# Female chronotype relates to lay date but not fitness in an island population of great tits

## TL;DR

This study found that the time of day when female great tits are active does not affect their fitness, but it does influence when they start breeding.

## Contribution

The study provides new insights into the relationship between chronotype and fitness in a wild bird population.

## Key findings

- Chronotype was not significantly related to fitness parameters like the number of fledglings or hatchlings.
- Extremely early and late chronotypes started breeding later in the studied population.
- Results suggest chronotype may be under fluctuating selection rather than strong selection.

## Abstract

Organisms use diel timing mechanisms to anticipate predictable daily environmental fluctuations, such as the start and end of the day light period. There is often ample intraspecific variation in this diel timing, with individuals being consistently active earlier or later than others in the population and therefore having early or late chronotypes. In tit species, early-active males have higher rates of extrapair paternity, and early-active females might have more offspring, thereby increasing their fitness. However, studies on these fitness consequences of chronotype tend to be inconclusive, based on small sample sizes and confounded with seasonal effects on activity timing. Here, we measured the fitness of standardised chronotype in female great tits (Parus major) across three breeding seasons. We extracted activity onsets, when females first left the nest in the morning, from recordings of nest temperatures during incubation and chick provisioning. To account for seasonal and daily variation in the timing of activity, we expressed these onsets relative to the conspecifics active on the same day. The chronotypes of 164 females were tested for differences in fitness and life-history parameters from brood monitoring data. We show that chronotype was not significantly related to fitness parameters, such as the number of fledglings and hatchlings, nor to offspring and female condition. However, extremely early and late chronotypes started breeding later in our population, but not in the re-analysed datasets from three other populations. Our findings suggest that chronotype is not under strong selection, or perhaps under fluctuating selection, allowing high between-individual variation to persist.

The online version contains supplementary material available at 10.1007/s00442-025-05857-3.