Across North America, hundreds of bird species are victims of the cowbird, which adds its own egg to other birds' nests, tricking another species into raising its offspring. One target, the yellow warbler (林莺) , has a special call to warn egg-warming females when cowbirds are approaching the area. Now, researchers have found the females act on that warning one day later一suggesting their long-term memories might be much better than thought.

"Animals are smart in the context in which they need to be smart," says Mark Hauber, an animal behavior researcher at the University of Illinois who co-authored the new study. He wanted to see whether yellow warblers had the capacity to remember their own important warning call known as a seet. The birds make the call only when a cowbird is near. When yellow warbler females hear it, they go back to their nests and sit tight. But it's been unclear whether they still remember the warning in the morning.

So the study team found 27 yellow warbler nests and exposed females to either silence, or one of two sounds: a recording of a seet call or a recording of a common warning- used for predators or competition-called a chip for 10 minutes. The next morning, the researchers observed the birds for 80 minutes: 20 minutes before sunrise and 60 minutes after, when the cowbirds are at their most active.

They found the warblers left their nests less often after hearing a seet call than if they had heard no warning. The chip call didn't seem to have any impact on how often they left the nest. Sixteen hours after the experiment, the birds were still behaving as if there's a cowbird threat. It allowed us to think that these kinds of signals carried long-term meaning. Of more than 200 species targeted by cowbirds, yellow warblers are the only so far known to have developed a warning call tailored to cowbirds.

Identifying the chemical makeup of pigment (色素) used in ancient documents, paintings, and watercolors is critical to restoring and conserving the precious artworks. However, despite numerous efforts, scientists had been unable to determine the source of folium, a popular blue dye used to color manuscripts (手稿) in Europe during the middle ages — from the 5th to the 15th century. Now, a team of researchers from Portugal has finally uncovered the mysterious ingredient responsible for the gorgeous blueish-purple color that helped bring ancient illustrations and texts to life.

The research team began by poring over instructions penned by European dye makers from the 12th, 14th, and 15A centuries. They found what they were seeking in a 15th-century text entitled The Book on How to Make All the Color Paints for Illuminating Books. However, translating the instructions was no easy task. It was written in the now extinct Judaeo-Porluguese language, and though the source of the dye was traced back to a plant, no name was mentioned.

However, by piecing together suggestions from the text the scientists were able to determine that the dye was made from the bluish-green berries of the chrozophora tinctoria plant. After an extensive search, the team found a few varieties of the plant growing along the roadside near the town of Monsaraz in south Portugal.

The detailed instructions gave the researchers critical clues — including the best time to pick the berries. "You need to squeeze the fruits, being careful not to break the seeds, and then to put them on linen. The scientist says the detail was important since broken seeds polluted the pigment producing an inferior quality ink. The dyed linen, which was left to dry, was an efficient way to store and transport the pigment during ancient times. When needed, the artist would simply cut off a piece of the cloth and dip it with water to squeeze out the blue color.

Once the key ingredient had been identified, the researchers began to determine the dye's molecular structure. To their surprise, they found that folium was not like any other known permanent blue dyes — it was an entirely new class of color, one they named chrozophoridin. "Chrozophoridin was used in ancient times to make a beautiful blue dye for painting. " the team wrote in the study. "Thus, we believe that this will not be our final word on this amazing plant and its story and that further discoveries will follow soon."

I travel a lot, and I find out different "styles" (风格)of directions every time I ask "How can I get to the post office?"

Foreign tourists are often confused(困惑的)in Japan because most streets there don't have names; in Japan, people use landmarks(地标)in their directions instead of street names. For example, the Japanese will say to travelers, "Go straight down to the corner. Turn left at the big hotel and go past a fruit market. The post office is across from the bus stop. "

In the countryside of the American Midwest, there are not usually many landmarks. There are no mountains, so the land is very flat; in many places there are no towns or buildings within miles. Instead of landmarks, people will tell you directions and distances. In Kansas or Iowa, for example, people will say, "Go north two miles. Turn east, and then go another mile. "

People in Los Angeles, California, have no idea of distance on the map; they measure distance in time, not miles. "How far away is the post office?" you ask. "Oh," they answer, "it's about five minutes from here. ''You say, "Yes, but how many miles away is it?" They don't know.

It's true that a person doesn't know the answer to your question sometimes. What happens in such a situation? A New Yorker might say, Sorry, I have no idea. But in Yucatan, Mexico, no one answers "I don't know. " People in Yucatan believe that "I don't know" is impolite. They usually give an answer, often a wrong one. A tourist can get very, very lost in Yucatan!