DOI: 10.5061/dryad.h9w0vt4qk
title: A long-term study of size variation in Northern Goshawk Accipiter gentilis across Scandinavia, with a focus on Norway

description: Changing climate and growing human impacts are resulting in globally
 rising temperatures and the widespread loss of habitats. How species will
 adapt to these changes is not well understood. The Northern Goshawk
 (Accipiter gentilis) can be found across the Holarctic but is coming under
 more intense pressure in many places. Studies of recent populations in
 Finland and Denmark have shown a marked decline in body size of Northern
 Goshawks over the past century. Here we investigate long-term changes to
 Norwegian populations of Northern Goshawk by including material from the
 Middle Ages. We measured 240 skeletons of modern Northern Goshawks from
 Norway, Sweden, Denmark and Finland, and 89 Medieval Goshawk bones. Our
 results show that Norwegian and Swedish female Goshawks have decreased in
 size over the past century, whilst males showed little decline. Medieval
 female Goshawks were larger than contemporary females. A decline in forest
 habitats and a concomitant shift towards smaller prey likely drove a shift
 to smaller body size in Northern Goshawks. Our study shows that
 significant body size changes in birds can occur over relatively short
 time spans in response to environmental factors, and that these effects
 can sometimes differ between sexes.
description: Modern comparative
 material To analyse changes in both modern and past
 populations of <em>A. gentilis</em>, metric data of the
 nominate <em>Accipiter</em> <em>gentilis</em>
 <em>gentilis</em> were collected from across the Nordic
 countries (Norway, Sweden, Denmark and Finland). The Northern Goshawk is a
 sedentary species, generally choosing to breed and winter in the same
 area. There are some exceptions to this in North America, Fennoscandia and
 Russia (Squires et al., 2020). However, individuals from Fennoscandia
 rarely migrate further than 300 Km (Squires et al., 2020). The nominate
 <em>A. g. gentilis</em> is distributed across Europe and east
 to the Urals, Caucasus, and Asia Minor, and southwards to NW Africa
 (Squires et al., 2020). The slightly larger subspecies
 <em>Accipiter</em> <em>gentilis</em>
 <em>buteoides</em> breeds in northern Fennoscandia and
 Siberia, wintering in south and central Eurasia (Ferguson-Lees &
 Christie, 2005). Goshawks found in northern Finland and northern Sweden
 likely represent the subspecies <em>A. g. buteoides</em>
 (Gladkov, 1941; Vaurie, 1965). Therefore, we excluded any material from
 these areas in our study. In addition, young Northern Goshawk from
 interior parts of Scandinavia are known to winter along the Norwegian
 coast in the north (Fransson & Petterson, 2001; Bakken et al.,
 2003). To avoid mixing of the larger subspecies <em>A. g.
 buteoides</em> within the modern comparative sample, we decided to
 use material from the south of Norway, Sweden and
 Finland.
       Modern comparative skeletons (collected over the past
 c. 150 years from 1861 to 2015) have been measured from the University Museum
 of Bergen, the Natural History Museum of Denmark and the Finnish Museum of
 Natural History (see Appendix 1). The specimens were originally
 collected from Norway (n = 65), Sweden (n = 30), Denmark (n = 93) and
 Finland (n = 52) (see Fig. 2). We measured all skeletal elements, except for
 the vertebrae and ribs of 240 modern partial and complete
 <em>Accipiter gentilis gentilis</em> skeletons of which 103
 were female and 137 were male. It is not clear how sex of the specimens
 was initially determined, however, we presume that for the majority it was
 through internal inspection. <em>Accipiter gentilis</em> are
 highly sexually dimorphic, with the female being larger than the male
 (i.e., reversed sexual size dimorphism). There is very little to no overlap
 between the sexes of Northern Goshawk (Kenward, 2006). As a result, it was
 possible to identify 5 modern specimens that were likely to have been
 mis-sexed at the time of collection. Wrongly identified sex can be common
 in <em>Accipiter</em> subspecies and especially in
 <em>Accipiter</em> <em>gentilis</em>, as the
 paired ovaries can sometimes be mistaken for testes (Storer, 1966). The
 specimens which were wrongly sexed have been reclassified as the correct
 sex based on our osteological analysis and included within this study. The
 museum numbers of mis-sexed specimens are as follows; B 4462 and KL 31303
 (originally recorded as males but fall within the female size range and
 have been reclassified as females), B 9016, NHMD 306590 and NHMD 306670
 (originally recorded as females but fall within the male size range and
 have therefore been reclassified as males). All specimens were measured by
 SJW using digital callipers. Measurements followed the conventions set out
 in Von den Driesch (1976). Three additional measurements were recorded:
 the smallest depth of the distal shaft of the humerus (KB) found in Kraft
 (1972), depth of the ulna proximal end (Tp) and height of the symphysis of
 the carpometacarpus (HS) taken from Otto (1981).
 Data suggest that Finnish
 populations of the nominate <em>A. g. gentilis</em> are
 slightly larger in their wing length and body mass than other Scandinavian
 populations (Tornberg et al., 2006), although the reasons for this remain
 unclear. It is possible that they represent a slightly more northern
 clinal population with bigger proportions. Alternatively, it may reflect a
 slightly more continental climate than Norway and Sweden (Tornberg pers.
 comm.). In addition, we noticed that the Danish specimens included here
 were on average smaller than the other Scandinavian specimens, again
 possibly due to clinal variation. ANOVA tests were used to detect
 statistical differences between the modern populations for each country.
 The results show that Norway and Sweden did not differ; Denmark was often
 statistically different to Norway, Sweden and Finland; Norway and Sweden
 rarely differed from Finland (full ANOVA results in Supporting Information
 File 1 (SIF1)). For these reasons, we have grouped Norway and Sweden
 together but kept Denmark and Finland separate when drawing
 comparisons. Archaeological material
 In general, <em>A.
 gentilis</em> is not frequent within the archaeological record for
 Norway (Walker et al.,
 2019). Morphologically,
 the osteology of <em>A. gentilis</em> is not easily confused
 with any other species, and we are confident of specimen identification.
 Despite this, all specimens were confirmed using the extensive modern
 comparative collections held at the University Museum of Bergen. In total
 89 Medieval specimens were included within this study (Table 1).
 It is worth noting here
 that the 89 bones do not represent 89 individuals, although there is a
 possibility that some of these bones would have come from the same
 individual. The Medieval
 bones
 date to 1030–1537 Common
 Era (CE) and come from only 8 sites, all from the urban contexts of Oslo,
 Bergen and Trondheim (Table 1). Most Medieval bones come from female individuals and are likely to
 be linked to the practice of falconry (Walker et al., 2019). Most of the
 archaeological bones were limb elements, this may be due to
 taphonomic bias as they are more robust than for example cranial remains.
 However, this limited the skeletal elements that could be used for
 comparison. Within this paper, we focus on the humerus, ulna,
 carpometacarpus, femur, tibiotarsus and
 tarsometatarsus. Data analysis 
 We first explored differences
 in size between groups using descriptive statistics in PAST 4.03 (Hammer
 et al., 2001). All data were tested for normality by looking at the
 variances and the Shapiro-Wilk test for normality (see SIF1). Principal Components Analysis (PCA) was used
 to establish which measurements were most responsible for the observed
 differences. Two separate
 PCAs were performed, one on the modern specimens only (including
 Norwegian, Swedish, Danish and Finnish modern specimens; Supporting
 Information File 2 (SIF2)) and a second with both modern and
 archaeological specimens (including Norwegian, Swedish, Danish and Finnish
 modern and Norwegian Medieval specimens; Supporting Information File 4
 (SIF4)). To test for main and interaction effects of time, sex and country
 on greatest lengths (GL) of the humerus and femur of modern Northern
 Goshawks from Norway, Sweden and Denmark, we performed an Analysis of
 Covariance (ANCOVA) in R Statistical Software (v4.1.1; R Core Team, 2021),
 with the factor Time as the covariate and the factors Sex (2 levels), and
 Country (2 levels: Denmark and (Norway + Sweden) grouped together). For
 the humerus, n = 132, of which n = 57 for Norway & Sweden combined
 (43 males, 14 females), and n = 75 from Denmark (46 males, 29 females).
 For the femur, n = 172, with n = 83 for Norway & Sweden combined
 (55 males and 28 females) and n = 89 from Denmark (49 males, 40 females).
 Finnish modern specimens were excluded from the linear regression (Fig. 3)
 and the ANCOVA because, of the 52 Finnish specimens available, only 7
 predate the year 2000, preventing a detailed look into the past
 century. To test for statistical differences between the
 mean greatest lengths of Norwegian Medieval specimens and modern Norwegian specimens of <em>A. g. gentilis</em>, we used
 a 10.000-iteration Fisher’s permutation test in R. The permutation test is somewhat similar to
 the bootstrap but differs from it in that a permutation test resamples
 without replacement. First, the sample means for each group and the
 difference between these means is computed. The data are then pooled and
 randomly permuted. The means and difference in mean for the permutated
 samples are computed. This process is then repeated <em>n</em>
 times for all possible permutations of the data, resulting in a frequency
 distribution of the mean difference. The 95% confidence interval and
 p-values can then be calculated. We considered p-values ≤ 0.05 statistically
 significant.
description: # A long-term study of size variation in Northern Goshawk Accipiter
 gentilis across Scandinavia, with a focus on Norway
 Changing climate and
 growing human impacts are resulting in globally rising temperatures and
 the widespread loss of habitats. How species will adapt to these changes
 is not well understood. The Northern Goshawk (*Accipiter gentilis*) can be
 found across the Holarctic but is coming under more intense pressure in
 many places. Studies of recent populations in Finland and Denmark have
 shown a marked decline in body size of Northern Goshawks over the past
 century. Here we investigate long-term changes to Norwegian populations of
 Northern Goshawk by including material from the Middle Ages. We measured
 240 skeletons of modern Northern Goshawks from Norway, Sweden, Denmark and
 Finland, and 89 Medieval Goshawk bones. Our results show that Norwegian
 and Swedish female Goshawks have decreased in size over the past century,
 whilst males showed little decline. Medieval female Goshawks were larger
 than contemporary females. A decline in forest habitats and a concomitant
 shift towards smaller prey likely drove a shift to smaller body size in
 Northern Goshawks. Our study shows that significant body size changes in
 birds can occur over relatively short time spans in response to
 environmental factors, and that these effects can sometimes differ between
 sexes. ## Description of the data and file structure The excel file
 provides the measurements and background information for 240 modern
 comparative Northern Goshawk (*Accipiter gentilis gentilis*) skeletons
 (collected over the past c. 150 years from 1861-2015) from the University
 Museum of Bergen, the Natural History Museum of Denmark and the Finnish
 Museum of Natural History. The specimens were originally collected from
 Norway (n = 65), Sweden (n = 30), Denmark (n = 93) and Finland (n = 52)
 (see Fig. 2). We measured all skeletal elements, except for the vertebrae
 and ribs of 240 modern partial and complete Northern Goshawk skeletons of
 which 103 were female and 137 were male. The spreadsheet also includes
 measurements of 89 Medieval Goshawk bones from Norway. All measurements
 are in millimeters. The following headings are included: | Heading |
 Description | | ---------------- |
 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Museum Number/ID | Individual accession number given by the museum to each specimen. | | Institution | Institution where the specimen is held. | | Date of Specimen | Date the specimen died. | | Country | Country the specimen was recovered in. | | Location | More detailed location specimen was recovered. Where available. | | Complete/Partial | Refers to whether the skeleton was complete or partial. Note: for some entries they are listed as complete but not all measurements are present, this was due to time restrictions and focusing on the key elements related to this study, meaning it was not always possible to measure the whole skeleton. | | Measurements | All measurements refer to those in Von den Driesch (1976), with a couple of exceptions which along with the abbreviations are detailed below this table. All measurements are taken in mm. (C) = Cranium measurements, (M) = Mandible measurements, (ST) = Sternum measurements, (COR) = Coracoid measurements, (SCA) = Scapula measurements, (HUM) = Humerus measurements, (RAD) = Radius measurements, (ULN) = Ulna measurements, (MC) = Carpometacarpus measurements, (PEL) = Pelvis measurements, (FEM) = Femur measurements, (TIB) = Tibiotarsus measurements, (MT) = Tarsometatarsus measurements, (FUR) = Furcula measurements, (PH1) = Proximal phalanx measurements. | | JS number | Individual number given to each excavated site which is held in the osteological collections at the University Museum of Bergen. | Detailed descriptions of how to take the measurements can be found in 'Von den Driesch, 1976, A guide to the measurement of animal bones from archaeological sites' for the measurement of bird bones please see pages 103- 136, document can be found freely available online. measurement abbreviations by element: Cranium (C): GL = greatest length, GB = greatest breadth, SBO = smallest breadth between the orbits, LI = length of the Incisivum. LP = length from the Protuberantia occipitalis externa to the most aboral point of the processus frontales, GH = greatest height, CBL = condylobasal length. Mandible (M): GL = greatest length, LaF = length from the most aboral point of the articular surface, LS = length of symphysis. Sternum measurements (ST): Lm = length from the Manubrium sterni to the Metasternum, dL = dorsal length, LC = length of the Crista sterni, SBF = smallest breadth between the facets. Coracoid measurements (COR): GL = greatest length, Lm = medial length, Bb = basal breadth, BF = breadth of the Facies articularis basalis. Scapula measurements (SCA): GL = greatest length, Dic = cranial diagonal, KC = smallest breadth of the Collum scapulae (measurement taken from Otto C. 1981. Vergleichend morphologische Untersuchungen an Einzelknochen in Zentraleuropa vorkommender mittelgrosser Accipitridae: Schädel, Brustbein, Schultergürtel und Vorderextremität. München: Ludwig-Maximilians-Uiversität. [German]). Humerus measurements (HUM): GL = greatest length, Bp = breadth of the proximal end, SC = smallest breadth of the corpus, Bd = breadth of the distal end, KB = smallest depth of the distal shaft (measurement taken from Kraft E. 1972. Vergleichend morphologische Untersuchungen an Einzelknochen nord-und mitteleuropaischer kleinerer Huhnervogel. München: Ludwig-Maximilians-Uiversität. [German].). Radius measurements (RAD): GL = greatest length, SC = smallest breadth of the corpus, Bd = breadth of the distal end. Ulna measurements (ULN): GL = greatest length, Dip = diagonal of the proximal end, Bp = breadth of the proximal end, Tp = depth of the proximal end, SC = smallest breadth of the corpus, Did = diagonal of the distal end. Carpometacarpus measurements (MC): GL = greatest length, Bp = breadth of the proximal end, Did = diagonal of the distal end, HS = height of the symphysis (HS measurement taken from Otto C. 1981. Vergleichend morphologische Untersuchungen an Einzelknochen in Zentraleuropa vorkommender mittelgrosser Accipitridae: Schädel, Brustbein, Schultergürtel und Vorderextremität. München: Ludwig-Maximilians-Uiversität. [German].). Pelvis measurements (PEL): GL = greatest length, LV = length along the vertebrae, CB = cranial breadth, SB = smallest breadth of the Partes glutaeae, AA = breadth between the borders of the acetabula, DiA = diameter of one acetabulum, BA = breadth across the two antitrochanter. Femur measurements (FEM): GL = greatest length, Bp = breadth of the proximal end, Dp = depth of the proximal end, SC = smallest breadth of the corpus, Bd = breadth of the distal end, Dd = depth of the distal end. Tibiotarsus measurements (TIB): GL = greatest length, Dip = diagonal of the proximal end (defined as, from the Condylus medialis femoralis to the Crista lateralis), Bp = breadth of the proximal end, SC = smallest breadth of the corpus, Bd = breadth of the distal end, Dd = depth of the distal end. Tarsometatarsus measurements (MT): GL = greatest length, Bp = breadth of the proximal end, SC = smallest breadth of the corpus, Bd = breadth of the distal end. Furcula measurements (FUR): GL = greatest length. Proximal phalanx measurements (PH1): GL = greatest length. ## Code/Software The scripts used within the associated article are also included in this repository.