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Have you been to the beach this summer? I took my daughter there the other day and got her a pasty for lunch. She was happily munching it while staring out to sea, back turned to the café where we bought the pasty. Suddenly, there was a flapping of wings behind us and something swooping down. Next thing I knew the pasty was gone, and my daughter was in tears. A seagull had struck and flown off with a free lunch, again.

I should have known to keep my eye on the herring gulls (the most common gull species in British seaside towns) perching on the café roof looking for an easy meal – my students and I showing that gazing at gulls deters them from stealing your food.

However, not all gulls you see are human-food thieves and my team’s recent research showed that gulls would much rather have fish than human-made food.

The 2019 research also showed that, when presented with human food, only a quarter of gulls actually touched the food. This suggests that a minority of urban-dwelling gulls are responsible for our ruined beach picnics and stolen ice-creams.

But it wasn’t clear what turns a gull into a human-food thief. Much like human parents , in , the young learn what, where and how to feed from their parents. So my team wanted to know when gull parents feed their chicks human food, does this make the gull chicks more likely to seek out human food too, when they grow up?

It is virtually impossible to figure out what wild gull parents feed their chicks, and test how this influences their chicks’ food preferences, without causing major disturbance in their breeding colonies.

So we with rescued herring gull chicks – ones who had been found on the ground by people and brought into a wildlife rehabilitation facility because they could not be reunited with their parents.

Only chicks that a specialist avian vet confirmed were healthy took part in our study. We made sure their participation did not delay their release back into the wild.

We reared these rescued chicks from when they were about five days old until they were about 25 days old on one of two diets: a “marine” diet of sprats, mackerel and mussels, or a “man-made” diet that consisted of cat food and bread. We chose these diets as they are similar to the opposite extremes of the foods to their chicks in the wild.

We also gave the chicks the alternative diet 20% of the time. Marine chicks were fed the man-made diet several hours a day, and vice versa, to ensure all chicks were familiar with all the types of food before we tested which they fancied.

To test whether the man-made diet chicks would grow up to prefer the foods they were reared on, rather than the marine prey gulls traditionally forage for, we presented each chick with a smorgasbord of foods when they had been in captivity for five days.

They could choose from a bowl of fish, a bowl of mussels, one with cat food and a bowl with diced bread. We recorded which food the chick pecked at first. We found that almost all of the 27 chicks in our study preferred to eat fish first, and avoided bread. Even chicks that had been reared on cat food and bread still preferred fish.

The chicks did not change their fish fancy as they grew up. We tested all of the chicks three more times (after they had been in captivity for ten, 15 and 35 days), and each time fish was the winner.

So if gull chicks prefer fish even when they are reared on man-made foods (like cat food and bread), why do some insist on ruining your beach picnic once they’ve grown up? We think these gulls might be making the best of a bad situation, rather than being obsessed with your chips.

Marine prey and fishery discards they would naturally feed on have been . Although rearing chicks on food from the land is likely their development, it is better to feed your young something than to let them go hungry.

The abundance of the UK’s natural-nesting herring gulls is than it was almost 40 years ago and the herring gull is now of .

Our 2023 study found that when more human food was available, with both peaking during lunch time. In other words, .

There are a few things you can do to deter gulls stealing your food. First, gaze directly at gulls, especially when food outlets have good perching spots. You could also eat with your back against a high wall, or underneath a roof, parasol or umbrella because gulls are more likely to steal when there’s a clear escape route.

And don’t feed the gulls. Where there’s more human food, there will be more gulls – who doesn’t like a free lunch?

Neeltje Boogert receives funding from The Royal Society.