Data from: Singing from North to South: latitudinal variation in timing of dawn singing under natural and artificial light conditions

1. Animals breeding at northern latitudes experience drastic changes in daily light conditions during the breeding season with decreasing periods of darkness, whereas those living at lower latitudes are exposed to naturally dark nights throughout the year. Nowadays, many animals are also exposed to artificial night lighting (often referred to as light pollution). 2. Animals strongly rely on variation in light levels to time their daily and seasonal behaviour. Previous work on passerine birds showed that artificial night lighting leads to earlier onset of dawn song. However, these studies were carried out at intermediate latitudes with more limited seasonal changes in daylength, and we still lack an understanding of the impact of artificial night lighting in relation to variation in natural light conditions. 3. We investigated the influence of natural and artificial light conditions on the timing of dawn singing in five common songbird species in each of three regions in Europe that differed in natural variation in daylength (northern Finland, 65°N; southern Germany, 48°N; southern Spain, 37°N). In each region, we selected five peri-urban forest sites with and five without street lighting, and recorded dawn singing at the beginning of the local breeding season. 4. Our results show that the earliest natural singers, i.e., European robins (Erithacus rubecula) and common blackbirds (Turdus merula), started dawn singing earlier along with the natural increase in night brightness in Finland, with no additional effects of artificial night lighting. In contrast, the later singers, i.e., great tits (Parus major), blue tits (Cyanistes caeruleus), and chaffinches (Fringilla coelebs), showed similar onsets of dawn song relative to sunrise across the season and similar effects of artificial night lighting at all latitudes. 5. Artificial night lighting affected great tits, blue tits and chaffinches even in northern Finland where nights became very bright. Proximate factors such as differential light sensitivities may explain why early singers showed more plastic behavioural responses to naturally and artificially bright nights. The maintenance of rhythmicity in the late singers during bright northern nights and under artificial night lighting may also be an adaptive response to predation risk or costs of sleep loss.1. 繁殖于高纬度地区的动物在繁殖季会经历每日光照条件的剧烈变化，黑暗时段持续缩短；而栖息于低纬度区域的动物全年都能享受到自然的暗夜环境。如今，诸多动物还会暴露在人工夜间照明（常被称为光污染）之下。 2. 动物高度依赖光照强度的变化来调控自身日常与季节性行为的时间节律。既往针对雀形目鸟类的研究表明，人工夜间照明会导致其晨鸣起始时间提前。然而，这些研究均在昼长季节变化较为有限的中纬度地区开展，目前我们仍未明确人工夜间照明在自然光照条件变异背景下的具体影响。 3. 本研究针对欧洲三个昼长自然变异程度各不相同的区域（芬兰北部，北纬65°；德国南部，北纬48°；西班牙南部，北纬37°），探究了自然与人工光照条件对五种常见雀形目鸟类晨鸣时间的影响。在每个区域中，我们各选取了5处设有街道照明的城郊森林样地与5处无街道照明的对照样地，并在当地繁殖季初期记录鸟类的晨鸣行为。 4. 研究结果显示，自然条件下最早开始晨鸣的欧亚鸲（Erithacus rubecula）与乌鸫（Turdus merula），会随着芬兰地区夜间亮度的自然升高而提前开启晨鸣，且未受到人工夜间照明的额外影响。与之相反，自然条件下较晚开始晨鸣的大山雀（Parus major）、蓝山雀（Cyanistes caeruleus）与苍头燕雀（Fringilla coelebs），在整个繁殖季内其晨鸣起始时间相对于日出的比例保持一致，且在所有纬度区域均表现出相似的人工夜间照明影响效应。 5. 即使在夜间亮度极高的芬兰北部地区，人工夜间照明仍对大山雀、蓝山雀和苍头燕雀产生了影响。诸如光敏感度差异这类近因因素，或可解释早鸣物种为何对自然与人工增亮的夜间表现出更强的行为可塑性响应。而晚鸣物种在高纬度明亮夜间以及人工夜间照明环境下仍维持行为节律，也可能是针对捕食风险或睡眠损失成本的适应性响应。