Coal Tits Periparus ater build larger nests than Blue Tits Cyanistes caeruleus and Great Tits Parus major

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1 ACTA ORNITHOLOGICA Vol. 51 (2016) No. 1 SHORT NOTES Coal Tits Periparus ater build larger nests than Blue Tits Cyanistes caeruleus and Great Tits Parus major living in the same Mediterranean coniferous woodland habitat Marcel M. LAMBRECHTS 1 *, Jacques HAUREZ 2, Gérard BODINEAU 3, Gilbert GAGLIARDI 2, Marie MAISTRE 1, Philippe PERRET 1, Pierre PIHAN 2, Bernard WILHELM 2, Josette WILHELM 2, Cyril BERNARD 1 & Jacques BLONDEL 1 1 Centre d Ecologie Fonctionnelle et Evolutive CEFE, UMR 5175, Campus CNRS, Université de Montpellier, Université Paul-Valéry Montpellier, EPHE, 1919 route de Mende, Montpellier Cedex 5, FRANCE 2 Draille de Bélézy, Bedoin, FRANCE 3 Petit chemin des Vergers, Bedoin, FRANCE *Corresponding author, marcel.lambrechts@cefe.cnrs.fr Lambrechts M. M., Haurez J., Bodineau G., Gagliardi G., Maistre M., Perret P., Pihan P., Wilhelm B., Wilhelm J., Bernard C., Blondel J Coal Tits Periparus ater build larger nests than Blue Tits Cyanistes caeruleus and Great Tits Parus major living in the same Mediterranean coniferous woodland habitat. Acta Ornithol. 51: DOI / AO Abstract. The size and shape of the nest are species-specific characteristics that are often associated with environmental factors at the time of breeding. Nests are expected to be larger or thicker in colder environments, although the relationships between nest design and weather differ between species. Here we present the results of an analysis of the external height of the nest wall in Paridae that accepted small standardized nesting boxes for breeding. The study populations were monitored in a relatively cold Mediterranean study area. We found that Coal Tits Periparus ater built higher external nest walls than Great Tits Parus major or Blue Tits Cyanistes caeruleus, after controlling for the first-egg date and clutch size which are assumed to reflect aspects of the quality of the nest builders. Our measures of nest size were not closely associated with the average ambient temperature, but nest walls tended to be shallower when there was more rain. Nest-shape asymmetry, as reflected in the difference in the external height of the nest measured closest to and farthest from the nest-chamber entrance, was observed in all three species, but the average asymmetry was highest in Coal Tits. In asymmetric nests, more nest material was added to the side that was closest to the front wall considered to be the coldest and least protected against harsh weather. Thus, nest size characteristics differ between three ecologically similar species inhabiting the same cavity type in the same coniferous woodland habitat, which would imply that different species do not respond in the same way to the same set of environmental factors. Key words: Coal Tit, Great Tit, Blue Tit, nest size, nestboxes, hole-nesting birds Received Feb. 2016, accepted May 2016 The size and shape of the nest are species-specific characteristics and are often associated with environmental factors expressed before the onset of egg-laying. Because breeding is assumed to be energy-expensive, breeders are expected to evolve reproductive strategies that minimize energy loss (e.g. Lack 1968). For instance, to minimize heat loss in cold environments, avian nests are expected to be thicker, and thus more insulated than in milder environments (Slagsvold 1989, Soler et al. 1998, Hansell 2000, Schaedelin & Taborsky 2009, Mainwaring et al. 2014, Møller et al. 2014). Breeders are therefore expected to build larger, more insulated nests at more northern latitudes, at higher altitudes, during colder years or earlier in the season. For instance, nests are expected only to be thicker and thus better insulated earlier in the season than nests built later in the season for those species inhabiting temperate environments in the northern hemisphere (e.g. Skowron & Kern 1980, Nager & van Noordwijk 1992, Britt & Deeming 2011, Deeming et al. 2012, Mainwaring et al. 2012). However, several species that are exposed to harsh or cold weather (e.g. seabirds,

2 124 SHORT NOTES Goodfellow 2011) do not build elaborated nests, suggesting that a large nest is not always essential for successful breeding in extreme environments (Hansell 2000, Mainwaring et al. 2014). Paridae, like Great Tits Parus major or Blue Tits Cyanistes caeruleus, are among the best-studied free-living non-domesticated birds, mostly because they readily breed inside man-made nest boxes erected in a large variety of habitat types (Perrins 1979, Glutz von Blotzheim & Bauer 1993, Lambrechts et al. 2010, Møller et al. 2014). At the between-species level, nest size can differ between Great Tits and Blue Tits after controlling nest-chamber size (e.g. Kaliński et al. 2014, Glądalski et al. 2016, Lambrechts et al. 2016), although some studies claimed that the two species build nests of similar size (e.g. Britt & Deeming 2011). At the within-species level, nest size is not only influenced by the size of the nestchamber (e.g. Glutz von Blotzheim & Bauer 1993, Møller et al. 2014), but is also associated with additional biotic (e.g. predators, parasites) or abiotic external environmental factors (e.g. weatherrelated factors)(wesołowski 2002, Deeming et al. 2012, Mainwaring et al. 2012, Møller et al. 2014, Lambrechts et al. 2016). Quantitative multi-year studies of aspects related to nest size in Paridae other than Great or Blue Tits remain rare (e.g. Perrins 1979, Glutz von Blotzheim & Bauer 1993, Wesołowski 2002, Surgey et al. 2012, Lambrechts et al. 2014, 2015, Møller et al. 2014). Because different species from the Paridae family generally occupy different habitat types, studies of parids other than Great and Blue Tits might reveal underexplored relationships between nest size and external environmental factors. Here we present the results of a 19-year study that compared aspects of nest size and nest shape in one species assumed to be primarily adapted to coniferous forest Coal Tit Periparus ater and in two species assumed to be primarily adapted to mixed deciduous forest Great and Blue Tits (Snow 1954a,b, Partridge 1979). To compare the nest-size data from the Coal Tits with the other two species that all nested in a relatively small standardized box type, we exploited field data from a mixed forest dominated by the cedar Cedrus atlantica at the Mont-Ventoux, southern France. This forest grew from a plantation that had been established in the late 1860s at an altitude of asl., and is considered as a relatively cold Mediterranean study site for Paridae (Blondel 1985). Other tree species, such as Pinus nigra, P. silvestris, Quercus lanuginosa, Acer opalus, and Amelanchier ovalis, are also present. If the three tit species would respond differently to the same breeding environment, we predicted higher, and thus thicker, external nest walls in the smaller species assumed to be more vulnerable to cold (e.g. see Lambrechts et al. 2014), and also predicted higher, and thus thicker, external nest walls during colder weather (e.g. see Nager & van Noordwijk 1992, Deeming et al. 2012). The nests were measured in a nest-box plot described in Blondel & Isenmann (1979), Michelland (1980) and Blondel (1985). We monitored the contents of wood-concrete Schwegler B1 boxes (Schorndorf, Germany) or concrete boxes all having ca. the same nest-chamber size (Lambrechts et al. 2010). The boxes had either an initial circular entrance hole with a diameter of 26 mm to attract smaller cavity-nesters (Coal Tits, Blue Tits) or a diameter of 32 mm to attract both smaller and larger cavity-nesters (all box-breeding Paridae from the region). The small-holed and large-holed boxes were spatially intermingled in the sense that neighboring boxes were ca. 50 m apart and that neighboring boxes always differed in entrance hole size within the study plot. The set-up of the two box types within this study plot did not differ from the nest-box set-ups in other Mediterranean study plots (e.g. Lambrechts et al. 2016). From the end of March to the end of June, the boxes were usually visited at least once a week to quantify in individual first breeding attempts the initiation and progress of nest building, the firstegg dates and/or the number of eggs observed (Blondel 1985, Blondel et al. 2006). The external height of the wall of the nest was measured following measure a presented in Hansell (2000) or measure B presented in Álvarez & Barba (2008), which is the external vertical distance between the nest bottom and the top of the nest rim. This measure was also named nest thickness (Hurtrez- Boussès et al. 1999), nest depth (Hansell 2000) or nest height (Álvarez & Barba 2008, Lambrechts et al. 2012, 2014). Most measures were taken at the external side of the nest that was closest to the entrance, but in some years measures were also taken at the external side of the nest that was farthest from the entrance. Nests were measured with a ruler following Lambrechts et al. (2012, 2014). Nest volume was estimated by multiplying the external height of the nest wall with the inner surface of the box floor (ca. 113 cm 2 ; Lambrechts et al. 2010) following procedures used in previous

3 SHORT NOTES 125 studies (e.g. Hurtrez-Boussès et al. 1999, Lambrechts et al. 2012, 2014, 2015). Nest measures taken up to two weeks before the first-egg dates were considered. The daily minimum temperature ( C), the daily maximum temperature ( C), and the daily rainfall (mm) were provided by Météo France from the meteorological station of Chalet-Reynard (1455 m altitude at the Mont-Ventoux) (44.15 N, E). The minimum and the maximum of the daily ambient temperatures were used to calculate the average of the daily ambient temperatures. Subsequently, we calculated for each nest for the week before the nest size measure the means of the average daily ambient temperatures and the daily amounts of rainfall (Table 1). Of the 387 nest measures of first breeding attempts from the three species available (Table 1), 93% (n = 359) were taken before the eggs were laid. Because we expected finished nest walls to be higher than unfinished ones, we calculated for each individual breeding attempt the difference between the date that the nest was measured and the first-egg date. For nests without eggs, we applied the mixed procedure (Type 3 tests of fixed effects, data not transformed, SAS 9.4; cf. Lambrechts et al. 2014, 2015) using for first breeding attempts the external height of the wall that was closest to the entrance, the external height of the wall that was farthest from the entrance, or the nest-shape asymmetry (i.e. the difference in the external heights of the nest walls measured closest and farthest from the entrance) as dependent variables. We considered as fixed effects the species (Great Tit versus Blue Tit versus Coal Tit), the time lag between the date the nest was measured and the first-egg date of the first clutch (in number of days), the first-egg date of the first clutch (in Julian dates), the size of the incubated first clutch and the average weather during the week preceding the nest measure (daily ambient temperature in C, daily rainfall in mm). The nest-box identity (n = 59) and year ( ) were considered as random factors. The first-egg date and the size of the incubated clutch were used as proxy for female or territory quality following Przybylo et al. (2001) and Lambrechts et al. (2014, 2015). We expected to find larger nests in earlier breeders and in birds being able to produce more eggs (e.g. Lambrechts et al. 2016). For the small sample of nests with eggs (n = 28), we applied the GLM procedure (Type 3, data not transformed, SAS 9.4) with the external height of the wall closest to the entrance as the dependent variable and species as the fixed effect. Female identity was not taken into account because most of the parents were not captured in this study area. During the 19-year study period, the nest builders in Great Tits, Blue Tits and Coal Tits were exposed to similar average weather conditions (species effect on weather measures, all p > 0.50), although the average weather at the time of nest building differed substantially across years (year effects on weather measures, all p < ) (Table 1). When the data from the three species were combined in a single analysis, also taking the other biotic and abiotic factors into account, the species effects on nest size and nest-shape asymmetry were highly significant (Table 2, 3). The Coal Tits built higher external nest walls compared to the other two tit species (Table 2). The species effect on the height of the external nest wall has been found for the measures from nests that did not contain eggs (Table 2, 3) and in those that contained at least one egg (F = 5.20, p = 0.013, n = 28, after removal of the non-significant effects of year and the year species interaction). The Coal Tits also had more asymmetric nests compared to the two other species (Table 2, 3). Covering the two weeks prior to the onset of egg-laying, the external nest walls were higher Table 1. Average weather conditions per day (mean ± SD, with ranges in brackets) during the week preceding nest measures in three tit species (see text for details). n number of nest measures. Species Daily rainfall Daily minimum Daily maximum Daily average (mm) temperature ( C) temperature ( C) temperature ( C) Great Tit 2.3 ± ± ± ± 2.7 n = 100 (0 11.3) ( ) ( ) ( ) Blue Tit 3.2 ± ± ± ± 2.6 n = 90 (0 17.8) ( ) ( ) ( ) Coal Tit 2.6 ± ± ± ± 2.4 n = 197 (0 16.1) ( ) ( ) ( )

4 126 SHORT NOTES Table 2. Means (with ± SD, and ranges in brackets) for the external heights of the nest walls, nest-shape asymmetry (i.e. the difference in the external heights of the nest walls measured closest and farthest from the entrance) the first-egg dates, and the size of incubated clutches in three tit species studied from a plantation dominated by the coniferous Cedrus atlantica. Data are from first clutches. n the number of nests measured. Species External height of nest wall Nest-shape asymmetry First-egg date Clutch size closest to farthest from (Jan 1 = 1) entrance entrance Nests without eggs Great Tit 4.9 ± ± ± ± ± 1.3 (1 10) (0.5 6) (0 6) (77 127) (3 10) n = 89 n = 46 n = 46 n = 89 n = 89 Blue Tit 4.7 ± ± ± ± ± 1.3 (1 10) (0 5) (0 6) (82 126) (6 12) n = 88 n = 45 n = 45 n = 88 n = 87 Coal Tit 6.6 ± ± ± ± ± 1.1 (1 12) (0 9) (-1 6) (85 127) (5 12) n = 188 n = 103 n = 102 n = 188 n = 186 Nests with eggs Great Tit 4.4 ± ± ± 1.3 n = 10 (3 6) (89 116) (6 10) Blue Tit 6.2 ± ± ± 1.8 n = 5 (5 8) (93 126) (8 12) Coal Tit 6.7 ± ± ± 0.9 n = 13 (4 12) (83 110) (7 10) when the nests were measured closer to the firstegg dates (Table 3). However, nest-shape asymmetry was statistically not associated with the measure date in relation to the first-egg date. As concerns the female characteristics, nests with higher walls before the eggs were laid had smaller incubated clutches (Table 3). However, the nest size measures were not associated with the first-egg date. The height of the external nest wall was negatively associated with rainfall, but was not associated with the ambient temperature at the time of nest building (Table 3). We observed that Coal Tits, Great Tits and Blue Tits that co-exist in the same coniferous woodland and that occupy the same box type differ in nest size and nest-shape asymmetry. After controlling for these species effects and other biotic or abiotic factors, we observed that the nests were larger when they were measured closer to the first eggdates, larger nests contained smaller incubated clutches, and nests were larger during drier weather conditions. We also found that nest size was not associated with the first-egg date and the ambient temperature at the time of nest building, supporting the results of other published studies Table 3. Factors affecting external height of the nest walls and nest-shape asymmetry (i.e. the difference in the external heights of the nest walls measured closest and farthest from the entrance) results of Type 3 tests of fixed effects, data not transformed, from nests from first breeding attempts without eggs (see text for details). External height of the nest wall Nest-shape asymmetry closest to entrance farthest from entrance Species F 2,274 = F 2,114 = F 2,113 = 4.74 p < p < p = Difference between first-egg F 1,274 = F 1,114 = F 1,113 = 1.55 date and nest measure date p < p = p = 0.21 First-egg date F 1,274 = 0.70 F 1,114 = 0.05 F 1,113 = 0.33 p = 0.40 p = 0.82 p = 0.57 Clutch size F 1,274 = 6.40 F 1,114 = 0.04 F 1,113 = 0.89 p = p = 0.84 p = 0.35 Ambient temperature F 1,274 = 0.26 F 1,114 = 0.01 F 1,113 = 1.63 p = 0.61 p = 0.93 p = 0.20 Rain F 1,274 = 6.53 F 1,114 = 0.89 F 1,113 = 0.53 p = p = 0.35 p = 0.47

5 SHORT NOTES 127 in other Mediterranean habitat types (e.g. Lambrechts et al. 2016). When the external height of the nest wall closest to the entrance was used to calculate the nest volume, average nest volumes per species varied between ca. 540 cm 3 (Great Tit) and ca. 800 cm 3 (Coal Tit). Other studies indicated that Coal Tits, Great Tits or Blue Tits are able to more than triple the nest volume in chambers that are substantially larger than those we used in our study plot (e.g., see Glutz von Blotzheim & Bauer 1993, Lambrechts et al. 2014, Møller et al. 2014, own observations). Why then do species differ in nest volume in small standardized nest-chambers? At a biogeographic scale, Coal Tits traditionally occupy coniferous habitat whereas other Paridae, like Great Tits and especially Blue Tits, traditionally occupy mixed deciduous and deciduous forest types (Snow 1954a,b, Partridge 1979, Perrins 1979, but see Blondel et al. 2006). We therefore do not exclude that the species-specific nest characteristics may have become adapted to species-specific habitat characteristics (cf. Snow 1954b, Partridge 1979). For instance, Coal Tits might have been selected to breed in cooler coniferous forest types that subsequently resulted in the evolution towards larger, more insulated, nests. Lambrechts et al. (2014) did not exclude that past environmental factors related to climate might have selected for genetically-based species-specific nest size chamber size interactions that are still maintained today in the absence of the selection pressures that produced them. We also might expect that Great and Blue Tits that traditionally occur in mixed deciduous or deciduous woods have lower breeding success in cedar woodland than Coal Tits that traditionally occupy coniferous forest types. We therefore do not exclude that speciesspecific associations between nest characteristics and breeding success might vary according to the degree of local adaptation to the cedar forest as reflected in species-specific local breeding success. Lambrechts et al. (2014) proposed that secondary cavity-nesting Paridae that differ in body size might perceive weather-related factors differently, which could be subsequently reflected in how the nest is designed. For instance, smaller species assumed to be more vulnerable to cold, might require larger, and thus more insulated, nests than larger species assumed to be less vulnerable to cold (e.g. Lambrechts et al. 2014). However, the two smallest species in our study (Coal and Blue Tits) differed substantially in the external height of the nest wall (Table 2). This strongly indicates that the aspects of the nest that we quantified are not strongly associated with species-specific body size. In addition, when the data from the three species were combined, we observed that the external height of the nest wall was not significantly related to the average daily ambient temperature measured during the time of nest building, although the nest walls tended to be shallower when there was more rain. These kinds of associations were also reported in studies that combined the data from Great and Blue Tits that occupied warmer Mediterranean study sites (Lambrechts et al. 2016). According to Wesołowski et al. (2002), larger nests might be more efficient in keeping the nest occupants away from water that might accumulate at the bottom of natural cavities, which is inconsistent with our findings from nest-boxes. Why then should there be a significant negative association between the efforts of nest building as reflected in the external height of the nest wall and the average daily rainfall at the time of nest building? Perhaps the simplest proximate explanation is that females were constrained from building larger nests by the adverse weather conditions (see also Lambrechts et al for other explanations). This could explain why nest building is slowed down during cold and rainy weather (e.g. Flanagan & Morris 1979). However, the causal relationships between precipitation and avian nest characteristics remain unclear because experimental evidence is lacking. Perhaps causal relationships between nest size and rainfall could be examined with birds building nests in outdoor aviaries (e.g. Lambrechts et al. 1997) and exposed to different levels of humidity or artificial rain, e.g. to make the nesting material or the nest-building conditions more or less wet. In forests, the foundation of the nest is most often mainly built from moss. That moss might protect the nest occupants against lower ambient temperatures experienced within the nest chamber is supported by observations briefly reported in Nager & van Noordwijk (1992). They found that Great Tits from Switzerland add moss against box walls that have been experimentally made colder with ice packages. Thus, the nest cup position apparently avoids colder box walls. We observed that during the nest building phase the external height of the nest wall closest to the entrance was higher than the external height of the nest wall farthest from the entrance. Of the 214 nest-box visits with two measures of the external nest wall, 91% had a higher external nest wall at the side where the nest touched the entrance

6 128 SHORT NOTES plate than at the side where it touched the main nest-box body, with a difference in nest height of up to 6 cm. Because the boxes, and thus the nestbox floors, were only weakly tilted and not more tilted than in other study areas, nest-box placement could not explain this unusual asymmetry in nest shape which has not been reported in other Mediterranean study areas. However, we used boxes with removable entrance plates that were substantially thinner (ca cm) than the thickness of the walls of the main nest-box bodies (ca. 3 cm) (see Appendix 1 in Lambrechts et al. 2010). In addition, although nest-chambers are assumed to protect occupants against wind (e.g. Wachob 1996), many of our boxes were quite old. For instance, entrance plates did not always hermetically close the boxes so that an air flow occasionally might have entered the nest chambers via the edges of the entrance plates. We therefore do not exclude that the nest builders added moss against the coldest and more deteriorated entrance plates to protect them against an incoming cold air flow. Females are known to roost in the nest-chamber during the egg-formation period (e.g. Kluijver 1950, Flanagan & Morris 1979, Perrins 1979), so a more insulated nest shape might protect roosting females against cold perhaps favoring the energy budgets and therefore the process of egg formation. We also found that nest-shape asymmetry was higher in the Coal Tits than in the other two species (Table 2, 3) indicating species-specific responses to the nest-chamber characteristics. Future experiments could manipulate nest-chamber microclimates to identify the underlying proximate mechanisms that link nestmicroclimate to nest shape and nest-shape asymmetry. Future studies could also focus on the currently unmeasured aspects of the nest, like the depth of the nest cup or the animal-based lining material (hetero-specific hair, fur, or feathers) used to finish the nest cup assumed to be associated with weather (Deeming et al. 2012, Mainwaring et al. 2012). To conclude, we report the exceptional pattern that three co-existing cavity-nesting species that face the same external conditions and that nest in the same type of small standardized nest-chamber, do not build nests in the same way, despite the fact that the three species are able to build substantially larger cavity nests in larger nestchambers. This suggests that the perception of, and the responses to, a given nest-building environment in wildlife conditions are species-specific, and can also be disassociated from energyrelated constraints. For instance, different species might perceive and explore the same abiotic nestbuilding environment in a different way, which might subsequently be expressed in how the nest is built (e.g. Surgey et al. 2012). Although speciesspecific responses to external environments are also expected to be found in open-nesting species, wildlife studies with open-nesters face the logistic difficulties that nest placements cannot be fully standardized perhaps complicating the interpretation of field data associated with nest size in some taxonomic groups. Studies of nest size in laboratory conditions could be an additional option to examine nest-building responses in different experimentally controlled environmental regimes to ultimately identify causal associations between nest size and biotic or abiotic nest-building environments. ACKNOWLEDGEMENTS We would like to thank all the people that assisted with field work and a reviewer that provided constructive comments on a former version of the manuscript. This study has been financially supported by the CNRS and the OSU (OREME), France. The community of Bedoin (Mont-Ventoux) and the National Office of Forests kindly allowed access to the Cedrus forest. REFERENCES Álvarez E., Barba E Nest quality in relation to adult bird condition and its impact on reproduction in Great Tits Parus major. Acta Ornithol. 43: 3 9. Blondel J Breeding strategies of the Blue Tit and the Coal Tit (Parus) in mainland and island Mediterranean habitats: a comparison. J. Anim. Ecol. 54: Blondel J., Isenmann P Insularité et démographie des Mésanges du genre Parus (Aves). C. R. Acad. Sci. Paris, Ser. D 289: Blondel J., Thomas D. W., Charmantier A., Perret P., Bourgault P., Lambrechts M. M A thirty-year study of phenotypic and genetic variation of Blue Tits in Mediterranean habitat mosaics. 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C Opportunistic use of a wool-like artificial material as lining of Tit (Paridae) nests. Condor 114: Wachob D. G A microclimate analysis of nest-site selection by Mountain Chickadees. J. Field Ornithol. 67: Wesołowski T Anti-predator adaptations in nesting Marsh Tits Parus palustris: the role of nest-site security. Ibis 144: Wesołowski T., Czeszczewik D., Rowiński P., Walankiewicz W Nest soaking in natural holes a serious cause of breeding failure? Ornis Fennica 79: STRESZCZENIE [Sosnówki budują większe gniazda niż współwystępujące z nimi bogatki i modraszki w śródziemnomorskich lasach iglastych] Wielkość oraz kształt gniazda są cechami charakterystycznymi dla gatunku. Obie te cechy są często powiązane z warunkami środowiskowymi występującymi przed etapem składania jaj. W pracy badano wielkość gniazda (mierzoną jako wysokość ścian) w pierwszych lęgach trzech gatunków sikor gniazdujących w skrzynkach lęgowych w stosunkowo chłodnych śródziemno - morskich lasach cedrowych. Stwierdzono, że warunki pogodowe w okresie budowy gniazda były podobne dla trzech bada - nych gatunków (Tab. 1). Jednak sosnówki budo - wały gniazda wyższe niż bogatki i modraszki (Tab. 2, 3). Nie stwierdzono istotnych związków wielkości gniazda z temperaturą, ale wysokość gniazda wiązała się ujemnie z wielkością opadów deszczu (Tab. 3). Asymetrię gniazda, tj. różnicę między wy sokością ściany przedniej i tylnej gniazda, zaobserwowano u wszystkich trzech gatun ków, ale była ona najwyższa u sosnówki (Tab. 2). W asymetrycznych gniazdach więcej materiału gniazdowego znajdowało się przy przedniej ściance skrzynki, czyli tej, która najsłabiej zabezpiecza ptaki przed trudnymi warunkami pogodowymi. Uzyskane wyniki wskazują, że nawet u gatunków o bardzo zbliżonej ekologii wielkość gniazda może się różnić.