Breeding phenology and postnatal development data of northern bats (Eptesicus nilssonii)

Bats inhabiting northern latitudes are faced with short reproductive seasons during which they must produce and rear pups before fattening up in time to survive the winter hibernation. Therefore, the timing of parturition has considerable impacts on future fitness prospects for mother and pup. However, little is known about individual variation in breeding phenology and its consequences for postnatal development within bat populations. Here, we studied the phenology of breeding in Eptesicus nilssonii across seven years using data collected by day-to-day monitoring of a breeding colony in Norway (60.1°N) for which the identity and age of each mother (N = 8) and pup (N = 28) were known. Using mixed-effect models, we found that arrival at the colony was influenced by temperature conditions from mid-April to mid-May across all females, but that there were strong and consistent individual differences in arrival- and parturition time across years. Females generally arrived ~32.1 days before g..., , , # Breeding phenology and postnatal development data of northern bats (Eptesicus nilssonii)

[https://doi.org/10.5061/dryad.8sf7m0czk](https://doi.org/10.5061/dryad.8sf7m0czk)

## Description of the data and file structure

Data on breeding phenology and postnatal development in Eptesicuss nilssonii, collected in a small maternity colony in Nittedal (Norway) between 2017-2023. Missing values in the dataset are indicated with NA. The excel-file contains 8 spread-sheets, described in more detail below:

**Spreadsheet Baby growth**

A dataset which contains details on the development (forearm-length and body mass) of each pup born during the study period. The variables are the following:

Year: the year the observation was made

TimeDate: the date and time of the observation

BabyBorn: the date the observed pup was born

Baby: ID of the pup

Sex: sex of the pup

HoursOld: age of pup at observation, reported in hours (negative values indicate time before pup was born)

DaysOld: age of pup at...