary antioxidants’, they say.
Turning their attention to the clutches of eggs laid by the birds after 6 weeks of flight training, they were intrigued to see that the eggs had lower levels of the antioxidant carotenoid lutein, suggesting that the mothers had expended the antioxidants for their own protection during flight training, although the levels of vitamin E passed on to the eggs were unaffected by the zebra finch's exertions. Furthermore, the eggs of birds that had received dietary supplements had lower levels of lipid-soluble antioxidants (lutein) than those of birds that had received no supplements, contrary to expectations. The team also found that the eggs increased in size as the birds expanded the clutch, which they suggest may compensate for the reduction in antioxidants in the eggs.
‘For the first time, our experiment demonstrated that antioxidant supplementation and endurance flight together produce effects on allocation of lipid-soluble antioxidants to eggs by female zebra finches’, say Skrip and colleagues, who suspect that the impact of the parents’ exertions on the quality of their eggs will extend to the next generation.