We used long-term data recorded on a population of blue tits breeding in nest boxes in Tammisaari, southwest Finland (60° 01′ N, 23° 31′ E). The data used in this study was collected between 2006 and 2018, during which time the site consisted of 319–470 nest boxes located in patches of mixed boreal forest in between areas of farmland. The nest boxes (W × H × D = 125 mm × 200 mm × 81 mm, bottom thickness = 22 mm; bottom surface area, 125 mm × 81 mm = 101 cm2; bottom-to-hole distance = 170 mm) were made of untreated spruce and had an entrance hole with a diameter of 26 mm.
The blue tit is a socially monogamous, secondary hole-nesting passerine. It readily builds a nest in a nest box if available. Female blue tits choose the nesting site and build the nest that typically consists of a base layer of moss and dry grass, and a lining of hair, fur, wool and feathers (Britt and Deeming 2011; Deeming and Mainwaring 2015). Other common nest components include bark, wool, pine needles and fresh plant matter. Small mammal fur, flower petals and human-made materials such as wool insulation are also occasionally incorporated.
Description of a nest ornament
Nest ornaments in the form of feathers were primarily defined by their location in the nest: they were located on top of the nest and on the rim and not in the nest cup, and they stood out of the bulk of the nest based on colour (as perceived by human eye under natural light) and/or size (Fig. 1). We focused on feathers because these were the typical ornaments observed in the nests. To differentiate between feather ornamentation and a general preference for feathers as nest material, we also considered non-feather ornaments, which were defined as any nest components that fit the above-described criteria for nest ornaments but were not feathers but, e.g., small mammal fur or colourful wool (described in detail and illustrated in the Online Resource).
Every breeding season starting from late April, the nest boxes were checked once a week until the eggs hatched. We estimated the laying date of an incomplete clutch by back-calculating with the assumption that one egg was produced daily. We calculated the expected hatching date based on the date of clutch completion and assumed an incubation period of 12 days (Kluen et al. 2011). We began to check the nest every afternoon on the day of the expected hatching and continued until the first egg hatched (day 0).
In 2018, we photographed each nest during 4 weeks of construction (starting from the last week of April). Due to asynchrony in nest building, each nest was photographed between one and four times (once: 7 nests, twice: 26 nests, three times: 38 nests and four times: 26 nests). The nests were photographed directly from above and were not removed from the nest box. In 2006–2018, the nests were photographed 2 days (day 2) after hatching. The height of the nest material was measured (2008–2018, except in 2011) and the proportions of different nest materials were scored also on day 2. The nest height was measured as the distance between the floor of the nest box and the top of the nest in all four corners of the nest with a knitting needle fitted with a millimetre paper (2006–2012) or electronic calliper (2013–2018). The average of these values was used as the measure of nest height. The nests were removed from the nest box and the proportions of different nest materials were estimated by eye. The base layer of the nest was separated from the nest cup. The base layer typically consists of moss. The nest cup lining typically contains ungulate hair (of moose Alces alces or white-tailed deer Odocoileus virginianus), combined with grass and/or strips of bark and/or feathers. Blue tits use feathers from a variety of species. The proportion of feathers in the nest lining is calculated as the proportion of feathers in the nest after excluding moss (i.e. the base layer), and this proportion was arcsine-square-root transformed to approximate a Gaussian distribution (Järvinen et al. 2017a, b).
We used both the photographs taken during the construction period in 2018 and those taken after hatching in 2006–2018 (excluding 2007 when only the nest cup was photographed) to record nest ornaments according to the criteria described in 2.3. To reduce inter-observer variability, PJ scored all the photographs. To minimise subjectivity, two other researchers (JB and K. Schreven) followed the criteria and scored a subset of the photographs. The scores were unanimous with those of PJ.
Adults were caught when providing care for their offspring. Adults were ringed (if unringed) with a metal ring to allow lifelong individual identification and aged as either yearling (hatched the previous breeding season) or older on the basis of the colour of the coverts of their primary feathers.
Our previous results indicate that the nest features post-hatching are a female trait (Järvinen et al. 2017a). However, if males provided the nest ornaments, these should optimally be introduced before or during the egg laying period. Considering the nest ornaments’ putative function in sexual signalling, before females finished laying eggs would be the time males could influence female reproductive investment and to increase the number of eggs they lay. Females, on the other hand, should signal to males around hatching time to elicit an increase in male parental care at the beginning of a period when it is most needed (Soler et al. 2007). Because our long-term data consist of nest measurements taken after hatching, we had to determine whether the nest ornament status after hatching corresponds to that of the pre-egg laying to the incubation period. Thus, in 2018, we studied the association between the scores of nest ornaments each week of the construction period and those of post-hatching. This data (see results) showed we could rely on the long-term post-hatching nest data to represent nest ornament status throughout the breeding season.
To minimise observer bias, collection of data on nest construction and nest ornamentation was blind with respect to the identity and traits of the parents.
All the statistical analyses were conducted in R (R Core Team 2019). Uncertainty of an estimate was provided as ± the standard error of an estimate unless indicated otherwise.
We aimed to determine whether nest ornamentation behaviour showed consistency specific to males or females in our study population. We used the number of nest ornaments (0–5) counted when the eldest nestlings were 2 days old as response variable in a Poisson generalised linear mixed model (GLMM) with a log link, which was conducted with ASReml-R (v184.108.40.206, Butler 2018). We included age (1 vs. ≥ 2 years) of the adult female and male as factorial fixed effects, because experience and learning can influence the choice of nest material (Muth and Healy 2011, 2014; Sergio et al. 2011; Bailey et al. 2014). Variation across years, nest boxes, females and males were estimated by including these variables as random effects. The adjusted repeatability (i.e. the repeatability conditional upon the fixed effects in the model; Nakagawa and Shielzeth 2010) of the Poisson regression on the latent scale was computed as the ratio of the between-individual variance over the sum of all variance components. The adjusted repeatability on the data scale was computed following formulas given by de Villemereuil et al. (2016). In general, transformation from the latent scale to the data scale includes additional variation induced by the Poisson distribution and results in lowered repeatability (Nakagawa and Shielzeth 2010).
Nest height and the proportion of feathers in the nest lining are repeatable and heritable female traits in our study population (Järvinen et al. 2017a). Here we analysed the covariance of nest ornaments, the proportion of feathers in the nest lining and height of the nest material in a multivariate mixed model. The number of nest ornaments was modelled as a Poisson distributed variable. Following Järvinen et al. (2017a), nest height and the arcsin-square-root of the proportion of feathers in the nest cup lining was modelled as Gaussian distributed variable. The multivariate mixed models were conducted using ASReml-R 4. Correlations were calculated based on the estimates provided by these models.
Association between ornaments during construction and after hatching
We measured the correlation between the scores of nest ornaments during each week of the construction period and those of post-hatching in 2018 by calculating Goodman-Kruskal gamma values with the R package ‘MESS’ (v0.5.5, Ekstrøm 2019).
The dataset generated and analysed during the current study along with the R code used to perform the analyses are available in the Zenodo repository, doi: https://doi.org/10.5281/zenodo.4021911.