Introduction

For many animals, the rearing microenvironment is a crucial factor influencing early offspring development and behaviour. Many bird offspring develop in nests and nest architecture is an extended phenotype that results from the interaction between the evolutionarily adaptive behaviour of birds and the environment (Dawkins 1982; Hansell 2000). In altricial birds, a nest may be considered as a product of niche construction provided by the parents and aimed to optimize living conditions of the offspring (Mainwaring 2017; Maziarz et al. 2017; Vanadzina et al. 2023). Nests reduce both heat loss and overheating and, therefore, may create a suitable microclimate, as well as they can render protection from predators and help to maintain optimal position of eggs during incubation (Heenan 2013). Nests may also protect nestlings from ectoparasites and pathogens or facilitate nestling care and, therefore, nest may improve nestling survival and increase fitness (Mainwaring et al. 2014; Lambrechts et al. 2017; Glądalski et al. 2020a, b; Nuhlíčková et al. 2021).

Nest parameters vary largely between and even within taxa according to their particular life-history (Alabrudzińska et al. 2003; Heenan 2013; Sonnenberg et al. 2020; Glądalski et al. 2024). In our study populations, a tit nest usually consists of two layers. The moss part (usually larger) is considered rather as a sanitary layer (mainly important after hatching), and it also provides a structural support for offspring and parents (Stenning 2018; Rydgren et al. 2023; but also note that some moss species were shown to have antimicrobial and antifungal properties, see Wolski et al. 2021). The lining part (usually thinner) of the nest was shown to be associated mainly with its insulating capacity (Soler et al. 1998; Heenan and Seymour 2011).

Nest size and structure have been shown to be positively related to reproductive parameters, such as clutch size, fledging and hatching success or recruitment success, as well as quality and condition of the nest-building female (Alabrudzińska et al. 2003; Álvarez and Barba 2008, 2011; Mainwaring and Hartley 2008; Álvarez et al. 2013; Glądalski et al. 2016a, b). On the other hand, several field studies showed that associations between breeding parameters and nest parameters were non-significant, rather weak or differed between years within study areas (Britt and Deeming 2011; Lambrechts et al. 2016; see Harnist et al. 2020). It was suggested that it is not nest size per se (volume or mass), but the mass of the lining material in and surrounding the nest cup that might directly impact on breeding success in all positive-effect studies. Järvinen and Brommer (2020) conclude their eight-season study on the nests of Eurasian Blue Tits (Cyanistes caeruleus) that a higher proportion of feathers in the nest lining is positively associated with the probability of offspring to recruit as a breeding adult later in life and, therefore, the choice of nest material may be a maternal behaviour with potential lifelong effects on her offspring. On the other hand, Sonnenberg et al. (2020) conclude their two-season study on the Mountain Chickadee (Poecile gambeli) that in some tit species, the large variation in nest size, both across and within sites, may be rather a result of consistent individual preferences, which may not have clear fitness consequences.

Studies examining the impact of nest parameters on breeding performance and success are not numerous (especially when compared to other stages of bird reproduction) and the number of nest-studies is slowly increasing in recent years (Mainwaring et al. 2014; Harnist et al. 2020). Especially, long-term studies are still rare and the vast majority of reports refer usually to one to two seasons of research and inferring from such a small number of seasons in some cases can give incorrect or even opposite results. The aim of this paper is to study the long-term relation between nest structure and reproductive performance, including hatching success and fledging success of Great Tits (Parus major) and Eurasian Blue Tits in two floristically and structurally contrasting study areas: a rich deciduous forest and an urban parkland during eleven breeding seasons (2012–2022). Our prediction is that in both tit species clutch size and the number of fledglings will be positively associated with lining mass (and that both breeding parameters will be affected by the study area). We also predict that breeding success will be positively affected by nest characteristics (nest height/mass or/and lining mass).

Materials and methods

This study was carried out as part of long-term research project concerning the breeding biology of secondary hole nesting birds occupying nest boxes in the city of Łódź (central Poland, Glądalski et al. 2018). Nests examined for this study were obtained between 2012 and 2022 (eleven breeding seasons) in two structurally and floristically contrasting study areas: a rich deciduous forest and an urban parkland (located c.10 km apart). Both study areas were supplied with standardized wooden nestboxes with a removable front wall (Lambrechts et al. 2010). A 30 mm diameter entrance is located 20 cm from the bottom of the nestbox and internal dimensions are: 11.5 cm (depth) × 11 cm (width) × 30 cm (height).

During all the breeding seasons, the nestboxes were checked at least once a week (or more often if needed) to monitor breeding performance of tits. Parameters measured: clutch size, the number of hatchlings, and the number of fledglings. Hatching success refers to the proportion of eggs resulting in hatchlings and fledging success refers to the proportion of hatchlings resulting in fledglings. Only first clutches were studied.

The height (depth) of the nest (as a distance from the nest base to the top rim of the nest cup), following Glądalski et al. (2016a, b), was measured at the end of tit laying stage with a ruler to the nearest 5 mm. Nest height of Eurasian Blue Tits were measured in 183 nests (114 from the parkland site and 69 from the forest site). Nest height of Great Tits was measured in 322 nests (175 from the parkland and 147 from the forest).

After the end of tit breeding season, all nests were collected from the nestboxes and placed in a freezer (− 80 °C) for 24 h in order to sterilize nests by deep freezing. After this procedure, the nests were dried for 24 h in a lab dryer (60 °C) until reaching constant dry mass and then deconstructed (by M.G.) into two basic structural parts: (1) the cup lining and (2) the moss (and each of the part was deconstructed to the basic structural elements, which took up to an hour per nest). The cup lining and the moss layer were weighed for every nest to the nearest 0.1 g (Glądalski et al. 2016a, b, 2021; Table 1). All dead nestlings, feaces, food remnants and all detectable organic powder fragments (skin like dust that is a byproduct of feather growth) were removed. The height of the nest was positively correlated with the nest mass in both Great and Eurasian Blue Tit species, therefore, we used only nest height parameter in the GLM (for Great Tits: r = 0.62, n = 322, p < 0.001, and for Eurasian Blue Tits: r = 0.66, n = 183, p < 0.001).

Table 1 Mean nest parameters (lining and moss masses) ± SD (g) in the parkland and forest study areas (2012–2022)
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The effects of nest characteristics on clutch size and brood size at fledging, treated as Poisson variables, were tested using generalized linear models (GLM) with the log-link function (Crawley 2002; Heck et al. 2012). The effects of nest parameters on hatching and fledging successes (hatching success refers to the proportion of eggs resulting in hatchlings, and fledging success refers to the proportion of hatchlings resulting in fledglings, with abandoned broods excluded from the analysis) were examined using GLMs with binomial error distribution and the logit link function (Crawley 2002; Heck et al. 2012). The GLM modeling was performed using IBM SPSS v. 22 software (IBM SPSS Statistics 222013).

Results

The number of fledglings (Table 2) of the Great Tit was positively associated with lining mass and differed between years (Table 3). Hatching success in Great Tits was positively associated with lining mass. In Eurasian Blue Tits, clutch size and the number of fledglings were positively associated lining mass (Table 3). Hatching success in Eurasian Blue Tits was also positively associated with lining mass (Table 4). Clutch size differed between urban parkland and a forest study area in Great Tits (being higher in the forest) and the number of fledglings differed between urban parkland and a forest study area in both Great and Eurasian Blue Tits being higher in the forest. In the Great Tit, hatching success and fledging success were higher in the forest and in the Eurasian Blue Tit fledging success was also higher in the forest. Hatching success and fledging success differed between years in both Great and Eurasian Blue Tit species (Table 4).

Table 2 Mean breeding parameters (clutch size and the number of fledglings) ± SD in the parkland and forest study areas (2012–2022)
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Table 3 Summary of generalized linear model tests for the relations between reproductive performance and nest characteristics (as shown by nest height and lining mass) of Great Tits and Eurasian Blue Tits in urban parkland and a forest study areas (Lodz, central Poland), between 2012 and 2022 (11 breeding seasons), nGT = 322, nBT = 183
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Table 4 Summary of generalized linear models showing a positive relationship between two binomial response variables, hatching success and fledging success and nest characteristics of Great Tits and Eurasian Blue Tits (nest height and mining mass) in an urban park and a forest study area, between 2012 and 2022 (eleven breeding seasons), nGT = 322, nBT = 183, (significant p values are in bold)
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Discussion

We showed that in Great and Eurasian Blue Tits the number of fledglings were positively associated with lining mass and were affected by the study area (being higher in the forest). In Eurasian Blue Tits, clutch size tended to be associated with lining mass. There was also a relationship between lining mass and higher hatching success in both tit species.

Nest characteristics (like size, mass or structure) were shown before to be positively related to reproductive parameters, such as clutch size, hatching and fledging success or recruitment success, as well as quality and condition of the nest-building female (Heenan 2013; Deeming and Pike 2015; Mainwaring 2017; Stenning 2018; Deeming 2023). It was also suggested that the most important function of the nest is thermal insulation, especially during egg incubation by the female and during the first week of the hatchlings’ life when they are ectothermic (Perrins 1979). Nest material buffers ambient temperature fluctuations and helps to maintain egg temperature and humidity (Mertens 1977; Deeming 2011). Several authors reported that tits are characterised by a selective behaviour in their choice of building elements collected for nest construction (Wesołowski and Wierzcholska 2018; Järvinen and Brommer 2020; Glądalski et al. 2021; Fäth et al. 2023) and recently Deeming et al. (2020) showed that, by selecting particular materials whilst constructing a nest, bird can manipulate the thermal characteristics of the nest. Also Deeming and Pike (2015) showed on tits that the insulatory properties of nests during incubation can correlate with offspring fitness. Therefore, in many recent studies, authors were linking a lining (nest) mass with breeding success as a natural consequence (Heenan 2013; Mainwaring 2017; Stenning 2018). But after almost 2 decades of studies increasingly focusing on nests, some evidence is available that simple links may exist between lining mass and breeding success, yet not so strong as presumed before. For example, some of the authors did not find any effects (or effects were rather weak) of nest characteristics on breeding performance (Tomás et al. 2006; Lambrechts et al. 2016; for experimental study, see Lambrechts et al. 2012). Some authors suggested that there might be effects (between nest characteristics and breeding performance) in some years but not in other years (e.g., Britt and Deeming 2011). In addition, Harnist et al. (2020) showed on Great Tits that nest mass increases over the course of the nesting cycle (and basically the vast majority of the nest studies in tit literature is based on the post-season collection of the nests, similar findings were shown by Britt and Deeming 2011). Between the incubation and post-fledging stages the authors recorded an increase in fresh mass of the nest by nearly 70% (some of other studies reported about 9%, see Cruz et al. 2016). The change in mass between both stages is probably caused by the accumulation of organic powder, food remnants and feaces (Harnist et al. 2020). Authors also suggest that a better method of quantifying the nest size is measuring of the nest height/height, but it is not perfect because nest height may differ between different sizes of nestboxes and between various populations, it may also differ due to predator attacks and many other factors (Kaliński et al. 2014). Recently, Marcel Lambrechts after reading Harnist et al. (2020) suggested that it is probable that skin produced by nestlings accumulate inside the lining part and it can make the lining part of the nest with a higher fledging success or a larger number of nestlings heavier (Lambrechts, public information, 2021). Therefore, it seems reasonable that if hatching success in many studies (including ours) refers to the proportion of eggs resulting in hatchlings—more hatchings may deposit more organic powder in the lining layer of the nest and cause higher factor of hatching success (in other words and more general: organic powder may be correlated with hatching success/number of fledglings/clutch size and not with the mass of the lining). This hypothesis should be tested in the near future using lining and moss from the fresh nests. In the present study all the dead nestlings, feaces, food remnants and all distinguishable organic powder fragments were removed from the nests but it is important to emphasize that indeed some of the powder was tightly bound with the fragments of lining, which were simply impossible to remove (Glądalski, personal observation)—that could at least partly support Harnist et al. (2020) suggestion. In other words, although our results indicated a positive correlation between lining mass in the nest and hatching success, it is nonetheless possible that a greater proportion of nestlings produce a greater quantity of organic debris which consequentially increased lining mass.

Harnist et al. (2020) suggest also that the nest height/depth is a reliable method of quantifying the characteristics of the nest. Recently Lambrechts and Caro (2022) showed experimentally that egg cooling is associated with nest size in Great Tits—eggs cooled more quickly in thinner nests and when ambient temperature was lower. In southern France (Mediterranean region), the temperatures during breeding period are relatively high and stable and this effect (of faster egg cooling) may not occur or be just negligible, but in colder parts of Europe, like Poland, it could be important. During the breeding season in Poland, we have witnessed large drops in the mean ambient temperature (while tits were incubating), and during some breeding seasons even, to about 2 °C to 3 °C (Glądalski et al. 2020b). Therefore, a larger nest may be probably beneficial for the female (less heat loss) but it also may be beneficial for the nestlings because incubation temperature affects growth and energy metabolism in tit nestlings (Nord and Nilsson 2011). In Glądalski et al. (2016a), we showed that nest height (depth) positively affected fledging success in Great Tits, but with a larger set of data the effect disappeared in the present paper (and there is no effect for both tit species). The reason may be that in a large set of different seasons with various weather characteristics, the positive effect of nest size may be lost, but the reason may also be that the amount of moss (which dominates the nest mass/height) is mainly related to individual variability and does not affect fitness as suggested by Sonnenberg et al. (2020).

We also showed that both clutch size and number of fledglings were higher in both tit species in the forest and those results are in line with our earlier results on both tit species (for Eurasian Blue Tits Glądalski et al. 2015, for Great Tits Wawrzyniak et al. 2015). The site factor also affected hatching success (for Great Tits higher in the forest) and fledging success (for Eurasian Blue Tits higher in the forest) (for Eurasian Blue Tits Glądalski et al. 2017, for Great Tits Wawrzyniak et al. 2020). The main reason for the differences is probably food availability. The total number of native oaks (Pedunculate Oak Quercus robur and Sessile Oak Quercus petraea) per nestbox plot was 19 times greater in the forest than in the parkland (for the forest—65.0 trees / nestbox plot and for the parkland—3.4 trees/nestbox plot, Glądalski et al. 2017). In addition, both study areas differ in the abundance of caterpillars (considered as the most suitable kind of food for nestlings), as the differences in the amount of caterpillar frassfall suggests, with the parkland being characterized by ca. two to three times lower abundance of caterpillars than the forest area (Nadolski et al. 2021). Reduced hatching success in the urban parkland may be related to feeding and incubation rhythm disturbances. Exposure to human visitors in some birds may disturb the regularity of incubating and feeding nestlings by parents and may affect breeding success (Ruiz et al. 2002). The parkland site is one of the largest recreation and entertainment areas in Łódź and the number of visitors is large compared to the forest (Glądalski et al. 2016b). We previously showed that non-lethal disturbance events from humans could influence breeding success negatively in the parkland study site, especially during sunny and warm days, when the numbers of visitors are high (Glądalski et al. 2016b).

We also showed that hatching success and fledging success differed between years in both Great and Eurasian Blue Tits. As we showed in Glądalski et al. (2017), one of the most important components of the year factor in are food availability and weather conditions (precipitations, temperature, extreme weather events). The abundance of caterpillars (considered as the most suitable kind of food for nestlings) differs not only between habitats but also differs largely between years as shown for our study areas in a long-term study by Nadolski et al. (2021). Unfavorable weather conditions (like low temperatures of heavy rain in longer periods of time) may cause tits (and other birds) breed later and in lower numbers, lay smaller clutches, increase incubation bout length and have much lower breeding success (Bordjan and Tome 2014; Glądalski et al. 2017; Stenning 2018).

Breeding success is influenced by a large cocktail of factors and nest parameters are only small part of that mosaic of factors. There is no doubt that avian nests are important to reproduction and materials used to construct nests contribute to many and miscellaneous functional properties like insulation, control of humidity or weather-proofing, but new nest-studies, especially experimental are to be welcome.