Data from: The difference between generalist and specialist: the effects of wide fluctuations in main food abundance on numbers and reproduction of two co-existing predators

Specialist individuals within animal populations have shown to be more efficient foragers and/or to have higher reproductive success than generalist individuals, but interspecific reproductive consequences of the degree of diet specialisation in vertebrate predators have remained unstudied. Eurasian pygmy owls (hereafter POs) have less vole-specialised diets than Tengmalm’s owls (TOs), both of which mainly subsist on temporally fluctuating food resources (voles). To test whether the specialist TO is more limited by the main prey abundance than the generalist PO, we studied breeding densities and reproductive traits of co-existing POs and TOs in central-western Finland during 2002-2019. Breeding densities of POs increased with augmenting densities of voles in the previous autumn, whereas breeding densities of TOs increased with higher vole densities in both the previous autumn and the current spring. In years of vole scarcity, PO females started egg-laying earlier than TOs, whereas in years of vole abundance TO females laid eggs substantially earlier than PO females. The yearly mean clutch size and number of fledglings produced of both POs and TOs increased with abundance of voles in the current spring. POs laid large clutches and produced large broods in years of both high and low vole abundance, whereas TOs were able to do so only in years of high vole abundance. POs were able to raise on average 73% of the eggs to fledglings whereas TOs only 44%. The generalist foraging strategy of POs including flexible switching from main prey to alternative prey (small birds) appeared to be more productive than the strictly vole-specialized foraging strategy of TOs. In contrast to earlier studies at the individual-level, specialist predators at the species level (in this case TOs) appear to be less effective than generalists (POs), but diet specialisation was particularly costly under conditions when scarcity of main foods limited offspring production.

动物种群中的特化个体相较于泛化个体，通常具备更优异的觅食效率与/或更高的繁殖成功率，但目前针对脊椎动物捕食者的食性特化程度所带来的种间繁殖后果，仍未得到系统研究。相较于腾氏猫头鹰（Tengmalm’s owls，以下简称TOs），欧亚鸺鹠（Eurasian pygmy owls，以下简称POs）的食性特化程度更低，二者均主要以随时间动态波动的田鼠类资源为主要食物来源。为验证特化型捕食者TO是否较泛化型捕食者PO更受主要猎物丰度的制约，我们于2002年至2019年期间，在芬兰中西部地区开展了同域分布POs与TOs的繁殖密度及繁殖性状研究。POs的繁殖密度随前一年秋季田鼠密度的升高而增加，而TOs的繁殖密度则同时受前一年秋季与当年春季的田鼠密度正向影响。在田鼠匮乏年份，PO雌性个体的产卵起始时间早于TO雌性；而在田鼠丰富年份，TO雌性的产卵时间则显著早于PO雌性。POs与TOs的年平均窝卵数及出飞幼鸟数量，均随当年春季田鼠丰度的提升而增加。无论田鼠丰度高低，POs均能产出较大窝卵数并抚育出规模可观的雏鸟群；而TOs仅在田鼠丰富的年份才能达到这一繁殖表现。POs平均可将73%的卵成功抚育至出飞阶段，而TOs的这一成活率仅为44%。POs所采用的泛化觅食策略——包括灵活切换主要猎物与替代猎物（小型鸟类）——相较TOs严格以田鼠为食的特化觅食策略，似乎具备更优异的繁殖成效。与此前基于个体水平的研究结果相悖，在物种水平上的特化捕食者（本研究中的TOs）似乎不如泛化捕食者（POs）高效，但当主要食物匮乏限制子代产出时，食性特化的代价尤为高昂。