Note that on 18 April 2020 at 19:00 GMT we replaced the blue tit csv file with a corrected version (see bottom of this page for details) **Instructions:** 1. Download these data and analyse them to find the answer to the question "To what extent is the growth of nestling blue tits (Cyanistes caeruleus) influenced by competition with siblings?" using what ever methods you feel are most appropriate. 2. Write up a summary of your methods as if you were writing it for publication in a journal. 3. When you have completed your analysis, email Tim Parker (parkerth@whitman.edu) and Hannah Fraser (hannahsfraser@gmail.com) to let us know. 4. Fill in the survey that we will email you asking a) about some of your analytic decisions, b) to upload the description of your methods, and c) to upload all of your analysis files (e.g. R code, reformatted data, excel spreadsheet, SPSS syntax, step by step detailed instructions on all the settings you selected in your analytic software of choice) 5. Look over the manuscript as we write it up - you are invited to be a co-author but can opt out if you would prefer. *NOTE: your analyses will be made public when we publish this article but we will ensure that all identifying information is removed so that no one will be able to determine which coauthor conducted which analysis* **Evolutionary Ecology Data Information** Our evolutionary ecology data set is relevant to a sub-discipline of life-history research which focuses on identifying costs and trade-offs associated with different phenotypic conditions. These data were derived from a brood-size manipulation experiment imposed on wild birds nesting in boxes provided by researchers in an intensively studied population. Understanding how the growth of nestlings is influenced by the numbers of siblings in the nest can give researchers insights into factors such as the evolution of clutch size, determination of provisioning rates by parents, and optimal levels of sibling competition. Researchers conducted brood size manipulations and population monitoring of blue tits (Cyanistes caeruleus) at Wytham Wood, a 380 ha woodland in Oxfordshire, U.K (1º 20’W, 51º 47’N). Researchers regularly checked approximately 1100 artificial nest boxes at the site and monitored the 330 to 450 blue tit pairs occupying those boxes in 2001-2003 during the experiments. Nearly all birds made only one breeding attempt during the April to June study period in a given year. At each blue tit nest, researchers recorded the date the first egg appeared, clutch size, and hatching date. For all chicks alive at age 14 days, researchers measured mass and tarsus length and fitted a uniquely numbered, British Trust for Ornithology (BTO) aluminium leg ring. Researchers attempted to capture all adults at their nests between day 6 and day 14 of the chick-rearing period. For these captured adults, researchers measured mass, tarsus length, and wing length and fitted a uniquely numbered BTO (British Trust for Ornithology) leg ring. During the 2001-2003 breeding seasons, researchers manipulated brood sizes using cross fostering. They matched broods for hatching date and brood size and moved chicks between these paired nests one or two days after hatching. They sought to either enlarge or reduce all manipulated broods by approximately one fourth. To control for effects of being moved, each reduced brood had a portion of its brood replaced by chicks from the paired increased brood, and vice versa. Net manipulations varied from plus or minus four chicks in broods of 12 to 16 to plus or minus one chick in broods of 4 or 5. Researchers left approximately one third of all broods unmanipulated. These unmanipulated broods were not selected systematically to match manipulated broods in clutch size or laying date. We have mass and tarsus length data from 3720 individual chicks divided among 167 experimentally enlarged broods, 165 experimentally reduced broods, and 120 unmanipulated broods. Note on corrected data set posted 18 April 2020: Two nest had the treatment (rear_nest_trt) mislabeled. One (rear_nest_breed_ID = 202091) was label as enlarged (rear_nest_trt = 5) that should have been marked as reduced (rear_nest_trt = 6), and one (rear_nest_breed_ID = 202066) was marked as reduced that should have been marked as enlarged. These have been corrected. Twelve nests that lacked data for number of chicks removed (rear_Cs_out), number of chicks added (rear_Cs_in), and the net change in number of chicks due to manipulation (net_rearing_manipulation) had ‘0’ listed for those three variables instead of ‘.’ indicating missing data. We have changed the ‘0’ to ‘.’ for those three variables for rows with the rear_nest_breed_ID of: 201259 201137 201105 201409 202295 203220 203243 203021 203235 203067 201129 201230