Animal Ecology Group Behavioural Ecology and Social Organization, Centre for Ecological and Evolutionary Studies, University of Groningen, Groningen, The Netherlands.
J Anim Ecol. 2012 Jul;81(4):827-37. doi: 10.1111/j.1365-2656.2012.01959.x. Epub 2012 Feb 6.
1. Habitat selection can affect individual fitness, and therefore, individuals are expected to assess habitat quality of potential breeding sites before settlement. 2. We investigated the role of social environment on juvenile dispersal behaviour in the great tit (Parus major). Two main contradictory hypotheses can be formulated regarding social effects on juvenile dispersal as follows: (i) High fledgling density and sex ratio may enhance the intensity of local (kin) competition and, therefore, reduce individual survival chance, enhance emigration and reduce settlement ('repulsion' hypothesis) (ii) Alternatively, high fledgling density and sex ratio may signal high-quality habitat or lead to aggregation and thus increase individual survival chance, reduce emigration and enhance settlement ('attraction' hypothesis). 3. To disentangle positive from negative effects of high density and male-biased sex ratio on dispersal, we manipulated the social composition of the fledgling population in 12 semi-isolated nest-box areas (plots) via a change of fledgling density (low/high) as well as fledgling sex ratio (female-biased/balanced/male-biased) across 3 years. We then tested whether experimental variation in male and female fledgling densities affected variation in local survival, emigration and settlement of juveniles, and whether social effects on survival and dispersal support the 'repulsion' or 'attraction' hypothesis. 4. We found no experimental effects on local survival and emigration probabilities. However, consistent with the 'attraction' hypothesis, settlement was significantly and positively affected by local experimental sex ratio in each of the study years: both male and female juveniles avoided female-biased plots and settled more in plots that were balanced and male-biased the previous year. 5. Our study provides unprecedented experimental evidence that local sex ratio plays a causal role in habitat selection. We suggest that settlers avoid female-biased plots because a high proportion of females may reflect the absence or the low quality of local resources in the habitat. Alternatively, male territory acquisition may be facilitated by a high local density of 'candidate' males, and therefore, juveniles were less successful in settling in female-biased plots.
栖息地选择会影响个体的适应度，因此个体在定居前会评估潜在繁殖地的栖息地质量。
我们研究了社会环境对幼鸟扩散行为的影响。对于社会效应对幼鸟扩散的影响，有两种主要的矛盾假说，如下所示：(i) 较高的幼鸟密度和性别比例可能会加剧局部（亲缘）竞争的强度，从而降低个体的生存机会，增加迁出率并减少定居（“排斥”假说）；(ii) 或者，较高的幼鸟密度和性别比例可能会表明栖息地质量较高，或者导致聚集，从而增加个体的生存机会，减少迁出率并增加定居（“吸引”假说）。
为了将高密度和雄性偏性性别比例对扩散的正效应与负效应分开，我们通过在 3 年内改变幼鸟密度（低/高）以及幼鸟性别比例（雌性偏性/平衡/雄性偏性），在 12 个半隔离巢箱区域（样区）中改变幼鸟种群的社会组成。然后，我们测试了实验中幼鸟雄性和雌性密度的变化是否会影响幼鸟的本地存活率、迁出率和定居率，以及社会效应对存活率和扩散的影响是否支持“排斥”或“吸引”假说。
我们没有发现实验对本地存活率和迁出率的影响。然而，与“吸引”假说一致的是，在研究的每一年中，本地实验性性别比例都显著且正向影响着定居率：雄性和雌性幼鸟都避免雌性偏性的样区，而更倾向于在去年平衡和雄性偏性的样区定居。
我们的研究提供了前所未有的实验证据，证明本地性别比例在栖息地选择中起着因果作用。我们认为，定居者避免雌性偏性的样区，因为高比例的雌性可能反映了栖息地中本地资源的缺乏或质量低下。或者，雄性领地获得可能会受到高本地“候选”雄性密度的促进，因此，幼鸟在雌性偏性的样区中定居的成功率较低。
