From her 19th-floor West End studio, Brittney Kirk can see clearly across the city, a commanding perch that sometimes inspires vertigo. Wednesday morning, as temperatures climbed, Kirk decided to crack a window to give her cat, Sugar, some air while she was at work. It was a decision she would regret. Around lunchtime, she received a frightening message from the Animal Rescue League of Boston: They had Sugar. But Sugar was just fine after falling from the open window - a surprisingly common result for cats who fall from great heights.