The five million bird eggs in the world’s museum collections are an invaluable and underused resource

Abstract

The ~1.97 million egg sets (~5 million eggs) housed in museums have not been used in proportion to their availability. We highlight the wide variety of scientific disciplines that have used egg collections and the geographic locations and sizes of these collections, to increase awareness of the importance of egg collections, improve their visibility to the scientific community, and suggest that they offer a wealth of data covering large spatial scales and long time series for broad investigations into avian biology. We provide a brief history of egg collections and an updated list of museums/institutions with egg collections worldwide. We also review the limitations, challenges, and management of egg collections, and summarize recent literature based on historical and recent museum egg materials.

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PERSPECTIVE
The five million bird eggs in the world’s museum collections are an
invaluable and underused resource
Miguel Ângelo Marini,1*, Linnea Hall,2 John Bates,3 Frank D. Steinheimer,4 Robert McGowan,5
Luis Fábio Silveira,6 Darío A. Lijtmaer,7 Pablo Luis Tubaro,7 Sergio Córdoba-Córdoba,8
Anita Gamauf,9† Harold F. Greeney,10 Manuel Schweizer,11 Pepijn Kamminga,12 Alice Cibois,13
Laurent Vallotton,13 Douglas Russell,14 Scott K. Robinson,15 Paul R. Sweet,16 Sylke Frahnert,17 René Corado,2
and Neander Marcel Heming1,18
1 Departamento de Zoologia, Universidade de Brasília, Brasília, DF, Brazil
2 Western Foundation of Vertebrate Zoology, Camarillo, California, USA
3 Integrative Research Center, Field Museum of Natural History, Chicago, Illinois, USA
4 Natural Science Collections, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany
5 Department of Natural Sciences, National Museums Scotland, Edinburgh, Scotland, United Kingdom
6 Museu de Zoologia da Universidade de São Paulo, São Paulo, SP, Brazil
7 Museo Argentino de Ciencias Naturales, MACN - CONICET, Buenos Aires, Argentina
8 Instituto de Investigación de Recursos Biológicos Alexander von Humboldt, Claustro de San Agustín, Villa de Leyva, Boyacá, Colombia
9 Naturhistorisches Museum Wien, Wien, Austria
10 Yanayacu Biological Station, Cosanga, Napo, Ecuador
11 Naturhistorisches Museum, Bern, Switzerland
12 Naturalis Biodiversity Center, Leiden, The Netherlands
13 Muséum d’histoire naturelle de la Ville de Genève, Genève, Switzerland
14 Natural History Museum, Department of Life Sciences, Tring, United Kingdom
15 Florida Museum of Natural History, Gainesville, Florida, USA
16 American Museum of Natural History, New York, New York, USA
17 Museum für Naturkunde, Berlin, Germany
18 Programa de Pós-Graduação em Ecologia e Conservação da Biodiversidade, Universidade Estadual de Santa Cruz, Ilhéus, BA, Brazil
† This author is deceased.
* Corresponding author: marini@unb.br
Submission Date: December 19, 2020; Editorial Acceptance Date: May 13, 2020; Published July 24, 2020
ABSTRACT
The ~1.97 million egg sets (~5 million eggs) housed in museums have not been used in proportion to their availability.
We highlight the wide variety of scientific disciplines that have used egg collections and the geographic locations and
sizes of these collections, to increase awareness of the importance of egg collections, improve their visibility to the
scientific community, and suggest that they offer a wealth of data covering large spatial scales and long time series for
broad investigations into avian biology. We provide a brief history of egg collections and an updated list of museums/
institutions with egg collections worldwide. We also review the limitations, challenges, and management of egg
collections, and summarize recent literature based on historical and recent museum egg materials.
Keywords: avian biology, collection-based science, egg collections, eggs, metadata, spatial scale, time series
applyparastyle "g//caption/p[1]" parastyle "FigCapt"
Lay summary
• The 5 million bird eggs in museum collections are an invaluable and underused resource that could be used for a
variety of studies.
• We describe briefly the history of eggs that were collected worldwide over the last 200 years.
• We show that eggs from collections can be used to study ecology, behavior, evolution, classification, and species conservation.
• Several of the 300 institutions with egg collections that we list are already making them digitally available and
physically accessible to scientists and the general public.
• We hope with this commentary to increase awareness of the importance of egg collections and improve their visibility
and support.
Volume 137, 2020, pp. 1–7
DOI: 10.1093/auk/ukaa036
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The Auk: Ornithological Advances 137:1–7, © 2020 American Ornithological Society
Five million bird eggs underused in museums M. Â. Marini, L. Hall, J. Bates, et al.
Los cinco millones de huevos de aves en las colecciones de museos del mundo son un recurso invaluable y
subutilizado
RESUMEN
Los casi 1.97 millones de conjuntos de huevos (~5 millones de huevos) depositados en colecciones no han sido utilizados
en proporción a su abundancia. En este trabajo, destacamos la subutilización de las colecciones de huevos a pesar de
generar datos en amplias escalas espaciales y temporales, para incrementar la conciencia sobre la importancia de las
colecciones de huevos, mejorar su visibilidad para la comunidad científica, y mostrar que las colecciones de huevos
ofrecen una abundancia de datos a gran escala espacial y temporal para la investigación de la biología de las aves.
Proporcionamos una breve historia de las colecciones de huevos y una lista actualizada de museos/instituciones con
colecciones de huevos en todo el mundo. También discutimos las limitaciones, los desafíos y el manejo de este tipo de
colecciones, y fornecemos un resumen da literatura que utiliza dados históricos y recientes de colecciones de huevos.
Palabras clave: biología de las aves, ciencia basada en colecciones, colecciones de huevos, escala espacial, escala
temporal, huevos, metadata
INTRODUCTION
Scientific collections provide critical data about biodiver-
sity, yet some types of specimens are overlooked by the sci-
entific community. We present an overview of a specimen
type that documents a critical part of the avian annual cycle:
eggs. Egg collecting has a long history and peaked between
the 1890s and 1930s (Figure1, Supplemental Material S1). In
this paper we conduct the first comprehensive review of egg
collections worldwide, including small, formerly overlooked
collections, and to our knowledge the first major review
since the North American inventory by Kiff and Hough
(1985). We located data for ~5 million bird eggs, or ~1.97
million egg sets, collected over the last 250 yr and housed
in hundreds of museums (Supplemental Material Table S1).
Although eggs are the second most numerous type of bird
specimen in museums after skins (Roselaar 2003, eBEAC
2020), they have not recently been studied in proportion
to their availability, as evidenced by a relatively low visit-
ation rate by scientists at even the larger institutions over
past decades (Supplemental Material Table S2). This lack
of use contrasts with an overall increase in use of biolog-
ical collections from 1980 to 2004 (Pyke and Ehrlich 2010).
In some museums, the use of egg collections has decreased
in recent decades, as seen in visitation rates at the large egg
collection at National Museums Scotland (NMS, Edinburgh,
Scotland) from 1980 to 2019 (Supplemental Material Figure
S1). One reason suggested for the low use of egg collections
is that egg structure and morphology are of limited use in
higher-level systematics (Mikhailov 1997), but we discuss a
multitude of studies, including systematics studies, that have
been and can be conducted on these collections, some of
which have gained recent widespread attention (Stoddard
etal. 2017, Birkhead etal. 2019). In an era of large datasets
and broad spatial modeling (Lister etal. 2011, Heming and
Marini 2015), egg collections, which are arguably one of
the largest and most readily available sources of long-term
avian breeding information, offer an important and relevant
source of data for analyses of long time series (Figure1) and
large spatial scales (Figure2).
USE OF EGG COLLECTIONS FOR RESEARCH
Most historical research using egg collections was related
to the description and study of the evolution of egg colors,
patterns, and morphology. Starting in the late 1960s it was
realized that eggs also can be reservoirs of critical infor-
mation on avian breeding biology (Harrison and Holyoak
1970, McNair 1987, Green and Scharlemann 2003) and en-
vironmental impacts (Hickey and Anderson 1968). Amuch
wider use of eggs in collections followed, resulting in
studies of many diverse topics (see review in Supplemental
Material Table S3).
Preserved avian eggshells should be viewed as “voucher”
specimens (i.e. those that help substantiate research
conclusions; Kageyama etal. 2007). Eggs are instantaneous,
well-preserved snapshots of bird breeding, contain infor-
mation about past environments, and hold unique biolog-
ical information not available in skins, skeletons, blood,
FIGURE 1. Distribution of 313,209 egg sets of birds collected
by decade between 1800 and 2017, from selected collections
available online or made available by curators/collection
managers (WFVZ-USA, RMNH-The Netherlands, FMNH-USA,
MNHG-Switzerland, AMNH-USA, NMBE-Switzerland, CAS-USA),
and from 40 museums from South and North America and Europe
with egg sets from the Neotropical region.
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M. Â. Marini, L. Hall, J. Bates, et al. Five million bird eggs underused in museums
fluid-preserved specimens, tissues, or feathers. More re-
cently, the usefulness of egg collections has increased
with the availability of advanced analytical technologies,
including scanning electron microscopy, spectropho-
tometry, several types of spectroscopy, chromatography,
photogrammetry, genetic sequencing, and stable isotope
analyses (reviewed by Burns etal. 2017). Digital photog-
raphy (Bridge etal. 2007, Troscianko 2014) and noninva-
sive measuring tools such as 3D scanners allow for detailed
measurements and modern image-processing. While data-
rich specimens (i.e. specimens with associated egg slips
with detailed data about the collecting event) are more
valuable, even data-poor eggs can be useful for research
requiring destructive analysis (Russell etal. 2010).
Eggs, because their dimensions do not change with time
(Väisänen 1969), are well suited to studies of morphology. For
example, a recent study of the eggs of ~1,400 bird species re-
vealed that egg shape variation in birds is related to constraints
induced by differences in flight (Stoddard etal. 2017). Olsen
and Marples (1993) used differences in egg measurements of
Australian raptors as evidence for subspeciation. Duursma
etal. (2018) used egg collection measurements to show that
passerine egg shape is influenced by climatic conditions such
as temperature and humidity.
One of the main uses of bird eggs from museum
collections is to study the evolution of egg colors and
patterns. Changes in egg color patterns on egg surfaces
have assisted in understanding the evolution of avian host/
nest parasite relations (Spottiswoode 2010, Jaeckle etal.
2012). Eggshells could also assist with studies of sexual-
signaling hypotheses (Hanley etal. 2010).
Eggs and eggshell traits are useful for phylogenetic
and taxonomic studies (Sibley 1970, Sibley and Ahlquist
1972, Olsen and Marples 1993, Grellet-Tinner etal. 2012).
Dried eggshells often hold material from the inner shell
membranes, embryos, or shell powder that is useful for ge-
netic analyses (Chilton and Sorenson 2007, Lee and Prŷs-
Jones 2008). Further testing of historical museum eggshells
for DNA fragments, as well as use of egg characteristics in
phylogeny construction and resolution, could be a fruitful
area of investigation.
Isotope analysis of eggshells of seabirds has expanded
what we know about their foraging habits, geographic dis-
tribution, and differential use of marine and freshwater
ecosystems (Schaffner and Swart 1991). Similarly, stable
carbon and nitrogen isotopes of fossil ostrich eggshells
were used to reconstruct Holocene paleoecological
parameters in Egypt (Johnson etal. 1993), and the use of
peptide markers to identify eggshells from a medieval ar-
chaeological site in the UK showed their potential to help
understand historical egg use in human diets (Presslee
etal. 2018).
Valuable natural history data for extinct, rare, and un-
common species can be obtained from egg collections
(Maurer etal. 2010, Oskam etal. 2010). Egg sets can provide
data about the historical demography of birds, including
reproductive effort (based on egg sizes and clutch sizes),
and breeding periods (Beissinger and Peery 2007, Blight
2011). New descriptions and updates on avian breeding
strategies continue to be published, some of which have
been based on formerly overlooked egg sets deposited in
museums for nearly a century (McGowan and Massa 1990,
Steinheimer 2004). Filling significant gaps in our knowledge
has been slow for many groups such as Neotropical raptors
(Monsalvo etal. 2018), and eggs in museums could be in-
strumental in assisting with additional information on his-
torical distribution and breeding phenology of thesebirds.
Egg collections can provide historical information about
early scientific exploration, and historical egg data can be
used in association with data from nest collections (Ingels
and Greeney 2011, Russell et al. 2013). Only 30% of the
~10,000 living bird species, mainly those from northern
temperate regions, have well-understood breeding biology,
with 39% moderately known and 31% still poorly known
(Xiao etal. 2016). In our review of egg collections world-
wide, we found broad geographical representation across
regions in North America and Europe (Figure 2). Other
FIGURE 2. Global coverage of 269,783 egg sets of birds of the
world collected since 1800 from selected collections available
online or made available by curators/collection managers (WFVZ-
USA, RMNH-The Netherlands, FMNH-USA, MNHG-Switzerland,
AMNH-USA, NMBE-Switzerland, CAS-USA), and from 40 museums
from South and North America and Europe with egg sets from the
Neotropical region. Top = individual records; bottom = kernel-
smoothed intensity function [e^1/2] from point pattern.
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Five million bird eggs underused in museums M. Â. Marini, L. Hall, J. Bates, et al.
geographical regions of the world, however, are largely un-
documented, and thus represent fruitful areas for investi-
gation. Filling these data gaps would increase opportunities
to study regional differences in breeding biology (Green
and Scharlemann 2003, Heming and Marini 2015).
The value of eggs to avian conservation is best
represented by their role in long-term environmental
biomonitoring, documenting the effects of chemical
pollutants on eggshells (Hickey and Anderson 1968).
During the period of heavy global DDT (dichlorodiphenylt
richloroethane) use from the late 1940s through the 1980s,
hundreds of studies were conducted on eggshell thinning,
egg content contamination, and embryo malformation
(Hickey and Anderson 1968, Morrison and Kiff 1979).
Egg sets from common species often consist of long tem-
poral series (up to 200 yr) with broad geographic coverage,
which can be used to study long-term changes in breeding
traits, habitat loss, and climatic shifts (Scharlemann 2001,
Green and Scharlemann 2003, Lister etal. 2011).
CHALLENGES, PREJUDICES, AND LIMITATIONS OF
EGG COLLECTIONS
The majority of the world’s egg collections have experienced
curation challenges caused by funding, time, and staffing
constraints (Dalton 2003). However, egg collections are, in
some ways, easier to curate than skin collections because
per specimen they need the same or less space; they are not
as vulnerable to climate oscillations (especially humidity);
and they need little or no protection against insect pests.
Data from few egg collections are catalogued, digitized,
and available online (but see CAS, FMNH, MVZ, NMBE,
WFVZ, and YPM [institutional acronyms in Supplemental
Material Table S1]). Furthermore, egg sets, like other bird
specimens, are generally scattered among hundreds of
institutions and countries. However, egg sets are concen-
trated in the 2 largest collections (NHMUK and WFVZ),
which combined hold ~40% of the world’s egg sets. This
concentration is probably related to the lack of interest
of many institutions in curating egg collections, and to
donations to these 2 major institutions.
In the decades after the regulation of egg collecting began
(i.e. approximately 1930s to the 1960s), a prejudice against
historical egg collecting may have led to the relatively low
use of egg collections by researchers (Birkhead 2016).
However, once eggshell thinning due to DDT metabolites
was shown to affect the breeding of many bird species,
the stigma decreased, leading to a resurgence in use of
museum eggshell materials. Despite this, some stigma re-
mains attached to eggs collected by market collectors, as
well as those that were sold and traded by private collectors
(Barrow 2000). The nonscientific, amateur origin of many
formerly private collections (Lightman 2000) can further
stigmatize egg collections; as for collectors of study skins,
some egg collectors have proven to be unreliable (Olson
2008). However, collections with problematic species
identification can be overcome using modern methods of
proving species identifications, such as genetic barcoding
(Chilton and Sorenson 2007). General localities can be
checked and improved with isotopic analyses (Hobson
2011). Egg sets have additional concerns that necessitate
careful review of the specimens, such as the inability to de-
termine or check species identification with skin vouchers,
and uncertain clutch sizes (i.e. because the collector did
not clearly state the size of the complete clutch), which can
limit their scientific usefulness (Rasmussen and Prŷs-Jones
2003). In some collections, the disassociation between
data (on original paper slips or egg cards) and egg sets can
make eggs less readily useful than skins for research (Fisher
and Warr 2003). However, digitization of field notes and
original egg records, as well as sharing of provenance data
among researchers and institutions, can facilitate realign-
ment of records with eggsets.
Eggshells in collections may change color over time
when compared with fresh eggs (Starling et al. 2006).
Exposure to any light (not just UV light) can affect egg
color. Furthermore, because their contents have been
removed, historical egg sets cannot be used for proteomic
analyses (Portugal et al. 2010), although proteins can be
extracted from freshly collected eggs (e.g., Sibley 1970,
Sibley and Ahlquist 1972) before they are made into egg-
shell specimens.
Fortunately, as for all museum specimens, researchers
can overcome limitations by concentrating on reli-
able collectors who recorded relevant data, and by using
large sample sizes of egg sets from several time periods,
collectors, localities, and museums (Burns et al. 2017).
While some biases have been reported in egg collections,
such as higher proportion of larger clutches and early
breeding dates, and a focus on mimics (Lack 1946; but
see McNair 1987, Starling etal. 2006), such biases can be
dealt with when recognized, as with almost any type of
collection.
THE FUTURE OF EGG COLLECTIONS
The digitization revolution has greatly benefitted
collections by improving data management, quality con-
trol, and data sharing (Peterson etal. 2005). For example,
starting in 2005, the WFVZ partnered with museums in
North America to put its egg and nest data online through
ORNIS and other versions of NSF-funded public access
portals (e.g., ORNIS2, VertNet, and iDigBio). As of 2018,
data for more than 200,000 of the WFVZ’s egg sets have
been shared online, along with scans of original record
cards and 83,000 photographs. Similar digitization projects
have been implemented by other large egg collections (e.g.,
NHMUK andCAS).
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The Auk: Ornithological Advances 137:1–7, © 2020 American Ornithological Society
M. Â. Marini, L. Hall, J. Bates, et al. Five million bird eggs underused in museums
Additional investments of time and resources can fur-
ther improve egg collections. Data computerization, error
checking, and taxonomic identification are needed, and
in many cases, better housing of specimens will prevent
damage over time (Stewart etal. 2015). Standardization of
data (Aubrecht and Malicky 2010) can make egg sets more
accessible to researchers. For example, databases should be
compliant with Darwin Core Data Standards so that they
can be shared online through existing data portals, and
should include updated as well as historical taxonomy (Gill
and Donsker 2018), specific collecting dates, specific lo-
cality data, georeferenced localities, and full names of the
collectors. As with study skin data, use of egg set data is
recommended only after validation of species identifica-
tion and collection locality and date (Steinheimer 2010).
The participation of all egg collections in the data-sharing
community would make information more widely available
for research and inter-museum collaborations (Peterson
and Navarro-Sigüenza 2003, Peterson et al. 2005). Once
digitized, these data should be made accessible via websites
at individual institutions or through online portals such as
the Arctos Collaborative Collection Management Solution
(arctos.database.museum), the Global Biodiversity
Information Facility (gbif.org), and VertNet (vertnet.org).
Although this appeal for improved availability may seem
obvious (Komen 1996), most egg collections are not yet
available online, including none in Latin America.
CONCLUSIONS
Egg collections have been relatively underused, despite
the investments in collecting and storing these eggs—with
their data—through the centuries. Making egg collections
more digitally and physically accessible will increase their
use, and can inform the controlled, ethical, and planned
collection of eggs that will be critical for many types of fu-
ture science and monitoring (Winker etal. 2010, Joseph
2011). Likewise, nest collections and the deposit of non-
specimen egg and nest data (i.e. photos with metadata) at
museums can build up archived and accessible breeding in-
formation (Russell etal. 2013); hundreds of bird species still
do not have a single clutch or nest represented in a collec-
tion. Furthermore, egg collections of some museums (e.g.,
FMNH, WFVZ, and YPM in the USA, and NHMUK and
NMS in the UK) are still receiving historical egg collections
from private collectors and small museums, and current
specimens from researchers conducting field projects.
Anotable example is the effort by the ANWC, which has
worked with Australian governmental authorities to le-
galize and transfer privately held egg collections to the mu-
seum (Joseph 2011). We hope that this commentary will
increase awareness of the importance of egg collections
and improve their visibility to the scientific community,
leading to greater use of a relatively untapped resource
documenting avian reproductive biology, life history
strategies, and application to species conservation.
SUPPLEMENTARY MATERIAL
Supplementary material is available at The Auk:
Ornithological Advances online.
ACKNOWLEDGMENTS
We thank collection curators and managers for providing in-
formation, and for their efforts to conserve, correct, and care-
take the egg collections of theworld.
Funding statement: Research funding was provided
by Conselho Nacional de Desenvolvimento Científico e
Tecnológico (# 473281/2013–9) and Fundação de Apoio
à Pesquisa do Distrito Federal (# 0193.000839/2015).
MÂM held researcher fellowship from Conselho Nacional
de Desenvolvimento Científico e Tecnológico. NMH
received a postdoc fellowship from Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior.
Author contributions: M.Â.M.conceived the idea for the
manuscript, and he and all other authors conducted the
writing.
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