The old wives are right — fish is good brain food. But only if it's the right fish. Kittiwake chicks fed on a diet of smelt, which is low in lipids, are stupider than those that get lots of oily fish.
This, coupled with climatic changes, could help explain why the gull species has been "in precipitous decline" in Alaska over the past few decades, say Alexander Kitaysky of the University of Alaska, Faribanks and his colleagues.
Kitaysky's team fed some captive red-legged kittiwake chicks as much fish as they could eat, but gave others more meagre rations. They also fed some chicks on oily silverside, and others or lipid-poor smelt.
At the end of the test, the birds on a restricted diet showed, not surprisingly, higher levels of nutritional stress, as revealed by their high levels of the hormone cortisone. Oily fish has more calories per-gram than other types, so the smelt-eaters had a harder time getting enough calories.
Hungry birds also showed cognitive impairments. Birds learnt how to open a plastic pot with food in, and then had to learn that black pots had food in, and white ones were empty. Those on the restricted diet did worse in these tests, and the birds on the poorest diets never learned to associate black pots with food.
A diet of smelt exacerbated the effects of nutritional stress alone, showing that both calories and lipid content seem to affect the chicks' learning ability.
Once they fledge, kittiwake chicks must fend for themselves, so slow learners might struggle to survive. Why kittiwakes have declined has been a puzzle — adults have been have having as many chicks as ever, but fewer birds survive to adulthood.
The change in diet is probably linked to a climatic change in the mid 1970s — a switch in a cycle called the Pacific decadal oscillation from cool to warm waters —that caused populations of oily fish— such as herring and mackerel — to decline, and favoured other fish such as pollock.
This finding also lines up alongside a theory to explain the similar decline of Steller's sea lions over the same period. The 'junk food' hypothesis advocated by Andrew Trites at the University of British Columbia and his colleagues argues that sea lions and seals have declined because they do worse on a diet of non-oily fish — experiments show that they have to eat more to get the same calories, and gain weight more quickly, for example. The whole issue of sea lion declines is controversial (registration required); some see this as an attempt to get the fishing industry off the hook, and view Trites' links with the industry with suspicion. (Although for what it's worth, the junk-food idea seems reasonable to me.)
Weirdly, Robert Winston, the avuncular face of British science communication, has recently got into trouble for advertising milk with added omega-3s (i.e. fish oil), and hinting that drinking it might make kids smarter. Perhaps he should market the magic milk to kittiwakes.