In May 2021, we opportunistically observed one Eurasian Jay (Garrulus glandarius) attacking an adult incubating Japanese Tit (Parus minor) and depredating nine tit eggs at a nestbox where a woodpecker had greatly enlarged the entrance. After the predation event, the Japanese tits abandoned the nest. We recommend that when using artificial nest boxes to protect hole-nesting birds, the appropriate entrance size should be proportional to the body size of the target species. This observation gives us a better understanding of the potential predators of secondary hole-nesting birds.

In 2021 from 27^th April to 9^th May, we used a mini digital video camera (CTS-K6, Chuangtianshi Technology Co., Ltd., Shenzhen, China) to monitor the incubating behaviors of Japanese tits in 22 nest boxes from 08:00 a.m. to 17:00 p.m. (GMT + 8). We fixed the camera on the inner lid of the box and adjusted the camera's angle to ensure the recording of the nest cup and entrance hole. The entrance hole of one of the nest boxes had been enlarged by a woodpecker (Dendrocopos spp.) pecking at it, such that the entrance to this box was 7.5 cm in diameter, and the depth from the entrance to the bottom was about 14.0 cm. On 1 May 2021, we recorded Eurasian jay attacking incubating adult and depredated eggs of Japanese tit with a nest hole size of 7.5 cm through the video cameras for the first time.