DOI: 10.5061/dryad.rxwdbrvq3
title: Supplemental feeding as experimental tool to understand why birds do not lay earlier in the season

description: 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,
 focusing on between-year variation in the response to supplemental feeding
 within the same studies. For 11 out of 19 species, at least two estimates
 were available, and effects of food supplementation clearly varied between
 species. The three studies done in the tropics showed a large effect of
 supplemental feeding, while in the temperate zone population, this effect
 was about one week for resident species, and small and non-significant for
 migrant species. We argue that it is crucial that studies report on the
 ecological conditions during the year(s) of the study to aid
 interpretation of the results. We conclude that, in general, there is an
 effect of supplemental feeding on the timing of reproduction in most
 resident birds, but that the variation in the response needs to be
 explored further.
description: # 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 open a spreadsheet,
 such as Excel, is recommended. #### Access information Other publicly
 accessible locations of the data: * Metadata of study
 populations: [https://spibirds.org/en](https://spibirds.org/en) Data was
 derived from the following sources: * Access database of NIOO-KNAW (for
 Great tits) * Access database of the University of Groningen (for Pied
 flycatchers)