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Parental Care IV: Incubation
JodyLee Estrada Duek, Ph.D.
With assistance from Dr. Gary Ritchison
http://people.eku.edu/ritchisong/avianreproduction2.html
Incubation
• During incubation, birds transfer heat to eggs. Arizona-Sonora Desert Museum
• For optimum development, egg temperature must be maintained at
about 37 - 38 degrees C (Gill 1995).
• Exposure to higher temperatures is lethal, while cooler temperatures
will, at minimum, slow down or stop development.
• Heat is transferred through brood, or incubation, patches - an area of
bare, flaccid skin on the abdomen and/or breast.
• Prior to the initiation of incubation, the skin in the area of the brood
patch loses its feathers.
• In addition, the dermis becomes spongy and richly supplied with
blood vessels (Welty and Baptista 1988).
Brood patch
• Birds of nearly all species temporarily shed their feathers on single or
paired areas of the breast or abdomen early in the breeding season.
• The bare skin increases in vascularity, which aids it in transferring
body heat for incubating the eggs and brooding the chicks.
• Development of these incubation (brood) patches is prompted by
rising levels of estrogen.
• They form in whichever sex cares for the eggs and young, usually
females but often males as well.
• The lost feathers are replaced in the complete molt following the
breeding season (Stettenheim 2000).
Incubation reduces microbial
growth on eggshells
• -- Avian eggshells harbor microbes shortly after laying, and under appropriate
ambient conditions they can multiply rapidly, penetrate through shell pores, infect
egg contents and cause embryo mortality.
• Cook et al. (2005) experimentally examined how incubation affects bacterial
processes on the eggshells of Pearl-eyed Thrashers (Margarops fuscatus) nesting in
tropical montane and lowland forests in Puerto Rico.
• Bacteria and fungi grew rapidly on shells of newly laid, unincubated eggs exposed
to ambient conditions, but declined to low levels on shells of eggs incubated by
thrashers.
• Divergence in bacterial growth between incubated and exposed eggs was more
marked at the montane forest than at the lowland site.
• Pathogenic microorganisms became increasingly dominant on shells of exposed
eggs, but these groups were relatively rare on incubated eggs, where more benign,
less invasive groups prevailed.
• Some incubation during laying may be necessary to decrease the probability of
trans-shell infection by reducing the growth of harmful bacteria and fungi on
eggshells, although it may increase hatching asynchrony and the likelihood of
brood reduction.
Energetic cost of incubation
• In birds, the annual peak of energy demand has long been thought to occur
when parents provision their offspring with food during the nestling phase.
• This led to idea selection on clutch size takes place during nestling phase.
• energetic demands during other reproductive phases –egg laying and
incubation – have been ignored.
• During incubation, avian eggs need external heat provisioning, regular
turning and favourable humidity for proper embryonic development; care
provided by one or both of parents.
• energetic costs of providing heat to the eggs thought to be negligible.
• Increasing evidence that below thermo-neutrality the metabolic rate
(energy spent per time unit) of an incubating female is higher than that of a
nonincubating female at rest.
• Because temperatures are normally below thermoneutrality at temperate
latitudes, energetic costs of incubation may substantially add to overall daily
energy expenditure of attending parents.
Energetic costs of incubation
• A nest-box modified into a metabolic chamber.
• To ensure the top of the nest-box was airtight, a sheet of rubber was
inserted between nest-box and lid (a) and a cork was placed in the
entrance hole (b).
• Reference air was measured close to the inflow of the nest-box
(arrows underneath nest-box; c), while sample air was drawn from
the nest-box via a tube near the entrance hole (d).
• The thickness of the nest was determined by the thickness of the
nest cup (e) and the height of the nest rim (f).
Clutch size and incubation energetics
• de Heij et al. (2007) manipulated the clutch sizes of female Great Tits (Parus
major) and monitored their metabolic rates during nocturnal incubation
using mobile oxygen analyzers.
• found clutch enlargement caused incubating females to expend more
energy, but clutch reduction did not lower energy expenditure.
• absence of an effect of clutch reduction can be explained by a limit to
number of eggs in direct contact with brood patch: threshold clutch size
• Above a threshold clutch size, eggs that are not in contact with the female's
brood patch cool.
• incubating birds will repeatedly rearrange eggs to rewarm
• Rewarming energetically more costly than maintaining eggs at incubation
temperatures.
• energetic costs increase when clutch size is above the threshold clutch size,
but do not change when clutch size is at or below threshold clutch size.
Nest insulation
• de Heij et al. (2007) also found that incubating birds with thicker
nests had lower energy expenditure, probably because thicker nests
were better insulated.
• The fact that not all birds build well-insulated nests suggests there is a
cost to thick nests.
• Knowing females expend more energy during nocturnal incubation
when incubating experimentally enlarged clutches is a first step
towards determining a potential mechanism underlying negative
selection on clutch size during the incubation.
• measurements on energy expenditure over a full 24 h are needed to
judge how important energy expenditure can be in explaining fitness
consequences of incubating experimentally enlarged clutches.
Incubation
• Most birds sit on their eggs to incubate, but there are exceptions.
• For example, male Emperor (D. Attenborough clip) and King penguins
place their egg on their feet, wrap their wing around the egg, and
incubate it while standing up.
• Male emperor penguins
http://www.youtube.com/watch?v=6AiCIZ9wM1o
• The egg is kept warm by the heat from the male's feet & wing.
Source: www.mbr-pwrc.usgs.gov/id/framlst/Photo/p1142.html
Foot-Mediated Incubation: Nazca Booby (Sula
granti) Feet as Surrogate Brood Patches
•Incubation in most birds involves transferring heat from
parent to egg through a highly vascularized brood patch.
•Some birds, however, do not develop a brood patch.
• hold their eggs under the webs of their feet
• webs are often positioned between the feathered abdomen and the egg during
incubation, suggesting that either the abdomen, the feet, or both could transfer
heat to the egg.
• Morgan et al. (2003) studied heat transfer from foot webs to eggs in Nazca boobies
by spatially separating the feet from the abdomen using an oversized egg.
• found feet transfer heat to eggs independently of any heat from the abdomen.
• found that incubating boobies had significantly greater vascularization in foot webs,
measured as percentage of web area covered by vessels
• males, whether incubating or nonincubating, had significantly less vascularization
• vascularized Nazca booby feet function during incubation as vascularized brood
patches
Micronesian megapode
Megapodes
• They are the only known birds which use
heat sources, other than the body, to
incubate their eggs.
• When the young birds hatch they are fully
able to defend for themselves and receive
no parental care.
• Megapodes are quite heavy-bodied birds
and forage on the forest floor, where they
search for insects, seeds and fruit.
• All members of the Megapode family can
fly, but most move around primarily by
walking.
Megapode
mound
•found in Southeast
Asia and Australia,
•use heat sources
•geothermal,
•solar, or
•decomposition
of organic
material.
Maleefowl 1
• Maleefowl mound, Australia
Malleefowl 2
• They spend up to eleven months of the year preparing, then
maintaining, the mound adapting to daily, as well as seasonal,
variations in temperature and rainfall.
• mounds, made largely of sand raked up by their powerful feet, are
each an immaculate circle approximately 4.5 metres in diameter.
• This labour is to establish an exact and constant 33oC - measured by
its heat-sensitive beak - in the central chamber of the mound.
• This chamber is literally carved out of consolidated layers of
vegetation, like compressed paper mixed with sand.
• All the eggs will be laid in this chamber - up to thirty at intervals of
two to five days apart.
• The key elements in incubation are solar heat and fermentation.
• malleefowl make the most of rainfall and rely on daily raking of the
mound to harness appropriate levels of solar energy.
Malleefowl 3
• In winter, when solar energy is minimal, they scratch together dead
leaves, twigs, sparse humus and rake them into a path that leads
directly into the now volcano-shaped centre of the mound.
• It is not an uncommon sight to see a path of sticks and leaves,
perhaps half a metre across and 20 metres long, leading from the
bush up and over the side of the mound into its centre.
• When wet, all the collected vegetation is sealed within the mound
and buried to rot and ferment.
• This process is repeated again and again and may take four months.
• As the season warms and eggs are laid and buried, the malleefowl
begin to uncover the mound, altering its architecture to reinforce the
slow fermentation with the heat of the sun.
• This process increases during the summer months until the mound
itself is shaped like a pointed cone and the incubation process
becomes dependent on solar energy alone.
Malleefowl 4 (Leipoa ocellata)
• dedicate 9-11 months per year building and maintaining a large
incubation mound of soil, leaves and twigs.
• The eggs are laid in the mound, buried and left to incubate by heat
generated from the composting litter.
• Malleefowl mounds may be used over many generations and can
attain an impressive size of 22 meters in circumference and 1
meter high.
• The birds maintain the mound temperature of 32-34 degrees C by
using their beak as a "thermometer" and adjusting soil cover to
either retain or expel heat from the egg chamber. 'Incubation'
typically takes 60 - 90 days.
(Source:http://www.malleefowl.com.au/Pages/TheMalleefowl.htm)
Incubation periods
• range from about 10 days for some passerines & woodpeckers to as
many as 80 days for albatross' and kiwis.
• time spent incubating is related to the size of the egg, state of
development (precocial vs. altricial), & ambient temperature.
Temperatures in an Eastern Bluebird
nest cup in Texas
(Cooper and Phillips 2002).
Nest predation appears to affect parental behavior 1
• Based on an analysis of incubation and provisioning behavior of 97
species of passerines, Conway and Martin (2000) suggested
environments with high risk of nest predation favor long on-bouts
(long periods on nest) and few foraging trips.
• This strategy may prevent frequent feeding by adults and thus
compromise future reproductive attempts.
• nest predation may influence evolution of avian life-history traits in
several ways.
• High nest predation favors a strategy of
• holding back reproductive effort for renesting attempts and survival,
• a short nesting cycle to minimize the time nests are susceptible to predation,
• small brood size to minimize noise of begging young
Nest predation appears to affect parental behavior 2
• Conway and Martin (2000) suggest nest predation may influence
passerines by placing constraints on parental activity and the way an
incubating female allocates her time between incubation and
foraging.
• with high nest predation, natural selection simultaneously favors
infrequent nest trips (to reduce the probability of predator detection)
and short off-bout duration (to maximize development rates and
reduce time of exposure to predators).
• These somewhat opposing constraints limit the range of effective
incubation strategies available to females in environments with high
nest predation.
Males Feeding Females during Incubation 1
• See graph on following slide of nest attentiveness (percentage of time
that the female is incubating on the nest) relative to the rate that
males bring food to the nest (incubation feeding).
• The relationship across 19 species of both open- and cavity-nesters is
curvilinear and significant.
• Nest attentiveness (percentage of time spent on the nest) during
incubation represents a parent-offspring conflict; incubating birds
must trade-off between caring for embryos by staying on the nest
versus caring for themselves by getting off the nest to forage.
• For species in which females are the sole incubator, males can
potentially affect this trade-off and increase nest attentiveness by
feeding incubating females on the nest (incubation feeding).
• (From: Martin and Ghalambor 1999).
Males Feeding Females during Incubation 2: Theory
(Martin and Ghalambor 1999)
• Increased nest attentiveness may be required when local microclimate is harsh and
requires more incubation feeding (microclimate hypothesis).
• Incubation feeding may be constrained by risk of attracting nest predators (nest
predation hypothesis), which in turn may constrain female nest attentiveness
because of energy limitation.
• incubation feeding rates are greater among cavity-nesting than open-nesting birds.
• Under microclimate hypothesis, the greater incubation feeding rates of cavitynesting birds generate the prediction that microclimate should be harsher than for
open-nesting birds.
• results reject this hypothesis because we found the opposite pattern; cavitynesting birds experienced more moderate (less variable) microclimates that were
less often below temperatures (i.e., 16°C) that can negatively impact eggs
compared with open-nesting species.
• In contrast, incubation feeding rates were highly negatively correlated with nest
predation both within and between the two nest types, supporting the nest
predation hypothesis.
• Incubation feeding in turn was positively correlated with nest attentiveness.
• Thus, nest predation may indirectly affect female incubation behavior by directly
affecting incubation feeding by the male (Check this short video of a male Blue Tit
feeding his mate).
Food availability and
nest attentiveness
across species and
latitudes
Chalfoun and Martin (2007)
• Both northern and southern species are expected to show proximate
increases in attentiveness in response to increased food availability,
• preliminary data further suggest that northern species show slightly
stronger responses.
• Proximate responses to food availability alone, however, cannot
explain why southern species generally show lower nest
attentiveness than similar northern species (grey arrows).
Latitudinal variation in avian incubation
attentiveness 1
• Avian incubation attentiveness has important fitness consequences:
•
•
•
number of young
quality of hatched young
energetic costs imposed on parents.
• Nest attentiveness is highly variable across species and geographical regions.
• Chalfoun and Martin (2007) reviewed the literature and found a worldwide pattern
that nest attentiveness of passerines is generally lower in south temperate and
tropical regions than in north temperate regions.
• conducted a food manipulation experiment to assess nest attentiveness: does it
reflect proximate responses or an evolved behaviour.
• Karoo Prinia (Prinia maculosa) in South Africa has very low nest attentiveness
(about 49%) compared with many passerines.
• provided supplemental food during early incubation to experimental females and
compared nest attentiveness and on- and off-bout lengths
• Nest attentiveness of females at food-provisioned nests was significantly higher
than control females (57% vs. 49%).
• Food-supplemented females spent significantly less time off nest than did control
females; mean on-bout lengths did not differ.
Latitudinal variation in avian incubation attentiveness
2
• mean nest attentiveness of food-provisioned females still substantially below
similar species worldwide.
• Food can be an important proximate influence on parental care behaviour, but
proximate influences of food do not explain latitudinal patterns of attentiveness.
• Climatic variation across latitudes may influence the amount of time that parents
spend on the nest, although temperatures at many south temperate sites often
approximate those at north temperate sites during the breeding season.
• One possible alternative explanation for geographical patterns in nest attentiveness
is variation in adult mortality across latitudes.
• According to classic life history theory, if southern birds experience lower adult
mortality, they should be less willing to invest as much in nest attentiveness
and other components of current reproduction.
• Testing for the existence of an adult mortality–nest attentiveness trade-off across
latitudes is therefore a critical next step in addressing geographical variation in
parental care strategies.
Winter habitat influences
reproductive success 1
• destruction of tropical forests is creating breeding problems for migratory birds.
• Norris et al. (2004) found quality of winter habitat affected ability of American
Redstarts to successfully reproduce when they returned north in the spring.
• Norris,noted that a relatively small geographic band – across the Caribbean,
Greater Antilles, and central America – is the annual destination of an estimated
five billion migratory birds flying south each year from Canada.
• Norris et al. (2004) measured stable carbon isotopes in blood samples collected
from American Redstarts after the birds arrived at their breeding grounds in
Ontario.
• Since the turnover time of the blood cells is from six to eight weeks, they provide a
good indicator of the quality of the birds' previous habitat on the wintering ground.
• The carbon signature of each redstart has been deposited by insects the bird ate,
and the insects in turn fed on vegetation growing in the winter habitat.
• It’s a 'food chain signature'
Winter habitat influences reproductive success 2
• Norris et al. (2004) first determined high and low quality habitats for redstarts,
which winter in the Caribbean and central America, and breed in deciduous forest
throughout Canada and the U.S.
• what makes a better winter habitat is primarily the degree of wetness, particularly
at the end of the season (late winter/early spring) when low quality habitats tend
to dry out.
• The second step was to develop isotope "markers" that identify habitat quality for
each type of habitat.
• Third, blood samples were taken from warblers in their breeding grounds
• Fourth, the birds' reproductive success was measured by counting the number of
"fledged" offspring to leave their nests.
• Analysis revealed that redstarts wintering in high quality habitats, such as
mangroves and lowland tropical forests, arrived earlier on the breeding grounds,
nested earlier, and were more successful in producing young.
• This study shows that destroying high quality winter habitat has a disproportionate
effect on the redstart populations: they lose the areas most capable of supporting
them.
Eggshell
• protects the embryo
• generally white in cavity-nesters & colored and patterned in open
nesters (for camouflage)
• color is added to the eggshell from pigments secreted by cells in the
wall of the uterus
• contains thousands of pores (see diagram) that permit gas exchange
Thousands of tiny pores like
the ones pictured, cover the
shell, providing a passage
for gas exchange.
(Source:
http://www.rit.edu/~tld089
8/SEM.html)
Eggshell pores
• An egg must exchange gases with its environment
• This allows for the removal of excess CO2 and the influx of “fresh air” with
additional O2 as gases diffuse through pores in the eggshell
• Water loss also occurs through these pores
• Water loss leads to a larger and larger air chamber at the blunt end of the
egg
• Just before hatching the chick needs to inflate its lungs and breathe
• This is accomplished as the chick breaks the membranes and has access to
the air chamber at the blunt end of the egg
• There is a fine line between
• not having enough air in the chamber to allow the chick to breathe long enough to break a
hole in the shell
• losing too much water and the embryo dehydrating before maturity
• The rate, and total amount, of water loss depends on
• Ambient temperature
• Humidity
• Elevation and subsequent air pressure
Egg water vapor conductance
• AR AND RAHN and others at SUNY Buffalo measured the water vapor
conductances of fresh eggs of 29 species
• showed how this total value increases with egg weight.
• change in egg shell thickness with egg weight also determined from
the literature
• measured periodically the weight loss under known conditions of
temperature, humidity, and barometric pressure
• rate of water loss per gram egg weight decreases as the eggs get
larger; large eggs can save more water than small eggs.
Incubation period and water loss
• Both Needham (1931) and Romanoff and Romanoff (1949) noted that
species with large eggs tend to have longer incubation periods and
thus would be exposed longer to water losses.
• This problem is discussed in detail by Rahn and Ar (1974) who
showed that the extended incubation period in larger eggs is
compensated for perfectIy by the reduction in water vapor
conductance per unit egg weight which they express as a common
water loss coefficient for all eggs.
• Whether the increase in pore area with weight is achieved by
increasing the number of pores or their diameter or some
combination of the two is still being explored.
Eggshell pores
http://www.cooper.org/pdf/p1977Rahn1974.pdf
• Gas exchange in the avian embryo has been shown to be dependent
on, and limited by, the diffusive properties of gases across the
resistance offered by the shell and shell membranes
• The diffusive rate of water loss from eggs depends on:
• permeability constant of the shell
– Pore size
– Number of pores per unit area
• surface area of the shell ( cm2)
• water vapor pressure difference across the eggshell
Maculation in eggshells
James P. Higham and Andrew G. Gosler 2006 Oecologia
•
•
•
•
•
•
Many small passerine birds worldwide lay white eggs speckled with red, brown and black
protoporphyrin pigment spots (maculation).
Unlike some patterns of avian eggshell pigmentation which clearly serve a crypsis or
signalling function, the ubiquity of maculation among passerines suggests that its origins lie
in another function, not specific to any particular ecological or behavioural group.
There is evidence that protoporphyrin pigments serve a structural function related to
eggshell thickness and calcium availability: eggshell maculation in the great tit Parus major
increases with decreasing soil calcium levels, pigments demarcate thinner areas of shell, and
both the pigment intensity and distribution are related to shell thickness.
maculation also affects the rate of water loss from the egg during incubation (≈ Mass Loss
per Day or MLD, which is critical to egg viability), but not that of unincubated eggs.
They demonstrate, both by observation and experiment, that the effect of female incubation
behaviour on MLD compensates in some way for variation in egg characteristics, and that
differences between females in the degree of such compensation are related to differences
in clutch maculation.
results suggest that, while a principal function of maculation in this species may be to
strengthen the eggshell, it may also reduce eggshell permeability when large amounts of
pigment are used, and that this necessitates a behavioural adjustment from the female
during incubation.
A special problem with mount nesters and gas
exchange
Condor, 1987
Elevation and water loss
• Among species found at wide ranges altitudinally, there is a local
variation in number of eggshell pores per unit area, OR in size of
individual pores
• Higher elevation results in more rapid diffusion; therefore fewer
pores, or smaller pores, are needed
• Birds that are moved altitudinally fairly quickly adjust pore numbers
and/or pore sizes in the eggs that they lay
Adaptation of the Avian Egg to High Altitude 1
CYNTHIA CAREY American Zoologist 1980
• Theoretical predictions and experiments on eggs of domesticated birds indicate
that the diffusion coefficient of gases is inversely proportional to barometric
pressure.
• Therefore potentially lethal losses of CO2 and water vapor from eggs laid at high
altitude might result if the increased tendency of gases to diffuse at reduced
barometric pressure were not counteracted
• data from two wild populations indicate water loss is independent of altitude over
a 3000 m gradient.
• Four different possibilities by which compensation for increased diffusion of water
vapor might be achieved at high elevations
– 1) a reduction in eggshell conductance (GH2O)
– 2) an increase in the initial water content of the eggs
– 3) an increase in shell thickness
– 4) alteration of water vapor pressure in the nest microenvironment or
incubation temperature by variation in parental behavior
Adaptation of the Avian Egg to High Altitude 2
• Mean GH2O of eggs of two precocial and four altricial species
breeding above 2800 m is significantly reduced below values of
related birds breeding at lower elevations, but no change in initial
water content or shell thickness has been observed in such eggs
• That data contradicts hypotheses 2 and 3
• Observations of parental behavior in species breeding over wide
elevational gradients have not yet been made (hypothesis 4)
• More research needed for identification of the mechanisms
– ways eggshell structure is modified to achieve a reduced GH2O
– environmental cues used by females to determine elevation of nest location
– the rapidity with which shell structure can be modified
Elevation and water loss
H. Rahn, T. Ledoux, C. V. Paganelli and A. H. Smith
J Appl Physiol 53: 1429-1431, 1982; 8750-7587
• Hens acclimated to an altitude of 3,800 m (PB 480 Torr) were
transferred to 1,200 m (PB 657 Torr).
• Eggs were collected before departure and daily after the transfer so
that changes in eggshell conductance could be studied.
• Over the next 2 mo eggshell conductance increased 30%, presumably
to compensate for the 37% reduction (from 657 to 480 Torr) in gas
diffusivity at the lower altitude.
• Measurements of shell thickness and number of pores in the shell
allow one to calculate that most of the change in total pore area
occurred by an increase in cross-sectional area of individual pores.
Altitude and eggshells
Monge-C. and F. León-Velarde (1993)
• At the end of incubation, partial pressures of oxygen and carbon dioxide in the air
cell of sea-level avian eggs are similar to those in the expiratory air of adult birds.
• At high altitude, changes in the permeability of the shell and probably in embryo
metabolism partially compensates the increase in the gas diffusion constant
resulting from the low barometric pressure.
• tested whether-despite of the adaptive responses of the high altitude avian
embryo-the air cell values would be similar to those of the alveolar air of high
altitude human natives.
• Air cell O2 (48.3±1.6 torr) and CO2 (20.9±0.85 torr) pressure values were obtained
by studying naturally incubated eggs of the Andean gull (Larus serranus) at 4650m.
• Sea-level chicken (Gallus gallus) air cell pressure values of O2 (102.3±2.7 torr) and
of CO2 (43.3±1.3 torr) were obtained from the literature for comparison.
• values similar to alveolar air of humans at sea level (O2: 104.4±0.4 torr, CO2: 40.1±
0.25 torr) and at high altitude (4540 m) (O2:50.5±0.53 torr, CO2: 29.1±0.37 torr).
• Despite large evolutionary changes in morphology and physiology of respiratory
organs, head pressure of O2 that oxygenates the blood keeps a constant value in
the pre-pipping avian embryo and in alveolar air of adult mammals.
• This constancy holds valid at high altitude.
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
MELANIE MASSARO, LLOYD S. DAVIS (2005) Ibis 147 (2) , 251–258
• Brood reduction in birds is frequently induced by hatching asynchrony.
• Crested penguins (genus Eudyptes) are obligate brood reducers, but in contrast to
most other birds, first-laid eggs are considerably smaller in size than second-laid
eggs; furthermore, first-laid eggs hatch after their siblings
• The mechanisms underlying this reversal in size and hatching order remain unclear.
• tested whether the second-laid eggs of Snares Penguins have a higher eggshell
porosity allowing them to maintain a higher metabolic rate throughout incubation
and to hatch before their first-laid siblings.
• investigated differences in egg size, shell thickness, pore density, pore diameter and
water vapour conductance between first and second eggs within clutches and
examined influence of shell characteristics on hatching asynchrony.
• First-laid eggs were approximately 78% the size of larger second eggs.
• Second-laid eggs had considerably thicker shells and more pores per cm2 than first
eggs; pore diameter did not differ
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
MELANIE MASSARO, LLOYD S. DAVIS (2005) Ibis 147 (2) , 251–258
• Water vapour conductance was greater in second- (16.8 mg/day/torr) than in firstlaid eggs (14.9 mg/day/torr).
• The difference in water vapour conductance between first- and second-laid eggs
within clutches was related to hatching patterns.
• In nests where second eggs hatched before first-laid eggs, second eggs had a
considerably greater water conductance than their sibling, whereas in nests where
both eggs hatched on the same day, the difference in water conductance between
eggs was very small, and in a few nests where small first eggs hatched before their
larger sibling, they had a greater water conductance than their larger second-laid
nestmate.
• Surprisingly few studies have investigated differences in shell characteristics
between eggs within clutches and associated effects on hatching asynchrony.
• This study has demonstrated that such differences exist between eggs within
clutches and that they can influence hatching patterns.
EGG SIZE, EGGSHELL POROSITY, AND INCUBATION
PERIOD IN THE MARINE BIRD FAMILY ALCIDAE
Karen Zimmermann and J. Mark Hipfner The Auk
•
•
•
•
•
•
•
ultimate factors that influence duration of avian incubation are well known, we know much
less about the proximate mechanisms by which birds adjust incubation period
tested the hypothesis that an adjustment in eggshell porosity is one such proximate
mechanism (i.e., that avian species with higher ratios of incubation period to egg size lay
eggs with less porous shells).
Eggshell porosity affects the rate of gaseous exchange between the developing embryo and
the external environment; thus, to the extent that embryonic metabolism is diffusionlimited, eggshell porosity could directly determine incubation period.
collected eggs from seven species of Alcidae, a family of marine birds that exhibits an
unusual degree of interspecific variation in incubation period, and measured egg mass and
eggshell porosity (determined by the number and size of pores and the thickness of the
shell). Incubation periods were obtained from the literature.
Egg mass and eggshell porosity combined explained 87% of variation in incubation period
among the species, which included at least one member of each of six main alcid lineages.
As predicted, eggshell porosity and incubation period were negatively related, after
controlling for egg mass.
results are consistent with the hypothesis that evolutionary changes in avian incubation
period may be attributed, at least in part, to adjustments in eggshell porosity.
Canada goose hatching
• http://www.youtube.com/watch?v=TYJiQ-03tBI