Large increase in nest size linked to climate change: An indicator of life history, senescence and condition Anders Pape Møller* Jan Tøttrup Nielsen2

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Large increase in nest size linked to climate change:

An indicator of life history, senescence and condition
Anders Pape Møller*,1 . Jan Tøttrup Nielsen2
1 Laboratoire d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud, Bâtiment 362, F-91405 Orsay Cedex, France;

2 Espedal 4, Tolne, DK-9870 Sindal, Denmark
Word count: 6194

Figures: 3

Tables: 3
Correspondence to APM:

Tel: (+33) 1 69 15 56 88

Fax: (+33) 1 69 15 56 96


Author Contributions: APM and JTN conceived and designed the study. JTN collected the data. APM analyzed the data and wrote the manuscript.

Abstract Many animals build extravagant nests that exceed the size required for successful reproduction. Large nests may signal the parenting ability of nest builders suggesting that nests may have a signaling function. In particular many raptors build very large nests for their body size. We studied nest size in the goshawk Accipiter gentilis, which is a top predator throughout most of the Nearctic. Both males and females build nests, and males provision their females and offspring with food. Nest volume in the goshawk is almost three-fold larger than predicted from their body size. Nest size in the goshawk is highly variable and may reach more than 600 kg for a bird that weighs ca. 1 kg. A fraction of 8.5% of nests fell down, but smaller nests fell down more often than large nests. There was a hump-shaped relationship between nest volume and female age with a decline in nest volume late in life as expected for senescence. Clutch size increased with nest volume. Nest volume increased during 1977-2014 in an accelerating fashion, linked to increasing spring temperature during April, when goshawks build and start reproduction. These findings are consistent with nest size being a reliable signal of parental ability, with large nest size signaling superior parenting ability, senescence, and with nest size indicating climate warming.
Keywords: Accipiter gentilis . Climate change . Extended phenotype . Goshawk . Senescence . Signal

Many animals build nests that are structures used for holding offspring (Collias and Collias 1984; Hansell 2000, 2007). Nest building has evolved numerous times independently in taxa as diverse as arachnids, crustaceans, insects, fish, reptiles, birds and mammals (Hansell 2007). Nests are often large and elaborate structures that may reach a height of several meters as in termites and ants and weigh several tons (Hansell 2007). The excessive amount of time and energy sometimes invested in nest building begs the question as to what are the advantages of such large nests. The traditional answer is that they function in protection and rearing of offspring although minimal structures required for efficient parental care can hardly explain exaggerated nest structures in many other taxa. Indeed, nest size is twice as large in birds species with biparental nest building compared to species with uniparental building, even when their clutch size is the same (Soler et al. 1988).

Nests constructed early during spring, at high latitudes and at high altitudes where the weather is colder protect offspring against inclement weather by being larger and better insulated (Schaefer 1980; Kern and Van Riper 1984; Møller 1984; Mainwaring and Hartley 2008; Crossman et al. 2011; Mainwaring et al. 2012). Such benefits of insulation may be traded against the higher risk of predation because large nests are more conspicuous and hence suffer more from predation (Møller 1990). Alternatively, specific types of nest material may provide protection of eggs and nestlings against bacteria and parasites (Wimberger 1984; Mennerat et al. 2009; Peralta-Sánchez et al. 2010). An optimal nest size will prevent excessive fouling of nests and the associated fitness costs of nestling death by allowing parents to keep the nest clean, as demonstrated experimentally for starlings Sturnus vulgaris (Erbeling-Denk and Trillmich 1990). Improved insulation provided by larger nests may benefit parent birds by preventing heat loss from eggs and nestlings. Large nests may reduce crowding of the offspring and prevent them from falling out of the nest (Slagsvold 1982, 1989; Heenan and Seymour 2011). Thus, reproductive success is positively correlated with nest size (Alabrudzinska et al. 2003; Álvarez and Barba 2008; Møller et al. 2014a, b). Nests may act as signals revealing features of the quality of the nest builder just as secondary sexual characters act as signals (Tortosa and Redondo 1992; Soler et al. 1998a, b; Møller 2006; Broggi and Senar 2009; Sanz and García-Navas 2011; Tomás et al. 2013). This hypothesis is supported by correlational and experimental studies showing that larger nests are favored because they result in differential parental investment by the partner (Soler et al. 1998b, 1999, 2001; de Neve et al. 2004; Tomás et al. 2013). Nests are extended phenotypes that reliably reveal information about quality features of nest builders. For example, bird species with more elaborate nests have a larger cerebellum compared to species with simpler nests (Hall, Street and Healy 2013). Thus nest structures may reliably reflect features of the cognitive abilities of the nest builder.

The objective of this study was to analyze long-term data on nest size in the goshawk Accipiter gentilis. We tested a number of predictions relating to the hypothetical signalling function of large nests using the goshawk as a model system. Nest size may act as an indicator of phenotypic quality as described above. If nests are larger than predicted for containers that can safely hold eggs and nestlings, we would expect that (1) nest size is larger than predicted from the allometric relationship between nest size and body size in birds. If nest size reflects individual reproductive ability, we should expect that (2) nest size is significantly repeatable among females and more repeatable than the effect of locality on nest size. If large nest size reflects superior quality, we should expect that (3) smaller nests were more likely to fall down, while the null hypothesis would be that a large and unwieldy nest would be more likely to fall to the ground. If the nest building ability of females is reflected in larger nests, but also greater reproductive investment, we should expect (4) clutch size to increase with nest size.

Senescence reflects the deterioration in phenotype at old age due to accumulation of mutations, disposable soma or deteriorating anti-oxidant capacity (e. g. Nussey et al. 2013). There are no previous studies investigating senescence effects in nest building. Senescence should be particularly visible for the most costly phenotypic characters. Hence nests of goshawks that may weigh several hundred kilograms may constitute a prime candidate for senescence. If nest building ability increases with age and subsequently decreases as a consequence of senescence, we should expect (5) a hump-shaped relationship between nest size and age.  

Finally, nest size may increase when there are benefits to be gained from insulation of nest contents (Schaefer 1980; Kern and Van Riper 1984; Møller 1984; Mainwaring and Hartley 2008; Crossman et al. 2011; Mainwaring et al. 2012). In contrast, nest size may increase with climate warming if increasing temperatures reduce the costs of nest construction. In that situation we should expect nest size to increase with increasing temperature. Therefore, (6) nest size should increase with increasing spring temperature during a period of climate warming. There are to the best of our knowledge no previous studies of this prediction.

Many species of raptors are suitable for studies of the factors affecting nest size, especially because nest sites are used repeatedly during long periods of time, the identity of individuals can be determined from feather coloration, and nest size is extraordinarily large for species of a given body size (Cramp and Simmons 1980; Kenward 2006; Newton 2010).
Materials and methods

Study species

The goshawk is a solitary, socially monogamous large raptor distributed across most of the Nearctic (Cramp and Simmons 1980; Kenward 2006). The large nest is built 10-20 m above ground in a large tree, and it is used in successive years although pairs may have two-three nests that are used in different years. The nest consists of twigs and branches lined with twigs with green needles and after leafing green twigs from larch Larix larix and deciduous trees (Cramp and Simmons 1980; Kenward 2006). It is constructed by both sexes although males contribute disproportionately, especially to new nests (Holstein 1942). Nest size reaches up to 100 cm in diameter and 100 cm in height (Cramp and Simmons 1980), which equals more than one cubic meter and weighs more than half a tonne, assuming that the specific gravity is less than 0.50 (Wikipedia: on 30 January 2015). Nest building starts up to 40 days before laying, and new material especially green material is added throughout the incubation and nestling periods. Clutch size is usually 3-4 eggs that are incubated by the female while the male provisions the female and small nestlings with food (Kenward 2006).
Methods for recording nest size

We retrieved information on nest height, nest diameter and body mass for all species of birds from Cramp and Perrins (1977-1994). Nest volume was calculated from the equation of an ellipsoid. The allometric relationship between log10-transformed nest size and log10-transformed body mass was used to calculate the expected nest size for a species with the average body mass of a goshawk (1140 g; Cramp and Simmons 1980).

Field methods

Jan Tøttrup Nielsen (JTN) systematically visited more than 120 localities that once held nests of goshawks in Northern Vendsyssel (57°10’ - 57°40’N, 9°50’ - 10°50’E), Denmark during April-August 1977-2014. This was part of a long-term population study. The study area is mainly agricultural habitats with mixed forests, villages and open heath. See Nielsen and Drachmann (2003) and Nielsen and Møller (2006) for a detailed description of the study areas. Each nest was visited at least three times during the breeding season. Not all pairs produced nestlings explaining the reduction in sample size from egg laying until the nestling period. A total of 569 nests built by 317 females were followed in this study. Not all nests were measured for logistic reasons since some nests were impossible to measure without endangering the measurer or the nest.

Goshawk nest size

JTN measured nest dimensions with a tape measure to the nearest cm recording maximum nest height, width and breadth when visiting the nest when nestlings were 18-30 days old.

Goshawk reproduction

JTN checked all nest sites of goshawks. Once having identified occupied nests these were all visited when nestlings were predicted to be 18-30 days old in an attempt to eliminate nest abandonment due to disturbance. We recorded wing length of the most developed nestling at the first visit to the nest, assuming that the first egg was laid 41 days before the first egg hatched (Holstein 1942). Hatching and laying date of the first eggs were subsequently determined based on wing length.

The identity of breeding females was confirmed from individual color, shape, length and pattern on their primaries and rectrices (Opdam and Müskens 1976; Kühnapfel and Brune 1995; Nielsen and Drachmann 2003; Kenward 2006). JTN kept all primaries and rectrices throughout the study, and these were used for subsequent identification of individual adults. Because all territories were visited annually, the age of reproducing females was estimated from the first year when a female was present in a territory, as yearlings, two years old, or at least three years old depending on plumage characters (Kenward 2006). Because the last category of individuals was at least three years old, we cannot be sure that this category did not include some individuals that were four years old. Hence age was the minimum age for these few individuals. These age estimates were corroborated by comparison with age as determined from 60 adults that had been ringed as nestlings and were later captured at the nest sites using standard traps. All these adults were correctly aged (3 one-year old, 12 two-years old, 9 three-years old, 7 four-years old, 6 five-years old, 3 six-years old, 6 seven-years old, 4 eight-years old, 3 nine-years old, 3 ten-years old, 1 eleven-years old, 1 thirteen-years old, 1 fourteen-years old, and 1 sixteen-years old). A total of 34 captured adults were not ringed as nestlings in the study area and hence must have immigrated from elsewhere. Finally, we emphasize that a total of only 16 individual goshawks that changed breeding forest during their lifetime were identified based on individual color, shape, length and pattern on their primaries and rectrices or from capture (three females). Although our study area of several hundred square kilometers allowed for tracking of individual females between territories, we cannot exclude the possibility that a few females dispersed outside our study area. Females of unknown age at the start of the study were excluded from analyses of age effect, ensuring that only females of known age contributed to the dataset.
Nest fate

Nest fate was recorded during visits as either ‘retained’ when the nest was still present, ‘fallen down’ when the nest was located on the ground below the nest tree, ‘cut down’ when nest trees or entire woodlots were cut, or the nest was ‘shot down’ as evidenced by gunshot.

Climatic conditions

Monthly mean temperatures from Aalborg were downloaded from the Danish Meteorological Institute. We have previously used winter (December–February) temperatures as a measure of winter severity (Herfindal et al. 2015), which is a period of the year when food scarcity may affect female condition and energy reserves needed for breeding. March temperature represents a measure of the period during initiation of breeding. April is the main period of egg laying and incubation during which climate can play a major role for reproductive success in birds (e.g. Herfindal et al. 2015; Nord et al. 2010 and references therein), whereas most eggs have hatched and environmental conditions can be important for chick survival in May.

Statistical analyses

We tested whether observed nest volume in the goshawk was larger than the predicted volume by analyzing the relationship between log10-transformed nest volume and log10-transformed body mass for 266 species of European birds. We compared the observed nest volume with the expected volume using a t statistic and the observed mean and SE.

We used General Linear Mixed Models (GLMM) to quantify variation in nest volume among females, localities and nests. We calculated the repeatability of nest volume among females and among localities using the intra-class correlation and the associated SE (Falconer and Mackay 1996; Becker 1984).

We tested whether nests with different fate differed in nest volume using a Welch ANOVA for unequal variances. We used logistic regression to test whether nests that fell down differed in nest volume and age from nests that did not fall down.

We analyzed the relationship between nest volume and female age and age squared, year, year squared as fixed effects and female identity and locality as random effects to account for variation among females and localities and for differences in sampling effort among females and localities. Female identity and locality were included as random effects because a few females changed nest site among years. We included year and year squared and female age and female age squared to account for linear and quadratic effects of these variables.

We analyzed clutch size in relation to female age, female age squared, nest volume, nest age and mean temperature during April and temperature squared as fixed effects and female identity and locality as random effects. The effect of age and age squared reflected the senescent effect with deterioration in nest building performance with increasing age, while nest volume accounted for the nest size effect, and nest age reflected any deterioration in nest condition with age or accumulation of parasites over time. The effects of spring temperature and spring temperature squared reflected the effects of climate change while controlling statistically for other potentially confounding variables in the model just described.

We estimated effect sizes by using Cohen’s (1988) guidelines for the magnitude of effects being small (Pearson r = 0.10, explaining 1% of the variance), intermediate (r = 0.30, explaining 9% of the variance) or large (r = 0.50, explaining 25% of the variance) relying on partial correlation coefficients. Values reported are means (with one SE in parentheses unless stated otherwise). All analyses were made using JMP (SAS 2012).

Comparative nest size

Nest volume increased isometrically with body mass in 266 species of European birds (F = 704.94, df = 1, 264, r2 = 0.73, P < 0.0001, estimate (SE) = 0.996 (0.038)) with a slope that did not differ significantly from the null hypothesis of a slope of one (t = 0.11, df = 264, P = 0.91). Nest volume of the goshawk was 89,094 cc, which equals 4.94985 in log10 units. The expected value from the allometry equation was 1.46965 (intercept) + 0.995979 x log10-transformed body mass of the goshawk = 3.0567 = 4.5141. This value back transformed gives 32,666 cc. Thus the observed relative nest size compared to the expected value is 89,094 cc / 32,666 cc, or 273% larger than predicted from body mass. This difference between observed and expected nest volume is highly significant (t = 61.62, df = 264, P < 0.0001).
Components of variation in nest size

Summary statistics for nest dimensions are reported in Table 1. The largest nest volume was 0.875 m3, which with a specific gravity of 0.5 equals 438 kg. The different nest measurements were highly variable as reflected by their high coefficients of variation. The number of years during which nests had been in use ranged from one to 33 years. Although change in nest volume from one year to the next ranged enormously among nests from a decrease by 0.459 m3 to an increase by 0.289 m3, the mean value of 0.007 m3 did not differ significantly from zero (t = -1.05, df = 241, P = 0.29). Thus an increase in nest size was not a simple consequence of the age of females changing over time. The frequency distributions of nest height, nest volume and nest age were strongly left-skewed with many more nests with large values than expected by chance (Table 1).

Nest volume was significantly repeatable among females (F = 2.22, df = 316, 225, P < 0.0001, R = 0.42 (SE = 0.06)), and repeatability among females was more than twice as large as that among localities (F = 2.42, df = 108, 460, P < 0.0001, R = 0.19 (SE = 0.03)).

A model of nest volume in relation to locality, female identity and nest identity as random factors explained 54% of the variance with most variation occurring among females (23.2%, 95% CI = 0.0043, 0.0250) followed by locality (14.3%, 95% CI = 0.0019, 0.04600) and least among nest sites (2.6%, 95% CI = -0.0013, 0.0046).

Small nests fall down more often than large nests

Out of 1877 nest years 159 nests or 8.5% fell down, while 16 nests fell down because the tree fell down (0.9%). A total of 129 nest trees were cut (6.9%) and four nests were shot down. Nests with different outcomes differed significantly in volume (F = 3.59, df = 4, 564, r2 = 0.02, P = 0.0066). Surprisingly, nests that fell down were 25% smaller (mean = 0.110, 95% confidence interval 0.092 to 0.131) than nests that did not fall down (mean = 0.147, 95% CI 0.140 to 0.155). Whether nests fell down or not did not depend on tree species (Wald 2 = 24.62, df = 19, P = 0.17). Whether nests fell down or not depended on nest volume (Wald 2 = 4.12, df = 1, P = 0.043, estimate (SE) = -3.01 (1.50)), but not on nest age (Wald 2 = 0.62, df = 1, P = 0.43).

Nest volume and life history

Nest volume was quadratically related to female age with nests being the smallest when females were either young or very old (Fig. 1; Table 2). In addition, nest volume increased over time, but particularly strongly in recent years (Table 2).

The model for clutch size in relation to nest volume, female age and April temperature accounted for 7% of the variance and had three significant predictors (Table 3). Clutch size increased with nest volume with a predicted increase from 2.7 to 3.2 eggs across the range of nest volumes, when controlling for the random effects of locality and female identity (Table 3). There was a quadratic effect of female age with clutch size peaking at intermediate age (Table 3). Finally there was a quadratic effect of April temperature with the smallest clutches occurring at intermediate temperatures (Table 3).
Nest volume and climate change

Nest volume changed over time with significant linear and quadratic terms (Fig. 3). Nest volume was linearly related to temperature in April (Fig. 2; F = 8.05, df = 1, 503.8, P = 0.0047, estimate (SE) = 0.019 (0.007)) with a small effect size of 0.17. Nest volume increased from an average of 120,226 cc to 173,780 cc, or an increase by 45%, when April temperature increased from 3.6 to 10.5ºC (Fig. 2). The relationship between nest volume on one hand and female age and April temperature on the other is shown in Fig. 3.


The main findings of this study were that nest size in the goshawk was excessive for a bird of its size, and that nest volume responded to phenotypic and environmental factors such as age, senescence, time and temporal change in temperature during spring. Several observations were consistent with expectations for nest volume being a reliable signal of phenotypic quality. These included exaggerated nest size relative to what would be expected for a species with the body mass of the goshawk, small nests being more likely to fall down that large nests, nest volume increasing with experience and then decreasing as predicted for senescence, and nest volume being positively associated with clutch size.

Goshawks often refurbish old nests (Cramp and Simmons 1980; Kenward 2006; this study), which at first glance would predict an increase in nest size from one year to the next although such an effect would not be additive because nests may shrink as a consequence of the nest material becoming more compact over time. However, differential loss of nests with respect to size (this study), differences in age of nests (this study) and differences in duration of occupation of nest sites (this study) all contribute to elimination of such a linear increase in nest size over time. In contrast to this expectation, we found that change in nest volume from one year to the next did not differ significantly from zero. Thus the findings reported here were independent of these potentially confounding variables. These results also imply that nest size reflects annual nest building effort and not accumulated effort of breeding pairs over time. Finally, we emphasize that we still include all these potentially confounding variables in the statistical analyses.

Nest size in the goshawk was related to climate change. A number of intraspecific and interspecific studies have shown that nest size and structure increase under colder conditions (Schaefer 1980; Kern and Van Riper 1984; Møller 1984, 2006; Mainwaring and Hartley 2008; Crossman et al. 2011; Mainwaring et al. 2012). For example, Mainwaring et al. (2012) showed in the blue tit Cyanistes caeruleus that nests at higher latitudes are bigger and better insulated than conspecific nests at lower latitudes. Therefore, we should expect that birds adjust their nest size and structure to climate change by reducing nest size as spring temperatures increase. In our study area temperature in April has increased from 3.6 to 10.5ºC since 1975, which was accompanied by an increase in nest size in the goshawk by 45%. This effect was controlled for the random effects of locality and female. This increase in nest size is unlikely to have occurred in response to demands for better nest insulation since nests were actually smaller when temperatures were lower during reproduction in the 1970’s and 1980’s. If nests were better insulated at low temperatures, we would expect that they would be larger at the beginning of the study in 1977. In contrast, the temporal increase in nest size is consistent with higher temperatures allowing breeders to allocate more effort to nest building even in the absence of a requirement for better nest insulation in years with high spring temperatures. Indeed, Herfindal et al. (2015) showed that climatic conditions during April were closely associated with food abundance and reproductive success. These findings suggest that nest size may serve as a sensitive biomarker of climate change in the goshawk and potentially other raptors with similar ecology. This is of general interest because nest size increased considerably in response to a tripling in temperature during April, when nest building and early reproduction takes place. This change in nest size in response to an increase in temperature is in contrast to laying date, clutch size, brood size and longevity that barely changed in response to the increase in temperature (J. T. Nielsen and A. P. Møller unpublished data). Likewise, life history traits in the sympatric sparrowhawk Accipiter nisus barely changed across years despite considerable change in phenology of prey and a general advancement in spring phenology caused by an increase in spring temperatures (Nielsen and Møller 2006; Møller et al. 2010).

Nests may function as signals if nests are much larger than required for holding young, and if only individuals in prime condition are able to construct large and elaborate nests (Tortosa and Redondo 1992; Soler, et al. 1998a, b; Møller 2006; Broggi and Senar 2009; Sanz and García-Navas 2011; Tomás et al. 2013). Several aspects of size of goshawk nests are consistent with this hypothesis. First, goshawk nests are very large for the body size of the bird being two- to three-fold larger than predicted. Maximum nest size in the present study reached 438 kg, which is large compared to the body mass of a goshawk (1140 g). Thus goshawks invested heavily in nest construction. Second, large nests were less likely to fall down than small nests, contrary to expectation. This was independent of the age of nests and tree species. Third, nest size reflected reproductive investment measured in terms of clutch size. Fourth, nest size decreased when individuals reached old age consistent with the expectation that nest size decreased in response to senescence.

Nest size in the goshawk was correlated with several life history characters. We showed for the first time that nest size is affected by senescence. While nest size increased with age in the first years of reproduction before reaching a plateau, it decreased in size among the oldest individuals when reaching an age of 10-15 years. Because nest size may reflect individual condition, and because nest size reflects considerable reproductive effort in terms of time and energy expenditure, we should expect nest size to deteriorate when goshawks reached very old age. We have already shown for the present goshawk population that laying date, clutch size and intensity of nest defense all show signs of senescent deterioration at old age (Møller and Nielsen 2014). Such senescent effects could be consistent with either of the current hypotheses of senescence including the mutation accumulation, the disposable soma or the antioxidant hypothesis (Nussey et al. 2013).

Clutch size and reproductive success have been shown to increase with nest size in secondary hole nesters (Alabrudzinska et al. 2003; Álvarez and Barba 2008; Møller et al. 2014a, b), but also in other species (review in Møller et al. 2014b). This effect has been interpreted to be a consequence of females and males investing more in reproduction when having built large nests. Here we have shown that goshawks with large nests have larger clutches than goshawks with smaller nests. In conclusion, nest size in the goshawk is a reliable indicator of climate change, reproductive success and senescence.

We thank the landowners for access.


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Legends to figures
Fig. 1 Nest volume (m3) in relation to age (years) of female goshawks
Fig. 2 Nest volume (m3) in goshawks in relation to (mean April temperature (ºC)). The line is the regression line with confidence intervals
Fig. 3 Nest volume (m3) in goshawks in relation to mean April temperature (ºC) and female age (years). The surface shows the predicted relationship and the dots individual observations

Fig. 1

Fig. 2

Fig. 3

Table 1 Summary statistics for dimensions and age of goshawk nests









Length (cm)








Width (cm)








Height (cm)








Volume (m3)








Nest age (years)








Change in nest volume (m3) from one year to the next








Table 2 Nest volume in relation to locality and female identity as random factors and female age and year as fixed factors. The random effect of locality had a variance component of 0.0064 (SE = 0.0030), 95% CI 0.00056 to 0.0122 accounting for 11.7% of the variance, while the random effect of female had a variance component of 0.0135 (SE = 0.0046), 95% CI 0.0044 to 0.0226 accounting for 22.7% of the variance


df denominator







< 0.0001



Female age






Female age squared









< 0.0001



Year squared






Table 3. Clutch size in relation to locality and female identity as random factors and female age, nest volume and age and April temperature as fixed factors. The random effect of locality had a variance component of 0.0323 (SE = 0.0269), 95% CI -0.0204 to 0.0851 accounting for 8.0% of the variance, while the random effect of female had a variance component of -0.0754 (SE = 0.0312), 95% CI -0.0874 to 0.0312 accounting for 0.0% of the variance


df denominator







< 0.0001



Female age






Female age squared






Nest volume






Nest age






April temperature






April temperature squared






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