Dave Enstrom can tell when sex is going on in the neighborhood by the sounds picked up from the tiny microphones he's planted on his randy subjects.

When they're doing "it," they invariably sing, whether it's really, really good or not.

They're birds, after all. Cardinals in particular – although this day Enstrom, a scientist with the Illinois Natural History Survey, is wearing a Chicago Cubs T-shirt.

Enstrom and Mike Ward, another survey research scientist and a University of Illinois natural resources and environmental sciences professor, are interested in the movement patterns of the birds – where they go and why, especially during the breeding season – and how the environment around them affects that.

The birds are supposed to be territorial during breeding and have been seen as sweetly monogamous. But recent genetic studies indicate there's a virtual bird Wisteria Lane lurking beneath the surface. Of offspring tested in various nests, something like 30 percent have different fathers.

Enstrom also is interested in the functions of cardinal songs. He's logged 30 different sounds so far for males and a couple dozen for females, some of them shared and others seemingly unique to gender.

Cardinals, along with orioles, are among the fairly rare birds whose females actually sing, which is mostly a male trait in the bird world.

"They both sing a lot in the early spring," Enstrom said. "They sing for hours every day."

The idea is to associate those songs with the birds' locations, proximity to each other and their interactions within and between territories, to get a better feeling for the purposes of the singing.

Besides songs, the microphone system also collects ambient noise, allowing things like vehicle traffic, hawks and coyotes to be factored into results from studying the data.

To track the birds' movements and listen to their singing, Enstrom, Ward and colleagues are fitting cardinals on the UI South Farms with miniature transmitters – the devices weigh 1.3 grams, slightly more than a dollar bill, battery included – that broadcast the position of the birds and their singing.

"Each bird is an AM radio station," Enstrom said.

They catch the birds in feeder traps or mist nets and basically glue the transmitters on with the same stuff used to attach false eyelashes. They're also testing an unobtrusive, break-away harness, which might be better for winter use.

Ward – who balances the team as a St. Louis Cardinals fan – said tests of stress hormones show the birds don't even notice the minimalist cargo after a day.

The transmitters simply fall off after a few weeks. Their Global Positioning System capability allows the researchers to recover and reuse them, however – a good thing at $300 a pop.

Enstrom has looked some at local cardinals in other locations, including Busey Woods in Urbana. But he and Ward are concentrating on the South Farms for a couple of reasons.

The area is representative of the environment faced by the birds all around Illinois, for one thing.

"It's what Illinois is," Ward said. "It's agriculture. It's little pieces of habitat here and there."

Moreover, past studies – and campus wireless communications projects – have equipped the South Farms with radio towers where the researchers can mount receivers to pick up the signals from the transmitters on the birds, which have a maximum range of about a mile. The sound files generated are big, and the UI locale also has the computer-related infrastructure needed to move all that data efficiently.

To analyze the data, Enstrom and Ward are getting help from data mining experts at the UI-based National Center for Supercomputing Applications and the Graduate School of Library and Information Science. That includes some researchers whose focus is automatic analysis and classification of music generated by humans.

"There are a lot of similarities," said UI Professor Stephen Downie, who's been working on ways to make digital music collections searchable by characteristics like brightness and loudness.

Among other things, birds, like musicians, produce well-pitched sound, and their songs have discernible themes or motifs, said Downie, a professor in the library school whose specialty is music information retrieval.

While cardinals are the focus at this point, Ward said the techniques that result could be employed to study other birds and other types of wildlife, and to examine issues such as the spread of animal-borne diseases, such as West Nile virus. They're already looking at robins in a limited fashion.

In the future, the transmitters also could be fitted with other types of sensors, to measure heart rate, for instance, barometric pressure, or light.

"There's all kinds of stuff with this technology you can address," Ward said.