Is hidden demographic heterogeneity pervasive? Harvested birds as an example
Individual demographic heterogeneity refers to the fact that individuals from a given population exposed to similar environmental conditions express different demographic traits or tactics. This heterogeneity can either be linked to factors/states such as age, sex or body condition, or be due to hidden factors/states not immediately apparent or easily characterized (hereinafter referred to as 'hidden heterogeneity'). Yet accounting for demographic heterogeneity, and more particularly heterogeneity in survival is crucial because it is an important mechanism underlying compensation processes between natural and harvest mortality. In this study, we used capture-mark-recapture-recovery data of 6 hunted species covering a large range of the demographic slow-fast continuum (61,432 individuals ringed from 1953 to 2020 among Common Teal Anas crecca, Mallard A. platyrhynchos, Common Pochard Aythya ferina, Common Snipe Gallinago gallinago, Common quail Coturnix coturnix, and Blackbird Turdus merula), to assess the presence of unexplained heterogeneity in survival with multievent models. We detected hidden survival heterogeneity in most species, even though the differences between the high and low categories of survival were often small. The probability for an individual to display a low vs high survival rate was not related to potentially important individual factors/states such as migration phenology or to morphology. The presence of hidden heterogeneity was pervasive, affecting species distributed along a relatively large spectrum of demographic strategies. This, in our opinion, supports that part of this heterogeneity in survival evidenced in our study is indeed hidden, resulting either from hidden intrinsic (inherent) individual differences of survival among individuals, or hidden extrinsic factors e.g., linked to spatial variations in hunting pressure. Our findings suggest that hidden heterogeneity needs to be explored more thoroughly in huntable species and accounted for in management policies.