Bitcoin miners have been forced to tap into their cryptocurrency stashes as a plunge in prices, rising energy costs and increased competition bite into profitability.The number of coins miners have transferred to crypto exchanges has steadily increased since June 7, researchers at MacroHive said.According to analysis by Arcane Research, several publicly traded bitcoin miners collectively sold more than 100% of their total production in May as bitcoin's value plummeted by 45%."The falling profitability of mining forced these miners to increase their sales rate to more than 100% of their production in May. Conditions worsened in June, which means they are likely to sell even more," Jaran Mellerud, an analyst at Arcane, said.According to CoinMetrics data, bitcoin miners, who operate computer networks to earn tokens by validating transactions on the blockchain, are typically staunch crypto HODLers and collectively own around 800,000 bitcoins.In 2021, the crypto-mining space expanded rapidly as Bitcoin more than quadrupled in value. However, the growth has put further pressure on margins as the process is expected to become more difficult as the number of miners increases."Over the past six months, hash rate and mining difficulty have increased while the price of bitcoin has dropped. These are both negatives for existing miners as both work to compress margins," Joe Burnett, analyst at bitcoin mining firm Blockware Solutions, said.High energy prices are also hitting miners.Bitfarms (BITF.TO), Riot Blockchain (RIOT.O) and Core Scientific (CORZ.O) are among companies that announced sales. Bitfarms' chief executive officer said the company is "no longer HODLing daily bitcoin production."Shares of publicly listed miners have been battered even more than bitcoin, with the Valkyrie Bitcoin Miners ETF falling by 59% this quarter compared to 53% drop for bitcoin.