Interpreting faecal nitrogen as a non-invasive indicator of diet quality and body condition in contexts of high ungulate density

Understanding how non-invasive indicators of diet quality, such as faecal nitrogen (FN), relate to population performance is a valuable asset when assessing the management of wild ungulates. However, the relationships between FN and protein intake may depend on the ingestion of less palatable plants containing high quantities of tannin (which is known to reduce degradable proteins) and management schemes (such as supplemental feeding). In order to describe the factors and potential use of FN as an indicator of diet quality, we selected a range of red deer (Cervus elaphus) densities in Mediterranean habitats in South Central Spain, which were classified under different management regimens (with and without food supply). We specifically aimed to (i) identify the main population (management, density) and diet quality factors related to FN levels, (ii) assess the relationship between individual performance (body condition measured as kidney fat index, KFI) and FN and (iii) provide evidence of the modulating role that management may play in this relationship. FN was positively associated with the faecal tannin concentration and the density of deer. Interestingly, a positive correlation was observed between FN and nitrogen in plants on hunting estates with a supplemental food supply, whereas this relationship was not evident in the absence of supplemental feeding. Moreover, KFI was negatively associated with FN, and this negative association was more marked in the presence of supplemental feeding. This study exemplifies that caution is necessary when the interpretation of individual FN values is used to monitor the performance of red deer populations, particularly in high density contexts with a reduced quality of forage, and when supplemental food is provided. However, when FN is interpreted in concomitance with body condition, population parameters and diet quality, it has been proved to be a potentially simple, cost-effective and reliable indicator of biological performance and diet quality.