DOI: 10.5061/dryad.b5mkkwhcb
title: The correlates of intraspecific variation in nest height and nest building duration in the Eurasian blue tit Cyanistes caeruleus

description: Birds build nests primarily as a receptacle to lay their eggs in, but they
 can also provide secondary benefits including structural support,
 camouflage, and adjustment of the microclimate surrounding the eggs and
 offspring. The factors underlying intraspecific variation in nest
 characteristics are poorly understood. In this study, we aim to identify
 the environmental factors that predict nest height variation and the
 duration of nest building in blue tits (Cyanistes caeruleus), evaluating
 latitude, elevation, temperature, and the timing of egg-laying as
 predictors of nest height, while also taking into account female and male
 parental identity. Using 713 nest height observations collected over a
 period of five years along a 220km transect in Scotland, we found that if
 the annual mean timing of egg-laying was earlier, nests were taller.
 However, there was no correlation between nest height and elevation,
 latitude, the minimum temperature in the 14 days pre-egg-laying or the
 phenology of birds within a year. Female parental identity accounted for a
 large amount of variation in nest height, suggesting that individual
 behaviour has an influence on nest structure. We also found that nest
 building duration was shorter in years when egg laying occurred earlier in
 the year, and that across all observations taller nests took longer to
 build. Overall, our results show that blue tits are able to alter their
 nest characteristics based on conditions environmental gradients like
 latitude (in the case of building duration) and the annual mean
 phenological variation timing of egg laying, and that birds build
 relatively taller nests faster.
description: Nest
 data were collected along a 220km transect spanning from Edinburgh
 (55.98°N -3.40°E) to Dornoch (57.89°N -4.08°E) in Scotland from 2014-2018.
 Six to eight nest boxes (front-opening Schwegler 1B with 26mm entrance
 holes located 141mm above the outer bottom of the nest box and a 120mm
 internal diameter) were placed at 44 different sites along this transect;
 40 sites were installed for the period 2014 – 2016 with a further four
 added in 2017. Pre-2017 all but one site had six nest boxes, and post-2017
 the number of nest boxes was increased to eight at most sites. Nest boxes
 were placed at approximately 40m intervals, ca 1.5m off the ground and
 facing away from the prevailing south-west wind. Nest box location was
 determined using a handheld GPS and site elevation was obtained from the
 Google Maps elevation API, with elevations ranging from 10 to 433
 m.a.s.l. Each site was visited every two days from mid-March (2014-2015)
 or early April (2016-2018) until the breeding season had finished (early-
 to mid-June). Nest height
 was measured at each visit from the first visit in which nesting material
 was found in a nest box (either the first or second day of nest building)
 until the first egg was laid, and the dates of commencement of both nest
 building and egg-laying were recorded in ordinal dates. Nest height was
 recorded using a ruler placed at the outside of the nest box to the
 topmost point of the nest, discounting straggly parts. In 2014, nest
 height was measured from the inside of the nest box, but this was changed
 to the current method to improve between-recorder accuracy as the bottom
 of the nest box is more repeatedly determinable than the top of the
 inside. To correct for this difference in technique, the average height of
 the lower ledge of the nest box (30mm) was added to the 2014 height
 measurements. The ordinal
 date of the first egg laid in every nest box was recorded based on whether
 one or two eggs were present, assuming a one-egg-per-day lay-rate. Adult
 birds were caught and individually identified by a metal ring when the
 chicks were over 10 days old. Temperature data was available for each site
 from two iButton dataloggers (Maxim: DS1922L) accurate to a resolution of
 0.0625°C and placed at opposite ends of each site from mid-February,
 recording every hour. They were placed in water-tight cylinders on the
 north side of trees, with a hole at the bottom to allow ambient air
 circulation. The "Nest
 Height Data" file includes three fixed effect predictors that
 capture various aspects of breeding phenology: the annual mean first egg
 date across all sites to control for early versus late seasons
 (yearmeanfedcentre), the annual mean phenological deviation of each site
 from the study-wide annual mean first egg date to capture spatial
 variation in phenology between sites (siteyearfeddevs), and the deviation
 of each nest box from the site-year mean to capture between-individual
 variation in phenology within a site and year (relfeddevs). Mean centred
 site latitude, elevation, and mean site minimum temperature in the 14 days
 before the first egg was laid are also included. Where a parental identity
 was unknown it was assigned a unique identifier, so that nest height
 observations with missing parental information could be retained (femaleID
 and maleID). The term "sitebox" corresponds to the nest box
 number at each site (1-8) and site ID. Also included are the year in which
 the nest was built, the site abbreviation, and the
 "nestrecorder" who measured the nest height. The
 "Building Duration Data" file includes the same "year,
 "nestrecorder", "femaleID", "sitebox", and
 meancentred elevation, and latitudevalues as in the "Nest Height
 Data" file. Also included in the "Building Duration Data"
 is meancentred nest height (heightmeancentre) and first egg date across
 all sites and years (fedmeancentre). The "buildingtime"
 variable was calculated as the date at which the final nest height +/- 3mm
 was reached minus the date of first recorded nest building. The
 "mintempsmeancentre" is the mean minimum site temperature in the
 7 days pre-egg-laying.