Science Q & A

Why do gulls like mall parking lots?

New York TimesDecember 1, 2013 

Q. Why do sea gulls like suburban mall parking lots so much, even when they are nowhere near the ocean?

A. The birds that ornithologists call gulls, not sea gulls, do not strictly live by the sea and often find the open spaces of parking lots a bounteous refuge, said Jessie Barry of the Cornell Lab of Ornithology.

Barry, who leads the Merlin Project to develop a better online bird-identification tool, said there are more than 50 species of gulls worldwide, with many found hundreds of miles from the nearest ocean. Indeed, some live primarily inland. The ring-billed gull thrives in suburban settings around the United States.

“Gulls are opportunistic omnivores,” Barry said.

Traditionally thought of as scavenging along beaches or swooping to the ocean surface to catch fish, gulls eat many things, including insects, earthworms, rodents, grains “and of course french fries,” she said.

For species like the ring-billed gull, a mall parking lot offers the chance of a handout, a left-behind takeout meal and Dumpsters filled with food scraps. Grassy medians and recently mowed fields can be a good source of invertebrates.

An empty parking lot or a big-box store’s roof can also offer a safe resting place between bouts of foraging.

“Gulls prefer open areas where they can spot predators and take off easily,” Barry said. “The roofs of shopping malls have even served as breeding colonies.”

Mold and water activity

Q. Why don’t sugar, molasses and honey ever rot?

A. “It’s almost all about water,” said Kathie Hodge, associate professor of mycology at Cornell University.

Molds and bacteria cannot survive on very sugary foods, she said, because the concentrated sugar, like salt, has the effect of drawing moisture out of cells.

“We talk about a food’s water activity, how much of its water is available,” Hodge said. “Sugar, honey, molasses and maple syrup have very low water activity. In fact, they will pull water right out of the air.”

That is why maple syrup left on the table will absorb water from a steamy kitchen and eventually become dilute enough to support mold growth, Hodge said.

Molasses is 50 to 75 percent sugar, preventing most bacteria and fungi from growing on it, but if it is diluted, microbes will eat it and spoil it. “That’s how we get rum,” Hodge said.

Honey has both low water activity and a very high acid level, she said, and also contains chemicals made by plants and bees that suppress the growth of fungi and bacteria.

A handful of organisms, especially some fungi, are good at growing on things with low water activity, Hodge said. Such organisms, which love dry places, are called xerophiles.

“But even for them, a sugar cube is too dry,” Hodge said.

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