Understanding trade-offs in wild populations is difficult, but important if we are to understand the evolution of life histories and the impact of ecological variables upon them. Markers that reflect physiological state and predict future survival would be of considerable benefit to unravelling such trade-offs and could provide insight into individual variation in senescence. However, currently used markers often yield inconsistent results. One underutilised measure is haematocrit, the proportional of blood comprising of erythrocytes, which relates to the blood’s oxygen-carrying capacity and viscosity, and to individual endurance. Haematocrit has been shown to decline with age in cross-sectional studies (which may be confounded by selective appearance/disappearance). However, few studies have tested whether haematocrit declines within-individuals or whether low haematocrit impacts survival in wild taxa. Using longitudinal data from the Seychelles warbler (Acrocephalus sechellensis), we demonstrated that haematocrit increases with age in young individuals (<1.5 years) but decreases with age in older individuals (1.5–13 years). In breeders, haematocrit was higher in males than females and varied relative to breeding stage. High haematocrit was associated with lower survival in young individuals, but not older individuals. Thus, while we did not find support for haematocrit as a marker of senescence, high haematocrit is indicative of poor condition in younger individuals. Possible explanations are that these individuals were experiencing dehydration and/or high endurance demands prior to capture, which warrants further investigation. Our study demonstrates that haematocrit can be an informative metric for life-history studies investigating trade-offs between survival, longevity and reproduction.

The dataset was collected from 2003 to 2017 following repeated (bi-annual) visits to the study site (Cousin Island, Seychelles). Haematocrit levels were obtained from individual birds (Seychelles warbler; Acrosephalus sechellensis) caught in mist nets. Indivudals were caught repeatedly across the study period - resulting in longitudinal haematocrit data. Sex of individuals was determined molecularly via blood sampling. Behavioural data (status, breed group, lay date) were deterimed from observatational montinoring of individuals (made possible by unique colour-banding) during bi-annual vivsits (1 - 3 months in duration). All data was inputted, managed and extracted using the central Access dataset of the Seychelles warbler project and analysed in Rstudio. 

Each row corresponds to a unique haematocrit value from a unique capture event of an individual. Column names are largely self-explanatory.

BirdID is a unique number assigned to each individual in the population

Status refers for the social status of an individual within its breed group (Dom = Dominant Breeder, Sub = Subordinate).

Age refers to the age of the individual at capture.

Missing values in the CatchTime column means that the time of capture was not recorded.

BreedGroupID is a unique number assigned to a group of associated individuals (i.e. interacting around the same territory/nest) during a field season.

NearestLayDate is the closest date from the capture date (OccasionDate) at which the domiant breeding female of the individuals breed group laid an egg. DaysFromEgg is the number of days between the capture date and lay date. Negative values of DaysFromEgg means that the individual was caught before the egg was laid, while positive values indicate that the individual was caught after. Missing values in these colums means that an egg was not observed for that breed group during that field season.

LastSeen is the last date of the last fieldseason for which the individual was observed (i.e. death date).