Spatiotemporal variation of the chick mass-brood size trade-off in the little owl

Resources are limited and cannot be used for multiple functions simultaneously, leading to trade-offs between life-history traits. The detection of such trade-offs depends on resource acquisition (total amount of resources) and allocation (distribution of resources between traits). When among-individual variation in acquisition exceeds that of allocation, trade-offs tend to be masked. The availability of food resources is shaped by environmental conditions, and thus is likely to vary over time and space. Thus, notable differences can arise in the strength but also the detection of trade-offs within a single species. Yet, studies spanning multiple years and assessing trade-offs among several natural populations within an acquisition-allocation framework remain relatively limited. We studied the well-known chick mass-brood size trade-off in more than 2,000 broods in three populations of little owl (Athene noctua) during 11 years, to better understand spatiotemporal changes in life-history trade-offs. We found that the negative correlations between brood size and chick mass were detected in most but not all years in the three populations. The larger the variation in allocation compared to variation in acquisition, the more pronounced the trade-off was, and the pattern was similar for all populations. These results indicate that trade-offs are detectable among individuals across different ecological conditions, and that the detection is strongly influenced by the variance in acquisition. A promising avenue of research would be to include intrinsic factors known to influence chick mass-brood size trade-offs, such as maternal age, breeding experience, or body condition.