Data from: Food availability and predation risk, rather than intrinsic attributes are the main factors shaping the reproductive decisions of a long-lived predator

Deciphering the causes of variation in reproductive success is a fundamental issue in ecology, as the number of offspring produced is an important driver of individual fitness and population dynamics. Little is known however, about how different factors interact to drive variation in reproduction, such as whether an individual's response to extrinsic conditions (e.g. food availability or predation) varies according to its intrinsic attributes (e.g. age, previous allocation of resources towards reproduction). We used 29 years of reproductive data from marked female tawny owls and natural variation in food availability (field vole) and predator abundance (northern goshawk) to quantify the extent to which extrinsic and intrinsic factors interact to influence owl reproductive traits (breeding propensity, clutch size and nest abandonment). Extrinsic and intrinsic factors appeared to interact to affect breeding propensity (which accounted for 83% of the variation in owl reproductive success). Breeding propensity increased with vole density, although increasing goshawk abundance reduced the strength of this relationship. Owls became slightly more likely to breed as they aged, although this was only apparent for individuals who had fledged chicks the year before. Owls laid larger clutches when food was more abundant. When owls were breeding in territories less exposed to goshawk predation, 99.5% of all breeding attempts reached the fledging stage. In contrast, the probability of breeding attempts reaching the fledging stage in territories more exposed to goshawk predation depended on the amount of resources an owl had already allocated towards reproduction (averaging 87.7% for owls with clutches of 1-2 eggs compared to 97.5% for owls with clutches of 4-6 eggs). Overall, our results suggested that changes in extrinsic conditions (predominantly food availability, but also predator abundance) had the greatest influence on owl reproduction. In response to deteriorating extrinsic conditions (fewer voles and more goshawks) owls appeared to breed more frequently, but allocated fewer resources per breeding attempt. However, intrinsic attributes also appeared to have a relatively small influence on how an individual responded to variation in extrinsic conditions, which indicates that reproductive decisions were shaped by a complex series of extrinsic and intrinsic trade-offs.

解析繁殖成功率（reproductive success）变异的成因，是生态学领域的基础性核心议题——生物产生的后代数量，是调控个体适合度（individual fitness）与种群动态（population dynamics）的关键驱动因素。然而目前学界对不同因子如何交互作用调控繁殖变异的认知仍较为匮乏，例如个体对外源环境条件（extrinsic conditions，如食物可获得性或捕食压力）的响应，是否会随其内在属性（intrinsic attributes，如年龄、过往繁殖资源分配情况）的不同而产生差异。本研究依托29年的环志雌性灰林鸮繁殖数据，结合食物可获得性（以田鼠（field vole）为指示物）与捕食者种群丰度（以北方苍鹰（northern goshawk）为指示物）的自然变异，量化了外源与内源因子交互作用对灰林鸮繁殖性状（繁殖倾向（breeding propensity）、窝卵数（clutch size）与弃巢行为（nest abandonment））的影响程度。研究发现，外源与内源因子的交互作用显著影响了灰林鸮的繁殖倾向，该性状可解释灰林鸮繁殖成功率变异的83%。繁殖倾向随田鼠种群密度升高而增强，但北方苍鹰种群丰度的提升会削弱这一正向关联。灰林鸮的繁殖概率会随年龄增长略有提升，但这一现象仅在前一年成功育出幼雏（fledged chicks）的个体中显著存在。食物资源越充足，灰林鸮的窝卵数越大。在捕食风险较低（受北方苍鹰捕食威胁更小）的繁殖领地中，灰林鸮99.5%的繁殖尝试均可成功让幼雏出飞；与之相反，在捕食风险较高的繁殖领地中，繁殖尝试成功让幼雏出飞的概率，取决于该灰林鸮此前已投入的繁殖资源量：窝卵数为1~2枚的个体，其繁殖成功率平均为87.7%，而窝卵数为4~6枚的个体则可达97.5%。总体而言，本研究结果表明，外源环境条件的变化（主要为食物可获得性，其次为捕食者种群丰度）对灰林鸮繁殖的影响最为显著。当外源环境条件恶化（田鼠数量减少、北方苍鹰种群增多）时，灰林鸮的繁殖频率看似有所提升，但单次繁殖尝试投入的资源量却有所减少。不过，内在属性对个体响应外源环境变异的影响相对较小，这表明繁殖决策是由一系列复杂的外源与内源权衡（trade-offs）过程共同塑造的。