Since 1997, a population of the Pine Processionary Moth Thaumetopoea pityocampa exhibited a shifted life cycle in a restricted area of a managed pine forest, at the central west coast of Portugal. Rather than during the regular winter period, larval development of this novel population occurs during the summer. We indicate the populations accordingly as the Winter Population (WP) and the Summer Population (SP). We quantified the numerical response of Great Tits Parus major to irruptions of the SP. In the years following an irruption, Great Tits were more abundant in the SP area, than in two similar non-infested forest areas. This was particularly pronounced during summer and autumn, at the time when moths, eggs and larvae of the SP were available for bird predation. A nestbox study allowed us to verify that the SP moth emergence period coincided with egg laying and raising of second broods in Great Tits, and higher reproductive rates were recorded in the SP than in the WP area. To our knowledge, this is the first study to document a response of a bird predator to a local T. pityocampa irruption, which was even more special as it concerned an insect population with a large shift in its life cycle. Our results show that the Great Tit is able to respond to local outbreaks of forest defoliator insects, and we suggest a potential impact of Great Tit predation on the population dynamics of the SP.