Embryonic eggshell thickness erosion: A literature survey re-assessing embryo-induced eggshell thinning in birds

Environmental Pollution 205 (2015) 218e224

Contents lists available at ScienceDirect

Environmental Pollution journal homepage: www.elsevier.com/locate/envpol

Review

Embryonic eggshell thickness erosion: A literature survey re-assessing embryo-induced eggshell thinning in birds Grzegorz Orłowski a, *, Lucyna Hałupka b a b


, Poland Institute of Agricultural and Forest Environment, Polish Academy of Sciences, Bukowska 19, 60-809 Poznan Ornithological Station, Faculty of Biology, University of Wroclaw, Sienkiewicza 21, 50-335 Wroclaw, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 February 2015 Received in revised form 1 June 2015 Accepted 3 June 2015 Available online

Although eggshell thinning has been described mainly in the context of environmental pollution, it can also be the effect of reproductive changes induced by a developing embryo. On the basis of a literature survey of 25 bird species (26 published papers) we reviewed data on embryo-induced eggshell thinning (EET) in three groups of birds: precocials, semi-precocials and altricials. The average EET at the equator of the eggs was 6.4% (median ¼ 4.7%). Our review did not confirm a general prediction of elevated EET at the egg equator in precocial species: altricial birds exhibited the highest EET (average ¼ 12.0%), followed by precocials (7.6%) and semi-precocials (4.2%). We make certain critical recommendations based on the results of this study. Studies aiming to assess variation in eggshell thickness should examine intrinsic factors affecting shell properties of avian eggs, like thickness, which are the result of anatomical or reproductive changes. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Avian eggs Embryo development Infertile eggs Embryo-induced eggshell thinning

1. Introduction Eggshell thinning has been associated mainly with failures in reproduction of wild birds, and explained as a serious ecotoxicological effect of contaminants (Ratcliffe, 1970; Newton, 1979; Ormerod et al., 1988; Henny and Bennett, 1990; Lundholm, 1997; Nybø et al., 1997; Hoffman et al., 2003; Jagannath et al., 2008). For almost half-a-century the insecticide DDT and one of its metabolites, the persistent p, p0 -DDE, were indicated as the principal culprit responsible for eggshell thinning in wild birds (Ratcliffe, 1970; Newton, 1979; Ormerod et al., 1988; Henny and Bennett, € and Sondell, 1990; Nybø et al., 1997; Jagannath et al., 2008; Odsjo 2014). However, eggshell thinning is also an effect of the reproductive changes induced by a developing embryo (hereafter: embryoinduced eggshell thinning, EET), which is an intrinsic feature of oviparous vertebrates, i.e. reptiles and birds (Pulliainen and Marjakangas, 1980; Deeming and Ferguson, 1991; Karlsson and Lilja, 2008; Castilla et al., 2010; Maurer et al., 2011a). This phenomenon is a consequence of the fact that the eggshell provides the embryo with the minerals and calcium needed for the development

* Corresponding author. E-mail address: [email protected] (G. Orłowski). http://dx.doi.org/10.1016/j.envpol.2015.06.001 0269-7491/© 2015 Elsevier Ltd. All rights reserved.

of calcium-consuming organs, including the skeleton, muscles and brain, which results in a progressive reduction in eggshell thickness during embryonic development (Simkiss, 1961; Blom and Lilja, 2004; reviewed by Karlsson and Lilja, 2008). The apparent EET occurs along the length of an egg: from the sharp pole through the equator (the middle part) and shoulders (the widest part) to the blunt end, where the decline is least pronounced (Pulliainen and Marjakangas, 1980; Booth, 1989; Kern et al., 1992; Castilla et al., 2010; Maurer et al., 2011a). Previous studies have shown that growth rate may play a fundamental role in the pattern of skeletal development in birds: the faster the growth, the less ossified the skeleton is at hatching (Bond et al., 1988; Karlsson and Lilja, 2008). A comparative study embracing 36 bird species from 18 orders provided evidence that slow-growing precocial and fast-growing altricial species lay eggs encased in shells with different structures adapted to support different rates of calcium removal by developing embryos (Karlsson and Lilja, 2008; cf. Castilla et al., 2010). In particular, calcium removal and the degree of eggshell erosion (indexed by the decrease in density of the mammillary tips per unit surface area at the egg equator) were more extensive in the shells of precocial birds than in those of altricial species (Karlsson and Lilja, 2008). In addition, it has been shown that calcium removal from the mammillary tips is associated with a number of effects on the shell: generally fewer changes occur in the shells of altricial than in

G. Orłowski, L. Hałupka / Environmental Pollution 205 (2015) 218e224

precocial bird species. Presumably, this difference is due to the fact that altricial embryos require less calcium for their development (Bond et al., 1988; Karlsson and Lilja, 2008). Overall, eggshell thickness seems to be influenced by a variety of factors: genetics, egg colour, maculation and size, the time the eggs spend in the uterus, female characteristics (condition, age, stress, health status), female diet or infection (Snyder and Meretsky, 2003; reviewed in Castilla et al., 2010; Maurer et al., 2012, 2011a). More importantly, despite the considerable body of research devoted to the erosion of embryonic shell thickness, the effect of developmental stage on eggshell thickness or strength remains unknown for most wild species (cf. Castilla et al., 2010). To the best of our knowledge, this issue has not been comprehensively reviewed, even though some results relating to EET in various species of birds have been published. Analytical limitations due to EET are rarely considered in studies of eggshell thickness variation (cf. Pulliainen and Marjakangas, 1980; Castilla et al., 2010). However, the failure to acknowledge this primary information in experimental and observational studies dealing with changes in thickness may undoubtedly lead to a false interpretation of the causes of eggshell thinning (cf. Gonzalez and Hiraldo, 1988; Bennett, 1995; Nygård, 1999; Castilla et al., 2007, 2010). In particular, eggshell samples (i.e. from eggs of an unknown embryonic developmental stage) as well as shells from abandoned eggs have been treated equally in many analyses (e.g., Ormerod et al., 1988; Falk et al., 2006; King et al., 2003; Jagannath € and Sondell, 2014). Likewise, comparisons of et al., 2008; Odsjo eggshell thickness data from museum eggs (unknown status of embryo presence/development) and those of hatched eggs (presumably thinner) (cf. King et al., 2003) could be particularly biased and does not allow a clear identification of the cause of eggshell thinning, as already noted by earlier researchers (e.g., Rothstein, 1972; Pulliainen and Marjakangas, 1980; Gonzalez and Hiraldo, 1988; Nygård, 1999). Based on the above framework, we have compiled here the findings of published studies on EET in wild and domestic birds, and re-analysed these data across three groups of birds categorized by developmental mode: precocial, semi-precocial and altricial. We hypothesized that the relative difference in shell thickness between infertile eggs (without an embryo or unincubated) and fertile ones (embryonated or hatched) would be greater in precocial species compared to the other groups. Conceptually, this prediction is an important complement to the earlier study of Karlsson and Lilja (2008). However, those authors gave no detailed analysis of eggshell thickness variation in relation to the formation/presence of an embryo. Furthermore, eggshell thickness is generally a function of egg mass and/or female mass in precocial and altricial species (Ar et al., 1974, 1979; Rahn et al., 1975; Rahn and Paganelli, 1989; Birchard and Deeming, 2009; Dyke and Kaiser, 2010). Precocial species generally lay larger eggs, but they have a smaller mass relative to female body mass than in altricial or semiprecocial birds (Dyke and Kaiser, 2010). Higher intrinsic demands and removal of greater amounts of calcium are expected from the eggshell in larger species. Therefore, in the light of previous studies of individual species, where the proportion of female mass to eggshell thickness was analysed (cf. Castilla et al., 2010; Hargitai et al., 2011), we determined the analogous relationship across three developmental modes of birds, between EET and female mass. Finally, on the basis of our results we formulated some critical recommendations regarding studies of eggshell thinning and shell thickness variation in birds. 2. Methods This systematic literature review of the effect of embryo

219

development on eggshell thickness in wild and domestic birds was based on a search of three scientific databases (Scopus, Web of Science and JSTOR) and an internet search (Google/Google Scholar). When papers were selected, their references (backward search) and citation records (forward search) were searched for other articles that could provide relevant data. We extracted from these papers the methodological details relating to the presence of an embryo and/or its stage of development. Unfortunately, however, such detailed information was often missing, especially in older works. In our statistical analysis we used only the results that revealed a difference (decrease) in shell thickness caused by embryonic development. Data that did not provide robust evidence for EET (e.g., Burnham et al., 1984; Falk and Møller, 1990; Bennett, 1995; Massaro and Davis, 2005) were omitted from the analysis. This failure to detect an EET effect most likely resulted from the examination of eggs with relatively well developed embryos and/or samples containing few or no infertile eggs (discussed by Castilla et al., 2010). Since data on EET in birds are generally scarce, and no information is available to show that domestic rearing could be a factor affecting avian eggshell properties, we included in our analyses and treated equally the results obtained for both wild and domestic birds. Since our major aim was to define EET-related differences in shell thickness (very likely to a large degree physiologically controlled), we assumed that the equal treatment of data for wild and domestic/hand-reared birds could also reflect potential differences occurring in natural conditions, where resources are subject to large fluctuations. Wherever available, data on shell thinning in infertile and fertile eggs, and in eggs with embryos at various developmental stages, were extracted from publications. The decrease in shell thickness due to the embryonic development of eggs was expressed as a percentage. The developmental mode of bird species was classified in two different ways. In the first classification, birds were divided into precocial and altricial species (according to Cracraft, 1988; Starck and Ricklefs, 1998). In the second classification, apart from precocial and altricial species, we introduced the additional category of semi-precocial species, which display mixed altricial and precocial traits (Starck and Ricklefs, 1998; Karlsson and Lilja, 2008). The latter division concurs with the classification applied by Karlsson and Lilja (2008). Data on female body mass (g) and egg mass (g) were gathered from the original publication, and if lacking, from the literature (Kotter, 1970; Ar et al., 1979; Sayce and Hunt, 1987; Cramp, 1998; Ramstack et al., 1998; Faquinello et al., 2004; Deeming, 2002; Dyke and Kaiser, 2010; Krist, 2011; del Hoyo et al., 2014). In most cases, we were able to find the missing data on female and egg masses for the geographical region where the eggshell thinning study was conducted. Our initial careful reading of the papers on embryonic eggshell thickness erosion in most cases revealed considerable inconsistencies in the description of the status of embryos in eggs. Since the egg content was not dissected, detailed information on the presence of an embryo was often missing, and the authors classed eggs without embryonic development as ‘unhatched’, ‘unincubated’, ‘freshly laid’, or occasionally as ‘infertile’ (when the presence or absence of an embryo was determined). Similarly, eggs with embryonic development were described as ‘hatched’, ‘posthatch’, ‘incubated’, ‘developed’ or ‘fertile’. Given these equivocal descriptions, we classified eggs into two developmental categories: eggs with an undeveloped embryo (i.e. infertile and without embryonic development) and embryonated/developed eggs (Castilla et al., 2010). Although such an approach combines heterogeneous data, it does seem to be valid within the main context of our review. In addition, it appears to agree with recommendations for a

220

G. Orłowski, L. Hałupka / Environmental Pollution 205 (2015) 218e224

Table 1 Results of studies on embryonic eggshell thickness erosion in three groups of birds representing different developmental modes: precocials (P), semi-precocials (SP) and altricials (A);1 egg region relates to the thickness at the equator unless stated otherwise.1 Egg region: blunt end (B), equator (E), sharp pole (S);A changes in eggshell thickness index. Species (developmental mode)

Body mass (g) Egg Embryo assessment Incubation state/egg characteristic mass (g) Presence Developmental only stage

Struthio camelus (P) Gallus domestica (P)

83500 1100

1461 35.9

No No

No No

Unhatched > hatched Unincubated > hatched

1.04% 7.9%

Numida meleagris (P)

1299

37.4

No

No

Unincubated > post-hatch

90 453

9.0 21.9

No No

No Yes

Unincubated > incubated Early incubated > late incubated

Yes No

No No

Infertile > fertile Unincubated > hatched

3.9% (B) 6.7% (E) 4.0% (S) 7.3% 1.7% (B) 4.4% (E) 0.4% (S) 20.8% 0.5% (B) 4.4% (E) 7.9% (S) 0.7% (B) 7.9% (E) 11.4% (S) 6.2% 8.2%

Coturnix c. japonica (P) Alectoris chukar (P)

173 345

Difference Source (reference) (% decrease) in shell thickness (egg region)1 Sahan et al., 2003 Pulliainen and Marjakangas, 1980 Ancel and Girard, 1992

Kreitzer, 1972  et al., 2010 Karabag

Leipoa ocellata (P) Cygnus olor (P)

1785 9670

Anas platyrhynchos (P)

1301

52.2

No

No

Unincubated > hatched

Larus occidentalis (P/SP) Larus ridibundus (P/SP)

950 260

89 36.3

Yes No

No No

Infertile (mostly) > hatched Unincubated > hatched

Larus ridibundus (P/SP)

260

36.3

No

Yes

Unincubated > developed

Sterna paradisea (P/SP) Aptenodytes patagonica (A/SP) Nycticorax nycticorax (A/SP) Plegadis chihi (A/SP) Falco p. peregrinus (A/SP)

110 13220

19.0 303

Yes No

No No

Unincubated > hatched Freshly laid > hatched

2.8% (B) 4.5% (E) 12.6% (S) 7.4e8.2% 4.2%

883

20.8

No

No

Unincubated > incubated

2.5%A

Bunck et al., 1985

546 1201

36.1 46.4

No Yes

No Yes/no

Unincubated > 11 d of incubation Infertile/aborted during 1 week of incubation > hatched or aborted with fully developed embryo Infertile/aborted during 1 week of incubation > hatched or aborted with fully developed embryo Infertile/aborted during 1 week of incubation > hatched or aborted with fully developed embryo Unincubated > incubated Fresh laid > hatched

4.3% 4.79%

Capen, 1977 Castilla et al., 2010

1.61%

Castilla et al., 2010

4.43%

Castilla et al., 2010

0.5%A 1.3% 3.8%A 4.7%A 5.6%

Bunck et al., 1985 Snyder and Meretsky, 2003 Bunck et al., 1985 Bunck et al., 1985 Sotherland et al., 1980

Falco p. babylonicus (A/SP)

850

44

Yes

Yes/no

Falco cherrug (A/SP)

1135

53

Yes

Yes/no

13.8 272

No No

No No

21.5 14 2.2

No No Yes

No No No

Falco sparverius (A/SP) 120 Gymnogyps californianus 10104 (A/SP) Tyto alba (A/SP) 303 Otus asio (A/SP) 190 Petrochelidon pyrrhonota (A) 22 Ficedula hypoleuca (A)

16

1.7

No

No

Unincubated > incubated Unincubated > incubated Undeveloped > hatched (presence of chorioallantois) Unincubated > hatched

Acrocephalus scirpaceus (A)

13

1.9

Yes

No

Non-embryonated > embryonated

Bombycilla cedrorum (A)

33

3.2

Yes

Yes

Unincubated > eggs with embryo

5.0% (B) 29.0% (E) 42.8% (S) 3.9% (B) 8.0% (E) 5.4%

Booth and Seymour, 1987 Booth, 1989

Balkan et al., 2006

Hunt and Hunt, 1973 Pulliainen and Marjakangas, 1980 Maurer et al., 2011b

Finnlund et al., 1985 Handrich, 1989

Kern et al., 1992

Orłowski et al., submit. Rothstein 1972

Sources of some data (if not given in the publication) on body mass and egg mass: Kotter, 1970; Ar et al., 1979, Sayce and Hunt, 1987; Dunning, 1993; Cramp, 1998; Ramstack et al., 1998; Faquinello et al., 2004; Dyke and Kaiser, 2010; Krist 2011.

research synthesis or systematic review at a biologically meaningful level (cf. Koricheva and Gurevitch, 2012). All the data were statistically analysed using Statistica 7.0 (StatSoft., 2006). The probability of P < 0.05 was assumed to be statistically significant. First, using the ANOVA KruskaleWallis test we compared the decrease (%) in shell thickness at the equator of infertile/unincubated and fertile/hatched eggs between precocial and altricial birds, including the semi-precocial group of species based on the results of all the studies (Table 1). To explore the relationship between the decrease (%) in eggshell thickness (EET)

and egg mass expressed as a percentage of female body mass, female mass and egg mass, we produced a linear regression with GLM (Statsoft, 2007) performing three separate models, for all 26 species analysed, i.e. a group of 12 precocial species and one of 14 altricial species (see Supporting Information for some additional analyses of EET in precocial and altricial species; Table S1 and Fig. S1). Before the analyses, percentage values were arcsine sqrt transformed and masses were log-transformed. All the data used in the statistical analyses met the assumption of normality indexing through the insignificant results of KolmogoroveSmirnov test.

G. Orłowski, L. Hałupka / Environmental Pollution 205 (2015) 218e224

3. Results Based on the literature survey (a total of 26 papers; Table 1), the average decrease in shell thickness as a result of EET at the equator of the eggs of 25 bird species (including one geographically distant subspecies) was 6.4% (95% CI ¼ 4.0% and 9.9%; range ¼ 0.5% and 29.0%), with a median of 4.7% (interquartile range ¼ 4.2% and 7.8%). The average decrease in shell thickness at the blunt end (7 species) was considerably smaller: 2.9% (median ¼ 3.9%) compared to a marked decline in shell thickness at the sharp end (6 species): 13.2% (median ¼ 9.7%) (Table 1). Overall, the studies of precocial species (n ¼ 12) showed a relatively greater EET at the equator between unincubated/infertile and embryonated eggs (average ¼ 7.3%, 95% CI ¼ 4.2% and 10.3%; median ¼ 7.8%) compared to altricial birds (average ¼ 5.7%, 95% CI ¼ 1.7% and 9.8%; median ¼ 4.4%; n ¼ 14), although this difference was barely not statistically significant (ANOVA KruskaleWallis test, H1,26 ¼ 3.62, P ¼ 0.057) (Table 1). Similarly, the difference in EET at the equator of unincubated/ infertile and embryonated eggs among precocial, semi-precocial and altricial species was marginally significant (ANOVA KruskaleWallis test, H2,26 ¼ 5.94, P ¼ 0.051). In 8 precocial species, the mean EET was 7.6% (median ¼ 7.0%), and 4.2% (median ¼ 4.4%) in 13 semi-precocial species (Table 1). The highest EET value was recorded in four typical altricial species (average ¼ 12.0%; median ¼ 6.8%) (Table 1). 3.1. Effect of egg mass and female mass on EET GLM analyses showed that in all the species (Table 1) EET at the equator was negatively correlated with female mass (r2 ¼ 0.157, F1,24 ¼ 4.47, P ¼ 0.045; Fig. 1) but not with egg mass (r2 ¼ 0.124,

Fig. 1. Relationship between embryo-induced eggshell thinning (EET) at the equator of eggs and log-transformed female body mass based on the results of 26 studies on precocial (C), semi-precocial ( ) and altricial (C) bird species (see Table S1 for Spearman rank correlation coefficients). The regression line is fitted for all the species: EET ¼ 1.0038  Log female mass þ12.726 (see more details in Result section). Comparison of egg mass expressed as a percentage of female body mass between the three groups of birds is depicted on the smaller graph along with results of the KruskaleWallis test. Species code: fc (Ficedula hypoleuca), le (Leipoa ocellata), lr (Larus ridibundus), as (Acrocephalus scirpaceus), g (Gallus domestica), an (Anas platyrhynchos), sp (Sterna paradisea), co (Coturnix c. japonica), nu (Numida meleagris), lo (Larus occidentalis), pp (Petrochelidon pyrrhonota), bc (Bombycilla cedrorum), fp (Falco p. peregrinus), oa (Otus asio), lr (Larus ridibundus), fh (Falco cherrug), ac (Alectoris chukar), cy (Cygnus olor), pc (Plegadis chihi), ap (Aptenodytes patagonica), ta (Tyto alba), ny (Nycticorax nycticorax), fb (Falco p. babylonicus), gc (Gymnogyps californianus), st (Struthio camelus), fs (Falco sparverius).

▫

221

F1,24 ¼ 3.40, P ¼ 0.078; Fig. S1) or with egg mass as a percentage of female body mass (r2 ¼ 0.128, F1,24 ¼ 3.51, P ¼ 0.073; Fig. S1). The corresponding GLM analyses explaining EET at the equator for the group of 11 precocial species (12 studies; cf. Table 1) did not yield any significant relationship: female mass (r2 ¼ 0.163, F1,10 ¼ 1.95, P ¼ 0.193; Fig. 1), egg mass (r2 ¼ 0.093, F1,10 ¼ 1.03, P ¼ 0.335; Fig. S1) or egg/female mass (r2 ¼ 0.113, F1,10 ¼ 1.28, P ¼ 0.284; Fig. S1). Finally, EET at the equator among the 14 altricial species was significantly negatively correlated with female mass (r2 ¼ 0.287, F1,12 ¼ 4.83, P ¼ 0.048; Fig. 1) and egg mass (r2 ¼ 0.325, F1,12 ¼ 5.79, P ¼ 0.033), but not with egg mass as a percentage of female body mass (r2 ¼ 0.147, F1,12 ¼ 2.07, P ¼ 0.176; Fig. S1). 4. Discussion Our literature survey, which for the first time collates evidence from a large set of papers, has shown that a decrease in eggshell thickness as a result of embryonic development occurs in precocial, semi-precocial and altricial birds, the average value of these changes across all studied species being 6.4%. These results suggest that EET is in fact the norm throughout the class Aves, and is a consequence of previously well-documented physiological/ anatomical changes or erosion of eggshell structure (Pulliainen and Marjakangas, 1980; Deeming and Ferguson, 1991; Karlson and Lilja, 2008; Castilla et al., 2010; Maurer et al., 2011a). The results of this review did not confirm our initial hypothesis that there would be a greater decrease in eggshell thickness resulting from embryonic development in precocial species. Indeed, we found that altricial birds exhibited the highest EET (average ¼ 12.0%), followed by precocials (7.6%) and semi-precocials (4.2%), however, the difference between these three groups only approached statistical significance. Generally, during the development of an avian embryo, calcium removal occurs both through the gradual erosion and the decrease in the number of mammillary tips per unit surface area of the egg; this is more pronounced in precocial species than in altricial ones (Karlsson and Lilja, 2008). Since our review seems to suggest a greater EET in altricial species compared to precocial ones, this may indicate that eggshell erosion due to embryonic development in altricial species can occur mostly through the decrease of shell thickness. It should be emphasized that in the context of EET, there are numerous differences between altricial, precocial and semiprecocial species. First of all, the female body mass of the precocial species covered by our review was approximately 6-fold and 590-fold greater compared to the semi-precocial and altricial species, respectively. Correspondingly, the egg mass was 4-fold and 24fold greater in precocials than in semi-precocial and altricial species, respectively. Secondly, altricial nestlings weight less and are less well-developed at hatching compared to precocial ones, but they have well-formed bones (but not muscles) (Starck and Ricklefs, 1998). In altricial nestlings the embryonic growth rate increases continuously during incubation, whereas in precocial species it declines shortly before hatching (Vleck and Vleck, 1987). On the other hand, as the above comparison of egg and female masses (Fig. S1) indirectly showed, the ratio of the initial body mass of hatchlings to female mass (undoubtedly similar to the eggfemale mass ratio) is greater in altricial species than in precocial ones (the difference was at the level of significance). This may partly explain the relatively higher rate of EET found in altricial species. In fact this high rate of EET found in altricial species is also a consequence of the extremely high value of EET found in the Pied Flycatcher (29%; Table 1). In comparison with other passerines or birds with a relatively small body mass, this high value of EET in the Pied Flycatcher indicates a disproportionately large disparity and

222

G. Orłowski, L. Hałupka / Environmental Pollution 205 (2015) 218e224

the need for a re-evaluation of EET in other populations of this species. Furthermore, although we found a statistically significant negative relationship between EET and female mass and egg mass in altricial species (see also Table S1), this resulted from the inclusion of the Pied Flycatcher data in our analysis. Re-analysis without this data yielded no significant correlation for this group of birds. Another important finding from our work seems to be the positive relationship between EET and egg mass as a function of female body mass revealed in the analysis of Spearman correlation for all the bird species analysed (see Table S1). In other words, species that lay large eggs relative to their body mass (like passerines and gulls/terns) are characterized by a more intensive EET. To conclude, our analysis showed a relatively high rate of EET in altricial species, and overall some of the presented relationships (e.g., between EET and female mass or egg mass as a function of female body mass) seem to be biologically justified in the context of the embryonic and post-hatch development of birds. Nevertheless, since a limited number of methods rather than a uniform method were applied to each group of birds in the works under review, further investigations to determine and evaluate the effect of environmental and physiological correlates of EET in a variety of bird species representing all developmental modes of birds are urgently needed. Such studies/analyses should be based on precisely defined protocols registering the developmental stages of embryos and accompanying changes in shell thicknesses (cf. Castilla et al., 2010). Finally, because none of the between-mode differences in EET was statistically significant, this suggests that EET at the equator of eggs occurs in all birds regardless of their developmental mode. Most importantly, the determination of the degree of EET should be treated as an output state in any investigation or assessment of the changes in eggshell thickness induced by external conditions, particularly pollution or environmental factors. 4.1. Implications for studies of eggshell thinning and variation in shell thickness in birds Some critical recommendations for eggshell thinning studies have emerged from our literature survey. To begin with, the results of many previous studies on pollution-induced eggshell thinning (including comparative long-term analyses of historical egg samples) are based on comparisons between museum eggs and hatched eggs, as in Ratcliffe (1970) classic study. Literally, this means that eggshell thinning resulting from the developmental changes of an embryo was ignored. Some authors were aware that they had neglected this factor (cf. Falk et al., 2006), but most of them apparently not (e.g., Ratcliffe, 1970; Newton, 1979; Green, 1998; Scharlemann, 2003). Therefore, an important conclusion resulting from our work is the need to re-assess some previous findings on eggshell thinning, especially those severely impacted by environmental changes, including pollution. For example, the most recent study (based on proper methodological foundations for detecting natural eggshell thinning) of the Peregrine Falcon Falco p. peregrinus, the eggs of which were highlighted as clear cases of severe pollution-induced shell thinning, showed a 4.8% decrease in shell thickness between infertile and hatched eggs or eggs with a fully-developed embryo (Castilla et al., 2010; cf. Table 1). Unfortunately, Ratcliffe (1970) assessment of a decrease in eggshell thickness in Peregrine Falcons was not based on direct measurements of shell thickness, but on a thickness index e a function of shell mass. Even allowing for the close correlation of these two values, i.e. shell thickness measurements and the shell thickness index (Nygård, 1999; Green, 2000; Mauer et al., 2012), the study by Bunck et al. (1985) showed contradictory changes between the index and measured shell thickness, which could mean

that the changes in these two values induced by embryo development are variable. On the other hand, the lowest average EETs were noted in semi-precocial species (4.2%), including predatory birds with the two highest decreases among these reported for Peregrine Falcon Falco peregrinus (4.8%) and Saker Falco cherrug (4.4%) (Castilla et al., 2010; cf. Table 1). This could suggest that the potential errors arising out of the neglect of embryonic eggshell erosion in raptors (including the numerous DDT studies of these species) would be relatively small compared to typical altricial or precocial species. This indirectly indicates that data relating to these species would be least affected by any misinterpretations of historical eggshell data. However, we believe that such differences in natural eggshell thinning between birds representing various developmental modes deserves greater detailed examination. On the other hand, museum eggs have been generally found to have relatively thick shells (Green, 1998; Scharlemann, 2003), although some of them must have experienced some embryonic development (Nygård, 1999). Evidence for this comes from the labels of museum eggs, or indirectly from the blow-hole size, which can be a useful indicator of the presence and/or size of an embryo (i.e. a larger blow-hole indicates a more developed embryo) (Kiff, 1989; Nygård, 1999). In addition, even if there are some analytical irregularities associated with the proper classification of museum eggshells (in the context of their embryonic development), it should be stressed that museum eggs are uniquely valuable in pollution studies, especially those comparing the levels of contaminants in historical samples (Gonzalez and Hiraldo, 1988; Henny and Bennett, 1990; Green and Scharlemann, 2003). Finally, since many avian pollution studies, including those of an experimental approach, have shown apparent negative correlations between pesticide residues and eggshell thickness (Ratcliffe, 1970; Ormerod et al., 1988; Henny and Bennett, 1990; Nybø et al., 1997; Jagannath et al., 2008; reviews in: Pulliainen and Marjakangas, 1980; Lundholm, 1997), the first principle in all eggshell thinning studies should be to eliminate the potential bias resulting from embryonic shell erosion. Furthermore, since the sources of variation in eggshell thickness, especially in large predatory birds, like Californian Condors Gymnogyps californianus (cf. Snyder and Meretsky, 2003; Burnett € and Sondell, 2014) et al., 2013), Ospreys Pandion haliaetus (Odsjo or Peregrine Falcons (Castilla et al., 2010), are still under intensive examination, determining the presence of an embryo (along with the proper identification of the region of an egg from which a shell fragment originates; e.g., Snyder and Meretsky, 2003) may be crucial to explain the causes of eggshell thinning per se. For instance, Snyder and Meretsky (2003) found in Californian Condors a small decrease in eggshell thickness between fresh and hatched eggs (1.3%), although those authors did not determine whether all the eggs were embryonated. However, inspection of the shell thickness data relating to Californian Condor eggs given by Kiff et al. (1979) showed that the shells of infertile eggs were much thicker (by up to c. 48%) than the other eggs examined. In addition, although the reduction of shell thickness found in the speckles of eggs of Black-headed Gull Larus ridibundus was minimal, compared both to the absolute shell thickness (on average a 1.2% thinner shell at the speckles) and to shell thinning in the course of embryonic growth (Maurer et al., 2011a), further studies to assess embryonic shell erosion in plain areas and the pigmented spots of eggs across different species or developmental modes of birds are desirable. This would to be an important contribution to the structuralfunctional hypothesis of avian eggshell colouration (Gosler et al., 2005, 2011), also in the context of changes in shell pigmentation induced by environmental pollution (Jagannath et al., 2008). Another important problem is that some previous studies did not ascribe shell fragments to the appropriate area of an egg. This

G. Orłowski, L. Hałupka / Environmental Pollution 205 (2015) 218e224

means that shell fragments from various parts of the egg (differing in their thickness, e.g., Maurer et al., 2011a, 2012; see Results) were analysed together (cf. Falk et al., 2006), which could have produced an overall artifact regarding the causes of eggshell thinning (discussed in Snyder and Meretsky, 2003). Such differences in shell thickness within an egg have been analysed rarely. For instance, Snyder and Meretsky (2003), studying shell fragments from the equator of Californian Condor eggs from the 1960s, observed the extreme shell thinning. However, this was likely the result of the smaller size of eggs at that time (Snyder and Meretsky, 2003). Hence, further detailed re-evaluation of studies covering all the potential factors affecting EET is desirable, including the analysis of the physiological state of a female or resource availability. In sum, our work clearly emphasizes the need to consider intrinsic factors affecting shell properties, such as thickness being an effect of anatomical or reproductive changes. This is crucial for the proper partitioning of external environmental effects (e.g., pollution) from natural changes in EET, which could explain a large part of the variability in shell thickness within an individual bird species. As the vast majority of previous studies of eggshell thinning have not considered variation in shell thickness resulting from EET, it should be assumed that the results of these studies may contain artifacts. This is particularly important in the case of studies comparing post-hatch eggs with intact, deserted eggs (with unknown embryo status) from museum collections. We would like to stress that there is a great need for the careful selection of egg samples (of similar developmental status) for comparative analyses in studies aiming to assess variation in eggshell thickness and any related shell traits that are a function of shell thickness. An initial, relatively simple, invasive examination for the presence of an embryo and/or its development stage should be applied in any experimental and observational studies dealing with changes in shell thickness (and any resulting physical features of avian eggshells) in order to avoid the false interpretation of causes of eggshell thinning. Alternatively, a non-invasive method of measuring eggshell thickness and the stage of embryonic development would be a very desirable tool. In particular, a method for determining the presence/status of embryos in museum eggs or eggshell remnants should be found. We consider that further studies to evaluate the effect of environmental and physiological correlates on EET of avian eggs are urgently required. Acknowledgements We are grateful to Dr Richard Broughton, Dr Stephan Schoech and anonymous reviewers who kindly provided valuable comments and corrected the English of the paper. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2015.06.001. References Ancel, A., Girard, H., 1992. Eggshell of the domestic guinea fowl. Bri. Poult. Sci. 33, 993e1001. Ar, A., Paganelli, C., Reeves, R., Greene, D., Rahn, H., 1974. The avian egg: water vapor conductance, shell thickness, and functional pore area. Condor 76, 153e158. Ar, A., Rahn, H., Paganelli, C., 1979. The avian egg: mass and strength. Condor 81, 331e337. Balkan, M., Karakas, R., Biricik, M., 2006. Changes in eggshell thickness, shell conductance and pore density during incubation in the Peking Duck (Anas platyrhynchos f. dom.). Ornis Fenn. 83, 117e123. Bennett, R.S., 1995. Relative sensitivity of several measures of eggshell quality to the stage of embryonic development. Bull. Environ. Contam. Toxicol 54, 428e431. Birchard, G.F., Deeming, D.C., 2009. Avian eggshell thickness: scaling and maximum body mass in birds. J. Zool. 279, 95e101.

223

Blom, J., Lilja, C., 2004. A comparative study of growth, skeletal development and eggshell composition in some species of birds. J. Zool. 262, 361e369. Bond, G.M., Board, R.G., Scott, V.D., 1988. A comparative study of changes in the structure of avian eggshells during incubation. Zool. J. Linn. Soc. 92, 105e113. Booth, D.T., 1989. Regional changes in shell thickness, shell conductance, and pore structure during incubation in eggs of the Mute Swan. Physiol. Zool. 62, 607e620. Booth, D.T., Seymour, R.S., 1987. Effect of eggshell thinning on water vapor conductance of Malleefowl eggs. Condor 89, 453e459. Bunck, C.M., Spann, J., Pattee, O., Fleming, W., 1985. Changes in eggshell thickness during incubation: implications for evaluating the impact of organochlorine contaminants on productivity. Bull. Environ. Contam. Toxicol. 35, 173e182. Burnham, W.A., Enderson, J., Boardman, T., 1984. Variation in peregrine falcon eggs. Auk 101, 578e583. Burnett, L.J., Sorenson, K., Brandt, J., Sandhaus, E., Ciani, D., Clark, M., David, C., Theule, J., Kasielke, S., Risebrough, R., 2013. Eggshell thinning and depressed hatching success of California Condors reintroduced to Central California. Condor 115, 477e491. Capen, D.E., 1977. Eggshell thickness variability in the White-faced Ibis. Wilson Bull. 89, 99e106. Castilla, A.M., Herrel, A., Díaz, G., Francesch, A., 2007. Developmental stage affects eggshell-breaking strength in two ground-nesting birds: the partridge (Alectoris rufa) and the quail (Coturnix japonica). J. Exp. Zool. 307A, 471e477. Castilla, A.M., Herrel, A., Robles, H., Malone, J., Negro, J., 2010. The effect of developmental stage on eggshell thickness variation in endangered falcons. Zoology 113, 184e188. Cracraft, J., 1988. The major clades of birds. In: Benton, M.J. (Ed.), The Phylogeny and Classification of the Tetrapods, vol. 1. Clarendon Press, Oxford, pp. 339e361. Cramp, S. (Ed.), 1998. The Complete Birds of the Western Palaearctic on CD-ROM. Oxford Univ. Press, Oxford. Deeming, D.C., 2002. Embryonic development and utilisation of egg components. In: Deeming, D.C. (Ed.), Avian Incubation: Behaviour, Environment and EvolutionOxford University Press, Oxford, pp. 63e87. Deeming, D.C., Ferguson, M., 1991. Egg incubation: its effects on embryonic development in birds and reptiles. Cambridge University Press, Cambridge. del Hoyo, J., Elliott, A., Sargatal, J., 2014. Handbook of the Birds of the World. Alive. Lynx Edicions, Barcelona available at: http://www.hbw.com/species. Dunning, J.B., 1993. CRC Handbook of Avian Body Masses. CRC Press, Boca Raton. Dyke, G.J., Kaiser, G., 2010. Cracking a developmental constraint: egg size and bird evolution. In: Boles, W.E., Worthy, T.H. (Eds.), Proceedings of the VII International Meeting of the Society of Avian Paleontology and Evolution. Rec. Austr. Mus, 62, pp. 207e216. Falk, K., Møller, S., 1990. Clutch size effects on eggshell thickness in the Peregrine Falcon and European Kestrel. Ornis Scand. 21, 265e279. Falk, K., Møller, S., Mattox, W., 2006. A long-term increase in eggshell thickness of Greenlandic Peregrine Falcons Falco peregrinus tundrius. Sci. Tot. Environ. 355, 127e134. Faquinello, P., Murakami, A., Cella, P., Franco, J., Sakamoto, M., Bruno, L., 2004. High tannin sorghum in diets of japanese quails (Coturnix coturnix japonica). Braz. J. Poult. Sci. 6, 81e86. Finnlund, M., Hissa, R., Koivusaari, J., Meril€ a, E., Nuuja, I., 1985. Eggshells of arctic terns from Finland e effects of incubation and geography. Condor 87, 79e86. Gosler, A.G., Higham, J.P., Reynolds, J., 2005. Why are birds' eggs speckled? Ecol. Lett. 8, 1105e1113. Gosler, A.G., Connor, O., Bonser, R., 2011. Protoporphyrin and eggshell strength: preliminary findings from a passerine bird. Avian Biol. Res. 4, 214e223. Gonzalez, L.M., Hiraldo, F., 1988. Organochlorine and heavy metal contamination in the eggs of the Spanish Imperial Eagle (Aquila (heliaca) adalberti) and accompanying changes in eggshell morphology and chemistry. Environ. Poll. 51, 241e258. Green, R.E., 1998. Long-term decline in the thickness of eggshells of thrushes, Turdus spp., in Britain. Proc. R. Soc. Lond. B 265, 679e684. Green, R.E., 2000. An evaluation of three indices of eggshell thickness. Ibis 142, 676e679. Green, R.E., Scharlemann, J., 2003. Egg and skin collections as a resource for longterm ecological studies. Bull. Brit. Orn. Club 123A, 165e176. Hunt, G.L., Hunt, M., 1973. Clutch size, hatching success, and eggshell-thinning in Western Gulls. Condor 75, 483e486. Handrich, Y., 1989. Incubation water loss in King penguin egg. I. Change in egg and brood pouch parameters. Physiol. Zool. 62, 96e118. € ro €k, J., 2011. Shell thickness and pore density in relation to Hargitai, R., Mateo, R., To shell colouration, female characteristics, and environmental factors in the Collared Flycatcher Ficedula albicollis. J. Ornithol. 152, 579e588. Henny, C.J., Bennett, J.K., 1990. Comparison of breaking strength and shell thickness as evaluators of white-faced ibis eggshell quality. Environ. Toxicol. Chem. 9, 797e805. Hoffman, D.J., Rattner, B., Burton, G., Cairns, J. (Eds.), 2003. Handbook of Ecotoxicology, Second ed. Lewis Publishers, D.C (A CRC Press Company, Boca, Raton, London, New York, Washington). Jagannath, A., Shore, R., Walker, L., Ferns, P., Gosler, A., 2008. Eggshell pigmentation indicates pesticide contamination. J. Appl. Ecol. 45, 133e140. Karlsson, O., Lilja, C., 2008. Eggshell structure, mode of development and growth rate in birds. Zoology 111, 494e502. , K., Mendes¸, M., Alkan, S., Balciog lu, M., 2010. An assessment of embryonic Karabag mortality stages in Chukar partridge (Alectoris chukar) by means of

224

G. Orłowski, L. Hałupka / Environmental Pollution 205 (2015) 218e224

classification tree method. Arch. Geflügelk 74, 269e273. Kern, M.D., Cowie, R., Yeager, M., 1992. Water loss, conductance, and structure of eggs of pied flycatchers during egg laying and incubation. Physiol. Zool. 65, 1162e1187. Kiff, L.F., Peakall, D., Wilbur, S., 1979. Recent changes in California condor eggshells. Condor 81, 166e172. Kiff, L.F., 1989. Techniques for preparing bird eggs and nests. In: Rogers, S.P., Wood, D.S. (Eds.), Notes from a Workshop on Bird Specimen Preparation Held at the Carnegie Museum of Natural History, in Conjunction with the 107th Stated Meeting of the American Ornithologists' Union, 7 August 1989, pp. 111e117. King, K.A., Zaun, B., Schotborgh, H., Hurt, C., 2003. DDE-induced eggshell thinning in white-faced ibis: a continuing problem in the western United States. Southwest. Nat. 48, 356e364. Koricheva, J., Gurevitch, J., 2012. Place of meta-analysis among other methods of research synthesis. In: Koricheva, J., Gurevitch, J., Mengersen, K. (Eds.), Handbook of Meta-analysis in Ecology and Evolution. Princeton Univ. Press, Princeton, New Jersey, pp. 3e13. Kotter, B.L., 1970. An Ecological Natural History of the White-faced Ibis (Plegadis Chihi) in Northern Utah. University of Utah, Salt Lake City, Utah. M.S. thesis. Kreitzer, J.F., 1972. The effect of embryonic development on the thickness of the eggshells of Coturnix Quail. Poult. Sci. 51, 1764e1765. Krist, M., 2011. Egg size and offspring quality: a meta-analysis in birds. Biol. Rev. 86, 692e716. Lundholm, C.E., 1997. DDE-Induced eggshell thinning in birds: effects of p,p’-DDE on the calcium and prostaglandin metabolism of the eggshell gland. Comp. Biochem. Physiol. 118C, 113e128. Massaro, M., Davis, L., 2005. Differences in egg size, shell thickness, pore density, pore diameter and water vapour conductance between first and second eggs of snares penguins Eudyptes robustus and their influence on hatching asynchrony. Ibis 147, 251e258. Maurer, G., Portugal, S., Miks, I., Cassey, P., 2011a. Speckles of cryptic black-headed gull eggs show no mechanical or conductance structural function. J. Zool. 285, 194e204. Maurer, G., Portugal, S., Boomer, I., Cassey, P., 2011b. Avian embryonic development does not change the stable isotope composition of the calcite eggshell. Reprod. Fert. Dev. 23, 339e345. Maurer, G., Portugal, S., Cassey, P., 2012. A comparison of indices and measured values of eggshell thickness of different shell regions using museum eggs of 230 European bird species. Ibis 154, 714e724. Newton, I., 1979. Population Ecology of Raptors. Berkhamsted. Poyser, London, UK. Nybø, S., Staurnes, M., Jerstad, K., 1997. Thinner eggshells of dipper (Cinclus cinclus) eggs from an acidified area compared to a non-acidified area in Norway. Water, Air Soil Pollut. 93, 255e266. Nygård, T., 1999. Correcting eggshell indices of raptor eggs for hole size and

eccentricity. Ibis 141, 85e90. €, T., Sondell, J., 2014. Eggshell thinning of osprey (Pandion haliaetus) breeding Odsjo in Sweden and its significance for egg breakage and breeding outcome. Sci. Tot. Environ. 470, 1023e1029. Ormerod, S.J., Bull, K., Cummins, C., Tyler, S., Vickery, J., 1988. Egg mass and shell thickness in Dippers Cinclus cinclus in relation to stream acidity in Wales and Scotland. Environ. Poll. 55, 107e121. Orłowski, G., Hałupka, L., Klimczuk, E., Sztwiertnia, H., 2015. Developmental Eggshell Thinning in a Small Passerine Bird in Relation to Hatching Failure and Egg Infertility. submitt. Pulliainen, E., Marjakangas, A., 1980. Eggshell thickness in eleven sea and shore bird species of the Bothnian Bay. Ornis Fenn. 57, 65e70. Rahn, H., Paganelli, C.V., Ar, A., 1975. Relation of avian egg weight to bidy weight. Auk 92, 750e765. Rahn, H., Paganelli, C., 1989. Shell mass, thickness and density of avian eggs derived € nwetter’s. J. Ornithol. 130, 59e68. from the tables of Scho Ramstack, J.M., Murphy, M., Palmer, M., 1998. Comparative reproductive biology of three species of swallows in a common environment. Wilson Bull. 110, 233e243. Ratcliffe, D.A., 1970. Changes attributable to pesticides in egg breakage frequency and eggshell thickness in some British birds. J. Appl. Ecol. 7, 67e115. Rothstein, S.I., 1972. Eggshell thickness and its variation in the Cedar Waxwing. Wilson Bull. 84, 469e474. Sahan, U., Altan, O., Ipek, A., Yilmaz, B., 2003. Effects of some egg characteristics on the mass loss and hatchability of ostrich (Struthio camelus) eggs. Brit. Poult. Sci. 44, 380e385. Sayce, J.R., Hunt, G., 1987. Sex ratios of prefledging Western Gulls. Auk 104, 33e37. Scharlemann, J.P.W., 2003. Long-term declines in eggshell thickness of Dutch thrushes Turdus spp. Ardea 91, 205e212. Simkiss, K., 1961. Calcium metabolism and avian reproduction. Biol. Rev. 36, 321e367. Snyder, N.F.R., Meretsky, V., 2003. California condors and DDE e a reevaluation. Ibis 145, 136e151. Sotherland, P.R., Packard, G., Taigen, T., Boardman, T., 1980. An altitudinal cline in conductance of cliff swallow (Petrochelidon pyrrhonota) eggs to water vapor. Auk 97, 177e185. Starck, J.M., Ricklefs, R.E., 1998. Patterns of development: the altricialeprecocial spectrum. In: Starck, J.M., Ricklefs, R.E. (Eds.), Avian Growth and Development. Evolution in the AltricialePrecocial Spectrum. Oxford University Press, Oxford, pp. 3e30. StatSoft, 2006. Statistica© (Data Analysis Software System), Version 7.1 (Tulsa: USA). Vleck, C.M., Vleck, D.J., 1987. Metabolism and energetics of avian embryos. J. Exp. Zool. S. 1, 111e125.