DOI: 10.5061/dryad.7wm37pvrp
title: Haematocrit, age and survival in a vertebrate population

description: 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 (&lt;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.
description: 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; <em>Acrosephalus sechellensis</em>) 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. 
description: 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).