(See Corrections & Amplifications item below.)  In May, Florida Marlins rookie Chris Coghlan realized an American dream, his first major-league home run.  The rude awakening came quickly when it landed in the glove of Nick Yohanek, a professional "ballhawk."  Zack Hample geared up to go after a fly ball during batting practice before a Philadelphia Phillies game against the Cincinnati Reds at Citizens Bank Park in Philadelphia.  Ballhawks go to games with the intention of grabbing any ball that goes in the stands, especially milestone home runs like a player's first or 500th. Most then refuse to give them back to the player unless he coughs up something valuable in return, from a signed bat or a jersey to up to $10,000. This breed of sophisticated collector now appears more determined than ever, stalking everyone from rookies like Mr. Coghlan to veteran stars such as Ken Griffey Jr.  When Marlins official Marty Sewell asked Mr. Yohanek to return Mr. Coghlan's ball, Mr. Yohanek said he asked for a bat and a ball signed by Mr. Coghlan plus a signed bat from Marlins star shortstop Hanley Ramirez. Mr. Coghlan had no idea there was such a thing as a ballhawk and was surprised by the intensity of the negotiations. Mr. Yohanek "wasn't the most polite or respectful guy about the whole process," Mr. Coghlan told the Palm Beach Post. "He told me he goes around a lot and catches these balls and holds them for ransom."  For his part, Mr. Yohanek says, "I knew I'd be able to ask for something in return....I wasn't going to give the ball away for nothing."  One weapon in the ballhawk's arsenal is research: which players are on the verge of landmark home runs, where they're likely to hit them based on which side of the plate they hit from, the dimensions of the ballpark and even details about the opposing pitcher. Mr. Yohanek, a 30-year-old law-enforcement officer in Milwaukee, updates a "milestone tracker" on his Web site (www.happyyoungster.com/milestone_tracker) so he and other ballhawks know which players to track.  Mr. Yohanek says he has caught 50 home-run balls since 1998. He has caught a few milestone balls, including Geoff Jenkins' 200th and Jeromy Burnitz's 100th, but says he has never traded them for anything more than signed bats, balls or tickets.  Fans who catch a ball in the stands have a right to keep it. In the past, when a player or team wanted a ball back, they'd find the fan who caught it and offer a handshake with a big leaguer and a different autographed ball in return. Team officials, however, say a boom in the sports-memorabilia business has caused fans who catch important balls to think they won the lottery.  After Mark McGwire hit his 70th home run in the 1998 season, Philip Ozersky, the man who caught it, got $3 million for it at auction. Before that, the most expensive ball ever auctioned was the first homer in the old Yankee Stadium, hit by Babe Ruth; it brought $126,500, according to Sports Collectors Digest.  There have been several cases of disputes between fans claiming ownership. When Barry Bonds was obliterating home-run records in the early 2000s, lawsuits erupted among fans over the ownership of his 73rd in a single season and the 700th of his career. This helped spur Major League Baseball's decision to employ two memorabilia authenticators at every park.  In San Diego last year, Adam Dunn, then an outfielder with the Cincinnati Reds, smashed his 250th homer. Tracy Collins, a self-described ballhawk and newspaper salesman, grabbed it. When a Cincinnati Reds representative came to him in the stands to ask for it back, Mr. Collins said he wanted Mr. Dunn to give him a signed ball. Mr. Collins said he was told the slugger wasn't available, so he didn't surrender it until he got a ball from Mr. Griffey, a Reds player at the time.  A Reds spokesman, Rob Butcher, doesn't remember that occasion, but said fans would "frequently" catch milestone balls and refuse to relinquish them until they got something signed by Mr. Griffey, regardless of who actually hit them. He said Mr. Dunn was always willing to sign items, no matter the circumstance.  A spokesman with Mr. Dunn's current team, the Washington Nationals, said the slugger doesn't remember the ball standoff. But when the fan who caught Mr. Dunn's 300th home run last week offered it back to the slugger for free, Mr. Dunn gave him a signed jersey, three signed balls and tickets for later games.  "Sometimes when they catch a valuable ball, the money is the important thing," said Mr. Griffey.  Joe Scherer, a fan from Fort Lauderdale, Fla., caught Mr. Griffey's 600th home run last year in Florida, but wouldn't hand it over. He got $42,000 for it in an auction, adding he sold it because he was strapped for cash. Mr. Scherer "wanted a few things that were out of hand," Mr. Griffey said, without giving specifics.  Zack Hample, a New York writer, has been to 46 stadiums to shag balls.  The record-breaking balls make headlines, but it's the lesser-known milestones that the ballhawks make a killing on. By tracking players' home run totals and where they hit them, ballhawks snatch balls that most fans don't realize are valuable.  In 2008 against the Marlins in Oakland, then-Athletics rookie Carlos Gonzalez, who had reached the majors just three weeks earlier, hit his first career dinger. Tom Snyder, a ballhawk, nabbed it. A 24-year-old from Northern California who's been hawking for about 12 years, Mr. Snyder thought he could get a jersey and two signed bats for his friend and brother in a swap for the ball.  An Oakland official, Bob Rose, said the team turned down the jersey request, saying uniforms take too long to replace. Mr. Rose said Mr. Snyder then asked for $10,000. When that was shot down, Mr. Snyder left with the ball before handing it over about a week later for a signed photo of himself with Mr. Gonzalez.  "I had nothing against him, I just really wanted a jersey and a couple bats," Mr. Snyder said. Added Mr. Rose: "We're not in the business of intense negotiations."  Mr. Yohanek says he gives away about half of the non-homers he reels in. He has season tickets in Milwaukee and moves around from section-to-section during games to get in premium position to catch balls. He is a charter member of the Ballhawk League, which formed this year. It has more than a dozen members, with competitors angling to top each other in total balls caught.  Zack Hample, 31, a New York City writer, says he has been to 46 stadiums and caught balls in every one, keeping most for his personal collection. He flew to Miami and Philadelphia last year to chase Mr. Griffey when he was on the cusp of 600. He charges hawk-hopefuls $500 to attend games with him, and said he averages a couple of clients a month.  Yet he's never grabbed a ball a player wanted back. "But I already know a couple things I'd ask for," he says. "We're already talking about A-Rod's 800th home run."  Write to David Biderman at David.Biderman@wsj.com  A photo that accompanied this article on "ballhawks" who stake out baseball games to snare home-run balls was taken by Brandon Sloter, not Scott Lewis as was incorrectly reported.
Ballhawks stake out stadiums with hopes of grabbing any balls that fly into the stands -- especially milestone balls. These fans will trade balls back to players, but for a price.