“大批出没”可以翻译为“appear in large numbers” 或者 “swarm in large quantities” 。
“appear in large numbers”强调大量出现这一现象，侧重于描述数量众多的主体出现在某个地方。例如：Strange birds have started to appear in large numbers in this area.（奇怪的鸟儿开始大批出没在这个地区 。）
“swarm in large quantities”中 “swarm” 本意有“成群移动；蜂拥”之意，该短语更生动地描绘出大量个体聚集、活动的状态。例如：Insects swarm in large quantities during the rainy season.（雨季时昆虫大批出没。 ）