Supplemental feeding as experimental tool to understand why birds do not lay earlier in the season

The timing of avian reproduction might be affected by the costs of egg production in early spring. To study these costs, supplemental feeding experiments have been conducted where birds are provided with food prior to egg laying. We discuss the opportunities and pitfalls of such experiments. We stress that supplemental feeding experiments alter the conditions for the birds and hence can be used to measure the response in laying date, but not to determine whether this response is adaptive. We present two experiments, one on pied and one on great tits, both showing no effect of supplemental feeding on laying date, but in great tits, laying was affected in an additional treatment where night temperatures in the nest boxes were manipulated. For both studies, we present ample data on the ecological conditions of the year in which the experiments were carried out. We furthermore present a mini-review of 31 studies on food supplementation in passerines that examined effects on laying date, foc..., , # Supplemental feeding as experimental tool to understand why birds do not lay earlier in the season

Dataset DOI: [10.5061/dryad.rxwdbrvq3](https://doi.org/10.5061/dryad.rxwdbrvq3)

#### Description of the data and file structure

Data on two food supplementation experiments on passerine birds and the effect on the date of egg laying.

Studies are:

Great tits (Parus major) at the population called Hoge Veluwe, in the year 1994.

Pied flycatchers (Ficedula hypoleuca) at the population called SW-Drenthe (NL) in the years 2015, 2021, and 2022.

#### Files and variables

**File: Both_Visser_Data_Food_Experiments_To_Dryad.xlsx**

Data are reported on the level of individual nests.

In the Excel sheet, there are four tables:

1. GreatTit_Data: raw data per nest
2. GT_Explanation: explanation of all the variable names
3. PiedFlycatcherData: raw data per nest
4. PF_Explanation: explanation of all the variable names

* Missing values are coded as NA

#### Code/software

Any program that will o...,