We investigated the consequence of no-night environment (constant light, LL) on reproductive performance in zebra finches in the parent (P) and subsequent (F1) generation. As a measure of the overall effects on metabolic reproductive health, we monitored daily activity behaviour, recorded song and cheek patch size in males, and measured body size and hormone levels. As compared with controls under 12 h light:12 h darkness (12 h:12 h LD), both P and F1 pairs showed a compromised reproductive success, as evidenced by fewer fledglings and fewer viable offspring with longer fledging durations, and increased offspring mortality with three successive clutches under LL. The overall negative effect of the no-night environment was increased in the F1 generation. As compared with P pairs, F1 pairs had more failed nesting and breeding attempts, took longer to initiate reproduction, incubated fewer eggs, produced fewer viable offspring with longer fledging duration, and showed increased offspring mortality. Consistent with negative reproductive effects, P males showed significant changes in the motif duration and other spectral features of song, and both F1 and F2 males copied poorly the song of their parent under LL. Plasma corticosterone and sex hormone (testosterone in males and oestradiol in females) levels were significantly lower under LL. Daily plasma melatonin rhythm persisted but with a reduced amplitude under LL. These results demonstrate the importance of night in reproduction in a continuously breeding diurnal species, and give insight into the possible impact on physiology of animals whose surrounding environment is consistently losing the darkness of night.

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