The tree sparrow passer montanus has lower egg hatching rate than other birds. In previous study, I found that unhatched eggs showed no sign of embryo development, in particular, male embryo failed to develop. Also, breeding density negatively affected embryo development success. These indicate that any factor resulted from breeding density impaired developing male embryo. Here, as one of the factors, I focus on nest site competition frequency. I predicted that nest visits of nest site competitors increases embryo mortality. I attached nest boxes in Ogata village, Akita prefecture, Japan. On the day 2nd or 3rd egg was laid, I recorded number of nest visits of the competitors for 3 hours in the morning. As a result, I found that increasing nest site frequency was negatively correlated with embryo development rate significantly. Moreover, I measured parent behavior; spending time on the nest box and number of nest visits as nest guard behavior. In the consequence, female parent which spent short time and often visited laid many undeveloped eggs. Additionally, when nest site competition was intensely, female parents showed more nest guard behavior. Such female parents might be in high stress level which has negative effect for offspring survive. Furthermore, brood sex ratio should be biased to female because most of mortal eggs were male. In summary, nest site competition frequency affected female parent behavior, maybe grow stress level, and thus male embryos failed to develop embryo. This suggests that secondary sex ratio could be skewed in according to nest site competition frequency.