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.

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.