Peregrine falcon chicks
http://www.uvm.edu/~vbba/images/Crystal%20Lake%20peregrine%20chicks_%202005%20SDF%20sm.JPG
Cliff swallows and a robin
Parental Care IV: Hatching to
Fledging
JodyLee Estrada Duek, Ph.D.
With assistance from Dr. Gary Ritchison
http://people.eku.edu/ritchisong/paren
talcare.html
Precocial & Altricial
• At hatching, some young birds are entirely dependent on their
parents, while others are able to leave the nest and begin finding
their own food within hours of hatching.
• Based on such differences, young birds are generally categorized as
either altricial or precocial. Because of variation within these two
broad categories, ornithologists more precisely classify young birds
into six categories (Gill 1995):
Hatching
•
•
•
•
•
•
•
•
•
•
•
•
About three days before hatching, embryo's head burrows beneath right shoulder so beak is
positioned under wing & against the membranes separating embryo from air space at large
end of shell.
same day beak pierces membranes into air space & pulmonary respiration begins.
a day later, with dwindling oxygen, embryo begins to kick, twist and thrust head and beak
backward; egg tooth pips first hole.
chick can now draw breath.
fresh air enters and circulates, membranes begin to dry, blood vessels within membranes
shrink.
embryo continues to pip, kick and twist.
Small cracks advance counter-clockwise by millimeters around big end of shell.
"hatching muscle" on back of neck (photo left) swells to several times normal size with influx
of fluid from lymphatic system.
swelling accentuates sensory signals sent through neck, stimulating further activity.
cap of the egg is cracked enough.
embryo pushes it off, unfolds from the tuck, and escapes from shell.
(See a Peregrine Falcon egg hatching! or a California Condor egg hatching - cracking egg &
emergence - or Budgerigars hatching)
Adelie penguin
• Click on the photo for a short video about life as a penguin parent.
Malleefowl (Leipoa ocellata) chick
(Source:
http://abc.net.au/science/scribblygum/October2000/gallery.htm)
superprecocial
• young are completely independent at hatching; no parental care
• examples include young megapodes
precocial
•
•
•
•
young leave the nest soon after hatching and follow parents
young can feed themselves almost immediately
examples include young waterfowl, shorebirds, and gallinaceous birds
Young wood ducks leaving the nest
http://www.youtube.com/watch?v=B5qnvZSg1XM
A clutch of Mandarin ducklings
Photo courtesy of Pete Akers
subprecocial
•
•
•
•
•
www.briansmallphoto.com/gallery/colo.html
young leave the nest at hatching and follow parents
young are fed by parents (or at least shown where food is located)
examples include young rails, grebes, & loons
Common loon http://www.youtube.com/watch?v=o4ofEAUXI6g
Great crested grebe http://www.youtube.com/watch?v=1hlwgIXqed0
semiprecocial
• young are somewhat mobile at hatching but remain & are fed by
their parents
• examples include young gulls and terns
• Black skimmer http://www.youtube.com/watch?v=JNtt1QBbX_U
semialtricial
• young not mobile at hatching & are fed and brooded by parents
• eyes of young open at hatching (semialtricial 1) or within a few days
(semialtricial 2, e.g., owls like the Eastern Screech-Owls in the photo
to the right)
• examples include young herons, hawks, & owls
altricial
• young are naked, blind (eyes closed), & helpless at hatching
• examples includes songbirds, woodpeckers, hummingbirds, and
pigeons
• American robins http://www.youtube.com/watch?v=fTPBMu1pM4Y
Summary of characteristics of young birds at hatching (Nice 1962):
Development
Down
present?
Eyes
open?
Mobile?
Feed
Themselves?
Parents
Examples
present?
Superprecocial
Yes
Yes
Yes
Yes
No
Yes
Yes (follow
parents & find Yes
own food)
Yes
Yes
Yes (may be
shown food by Yes
parents)
No
Yes
Type of
Precocial
Subprecocial
Yes
Yes
Yes
megapodes
waterfowl,
shorebirds
grebes, rails,
cranes, &
loons
gulls &
terns,
penguins
hawks,
herons &
egrets
Semiprecocial
Yes
Yes
Yes, but
remain in
nest
Semialtricial 1
Yes
Yes
No
No
Yes
Semialtricial 2
Yes
No
No
No
Yes
owls
Altricial
No
No
No
No
Yes
songbirds
Precocial & altricial
• Precocial development is the primitive, or original, mode, with altricial mode
developing independently in several groups (Ricklefs 1983, Gill 1995).
• Precociality puts a premium on the ability of females to obtain abundant resources
before laying.
• produce energy-rich eggs to support the greater in-egg development (eggs of
precocial birds contain almost twice the calories per unit weight as altricial birds).
• Females of altricial species do not have such large nutritional demands before egg
laying, but must be able (with their mates) to find sufficient food to rush their
helpless young (see drawing next slide) through to fledging.
• While the young are in nest, entire brood is vulnerable to predation and dependent
on concealment and parental defense.
• precocial young have some ability to avoid predation, much smaller chance of
entire brood being devoured.
• evolutionary trade-off in brain sizes related to the degree of precocity.
• Precocial species have relatively large brains at hatching and fend for themselves.
• precocial species trade-off: adult brain is small in relation to body size.
• Altricial young born small-brained, but on the protein-rich diet provided by adults
(and with their highly efficient digestive tracts) postnatal brain growth is great, and
the adults have proportionally larger brains than precocial species" (Ehrlich et al.
1988).
• Spectrum of developmental stages from super-precocial brush turkeys to
super-altricial songbirds.
• Parental care necessarily varies with categories of development at hatching.
• Precocial and superprecocial birds characterized by simple parental care,
minimal nest attendance, and simple nest structure; features considered
phylogenetically primitive.
• Galliformes and Anseriformes seek their own food the day that they hatch
but depend on parents for some degree of brooding and protection.
• altricial species characterized by sophisticated parental care; includes
complex nest building and high attendance to offspring.
• traits associated with altricial development (e.g. complex nest construction
and strong parental care) are also correlated with increase in range of flight
styles, flight speeds, and ecological habits (Dial 2003).
After hatching
• avian parental care may involve brooding and feeding nestlings as
well as protecting young from predators. Only a few species (brood
parasites and the moundbuilders or megapodes) exhibit no posthatching parental care.
• For those that provide care:
• biparental care is predominant
• female-care only has been described in about 85 species (those
with lek polygyny mating systems plus promiscuous species like
hummingbirds)
• male-care only has been described in about 30
species (polyandrous species)
Indigo buntings
• Female Indigo Buntings provision nestlings with little or no assistance
from their mate (Payne 2006).
http://people.eku.edu/ritchisong/parentalcare.html video about 20%
down page
Altricial young unable to control body temperature
• must be kept warm when ambient temperatures low, or cool when nest is
in sunlight.
• Parents help nestlings maintain body temperatures by brooding - covering
them to keep them warm or shielding them from the sun or rain.
• duration of the brooding period depends on (Pettingill 1985):
• the time needed for young to develop ability to thermoregulate - many young
songbirds are able to maintain their body temperature 5 - 7 days after hatching
• weather - young are brooded longer during cool, wet weather
• nest type - birds that nest in cavities, with more stable microclimates, typically
brood for shorter periods than open-nesting species
• Altricial young typically brooded almost constantly during first day or two
• next several days young brooded less and less.
• Toward the end of the brooding period, young may only be brooded at
night (or, for nocturnal birds, during the day).
• as time of fledging approaches, parents do not brood
• European Shag (Phalacrocorax aristotelis) nestlings had incipient
endothermic response at 9 days, homeothermic at 15–18 days.
• index of homeothermy (HI) calculated by dividing final temperature
differences between nestlings and surroundings by initial
temperature differences, using the formula: HI = (Tf - Ta)/(Ti Ta) where Tf and Ti are final and initial body temperatures,
respectively, and Ta is ambient temperature.
• The index is equal to 1 when Tb (body temp) is maintained without
change, and 0 when Tb falls to Ta within 45 min (Østnes et al. 2001).
Growth rates of passerines
Growth rates of passerines
•
•
•
•
•
•
•
•
•
reasons why growth and developmental rates vary widely among species have remained unclear.
Previous examinations of possible environmental influences on growth rates of birds yielded few
correlations, leading to suggestions that young may be growing at maximum rates allowed within
physiological constraints.
However, estimations of growth rates can be confounded by variation in relative developmental
stage at fledging.
Remes and Martin (2002) re-estimated growth rates to control for developmental stage. They
used these data to examine the potential covariation of growth and development with
environmental variation across a sample of 115 North American passerines.
Contrary to previous results, Remes and Martin (2002) found that growth rates of altricial
nestlings were strongly positively correlated to daily nest predation rates, even after controlling
for adult body mass and phylogeny.
In addition, nestlings of species under stronger predation pressure remained in the nest for a
shorter period, and they left the nest at lower body mass relative to adult body mass.
Thus, nestlings both grew faster and left the nest at an earlier developmental stage in species with
higher risk of predation.
Growth patterns were also related to food (aerial foragers tend to have slower growth rates),
clutch size (growth rates are slower in species with larger clutches), and latitude (faster growth at
higher latitudes).
These results support a view that growth and developmental rates of altricial nestlings are
strongly influenced by the environmental conditions experienced by species (Figure from Erickson
2005).
Feeding Young
• Among altricial (and semi- and subprecocial) species, one or both adults
begin to feed young (or show young where food can be obtained) soon
after hatching.
• In most socially monogamous species & some polygynous species, both
sexes help
• Among birds that deliver food, food may be transferred to young in several
ways (Pettingill 1985):
• carried in the bill & placed in open mouths (most passerines)
• swallowed by adults & later delivered to the young by:
– regurgitating into mouth or throat (e.g., waxwings, hummingbirds, &
herons)
– regurgitating food into nest or nearby where young can pick it up
(e.g., gulls)
– opening mouth and letting young reach in & retrieve food (e.g.,
pelicans & cormorants)
• food consists of 'milk' (produced in the crop) regurgitated into mouths
of young (e.g., pigeons & doves)
• carried in the talons to nest & then either torn into smaller pieces & fed
to young (e.g., raptors with young nestlings) or given to young whole
(e.g., raptors with older nestlings)
Feeding videos
• Cedar waxwing video about 30% down page
http://people.eku.edu/ritchisong/parentalcare.html
• Bald eagle video http://www.youtube.com/watch?v=7fFmnijb_AQ
• White-bellied sea eagle vs. sea snake
http://www.youtube.com/watch?v=Xf240LcsPno
House Wren nest
•
Feeding visits to nests are typically quick (less time at the nest means
less activity that might attract the attention of predators). However,
particularly early in the nestling period, females may remain at the
nest after feeding nestlings to brood the young.
• House wren video about 30% down page
http://people.eku.edu/ritchisong/parentalcare.html
Fecal sacs
Great tit fecal sac
Used with permission of Takashi Koike
Source:
http://www.fsinet.or.jp/~bird/bird/greattit/kara96.html
• After feeding nestlings, adults often pick up fecal sacs (packages of
excrement surrounded by a gelatinous membrane) that may be eaten
(particularly when nestlings are very young) or carried from the nest
for disposal. Older nestlings may 'shoot' their feces away from the
nest (e.g., see Bald Eagle video next slide).
• American robins and fecal sacs
http://www.youtube.com/watch?v=uS6cA0lxBw4
• Bald eagle and defecation from nest
http://www.youtube.com/watch?v=03G7CZt3JQc
Tucson, AZ
Distribution of parental care in shorebirds
(next slide)
• Male-only care is often associated with polyandrous mating systems,
whereas female-only care is associated with polygyny and leks
(modified from Székely and Reynolds 1995).
• The species pictured above the graph are, from left to right,
•
•
•
•
•
•
Greater Painted Snipe (Rostratula benghalensis),
Wattled Jacana (Jacana jacana),
Eurasian Thick-knee (Burhinus oedicnemus),
Eurasian Oystercatcher (Haematopus ostralegus),
White-rumped Sandpiper (Calidris fuscicollis),
Ruff (Philomachus pugnax).
Source: Szekely et al. (2006).
Distribution of parental care in shorebirds
Modes of parental care
(Cockburn 2006 -- see Table next slide)
• Estimates of major classes of parental care by birds have been drawn from
classical studies that preceded publication of a massive secondary literature
and the revolution driven by molecular approaches to avian phylogeny.
• Cockburn (2006) reviewed this literature in the light of new phylogenetic
hypotheses and estimated prevalence of six distinct modes of care:
1.
2.
3.
4.
5.
6.
use of geothermal / solar / composting heat to incubate eggs,
brood parasitism,
male only care,
female only care,
biparental care,
cooperative breeding.
• Female only care and cooperative breeding are more common than has
previously been recognized, occurring in 8 and 9% of species, respectively
• Biparental care by a pair bonded male and female is most common pattern,
but, at 81% of species, it is less common than once believed
The number of bird species known and inferred to
exhibit different modes of parental care.
Male only care
Red-necked phalarope
American avocet
• difficult to identify a common pattern in groups where males are
predominant carers.
• Even the best-known correlate, with precocial young, is now known to have
at least one exception (Andersson 1995).
• Owens (2002) argued that contrasts between families exhibiting male and
female only care support a low-density hypothesis, which proposes that
males should care if density is sufficiently low to prevent any benefit by
desertion, as they are unlikely to find alternative mates.
• The basis for this contrast is motivated by the dynamic desertion strategy of
Holarctic waders (Rostratulidae, Haematopodidae, Ibidorhynchidae, Recurvirostridae, Dromadidae, Burhinidae, Glareolidae,
Charadriidae and Scolopacidae regularly breeding within the Holarctic ) though this strategy appears to
occur when food is abundant late in the season, and not earlier
• single hypothesis is unlikely to encompass all cases
Female only care 1
• abundant evidence that common selection pressures have driven convergent
evolution of female-only care.
• numerous origins of female-only care among taxa with nidicolous, altricial young.
• In such taxa, female-only care evolved in birds that feed largely on tropical fruit and
nectar.
• correlation explained in complementary ways from female and male perspectives.
• Because tropical fruit and flowers can be massively abundant, yet availability can
be patchy on short-term spatial and temporal scales, males may gain advantage
from the defense of fruiting trees or geographical locations that females frequently
traverse in order to find fruiting or flowering trees (the hotspot hypothesis).
• From the female perspective, the limitation on reproduction is likely to be
associated with the ability of the young to extract nutrition from abundant but low
quality food.
• Hence male care is of limited value, allowing females to choose freely among males
for good genes rather than for direct benefits from the male such as a high quality
territory or paternal provisioning (the constrained female hypothesis).
Female only care 2
• What about primarily insectivorous taxa, where male care should be at a premium?
• Many insectivorous taxa with female-only care occur in dense nesting aggregations
in rich marshlands in which high abundance of food occurs because of seasonal
irruptions of aquatic insects.
• This reduces the need for females to obtain care and together with high female
densities, facilitates evolution of polygyny.
• However, a variety of taxa cannot be explained via this approach, particularly some
insectivorous denizens of rainforests.
• Predation might be important in these species because, as originally suggested for
frugivores, males might enhance detection of the nest by predators because
– any attempt by males to guard a single female against extra-pair mating would impose
impossible costs from the sit-and-wait predators that predominate in rainforest interiors
– the intrinsic mortality schedules of long-lived tropical species may make parents
reluctant to take risk during reproduction.
• Further investigations of these cases will be extremely valuable.
Cooperative breeding
• In the 1097 species of New World suboscines, cooperative breeding is
rare, inferred in just 16 species.
• By contrast, a larger proportion of oscines are cooperative breeders
(577 of 4456 species; 13%).
• It is unlikely that there is a simple ecological or life history
explanation for this difference.
• Both clades have diversified into an enormous range of niches and
show overlapping variation in life history.
• The low prevalence in suboscines is unlikely to be a result of the
environment they occupy.
• Several oscine taxa have primarily radiated in the Neotropics and
hence overlap the range of the New World suboscines.
• Many of these have a high incidence of cooperation (e.g., New World
jays, mimids, emberizids, icterids and wrens).
Parental conflict in birds 1
• Parents often conflict over how much care to provide offspring because
care requires time and energy and reduces parental survival and
opportunities for additional mating or polygamy.
• Therefore, each parent prefers the other to provide more care.
• This conflict is expected to produce a negative relationship between male
and female parental care, the strength of which may be mediated by both
ecological and life-history variables.
• In a broad-scale comparative study of parental conflict using 193 species
from 41 families of birds, Olson et al. (2007) found male and female
parental care were negatively correlated across a broad range of bird taxa.
• This indicates that there is an evolutionary tug-of-war between males and
females over the care of offspring, a result consistent with the prediction of
sexual conflict theory.
• Olson et al. (2007) found strong influences of both male and female mating
opportunities on patterns of parental care disparity.
Parental conflict in birds 2
• analysis revealed that developmental mode of the young has a strong
influence on relationship between parental care and mating
opportunities.
• the sexes appear to play the same strategies, at least in birds: if their
young need little care, then both males and females respond to
enhanced mating opportunities.
• these results suggest that sexual conflict is a key element in the
evolution of parental care systems.
• also support the view that major correlates of intersexual conflict are
mating opportunities for both sexes.
Begging
• When a parent arrives at the nest (e.g., an adult Velvet Asity, Philepitta
castenea, visiting its nest; Williams 2001), young typically respond (although
to varying degrees depending on how hungry they are) by begging --uttering 'begging' calls with mouth wide open.
• In many species, young have brightly colored mouth linings to help parents
direct the food into their mouths.
• Feeding rates typically increase with the increasing age of nestlings.
• Parents may make more trips to the nest, deliver more food per trip or
both.
• Of course, frequency of provisioning visits also varies with species and
number of young. For example (Welty and Baptista 1988):
•
•
•
•
young Golden Eagles are fed a hare or grouse twice daily,
young Bald Eagles are fed 4 or 5 times a day,
half-grown Barn Owls are fed about 10 times a night,
young Great Tits are fed several hundred times a day.
Begging
• Blue tits begging http://www.youtube.com/watch?v=WCGi8uhFTu4
• Young barn swallows being fed http://www.vimeo.com/80228
Food Begging: Red or Dead 1
cspottiswoode.free.fr/Anders/Research.htm
• many young birds beg for food by making lots of noise & opening their
beaks wide to reveal brightly colored 'gapes'.
• Among Barn Swallows (Hirundo rustica) Saino et al. (2000) found parent
swallows apportion food according to how healthy they judge nestlings to
be, and only the fit get fed.
• adult birds base this health audit on redness of the wide-open beaks.
• If food is short, no red gape can mean no dinner for a needy nestling.
• Saino et al drew this conclusion from experiments with baby Barn Swallows.
• First they dyed some nestlings' gapes with red food coloring; these got
more food.
• when they challenged the immune systems of swallow nestlings (with
sheep red blood cells), the color of the birds' gapes dulled -- sometimes
even to green or yellow -- and they were overlooked by their parents.
Food Begging: Red or Dead 2
• Pigments called carotenoids are largely responsible for gape hue, especially lutein.
• In birds and mammals, carotenoids also stimulate and regulate the immune
system.
• Saino's group wondered whether swallows whose immune systems are wrestling
with something -- a bacterial infection, say -- have dowdier gapes because they
cannot spare carotenoids for the less important business of imbuing the inner
lining of their throats with color.
• They tested this by giving extra lutein to swallows infected with sheep red blood
cells.
• lutein-boosted, but ailing nestlings, developed gapes just as red - and were fed
equally well by their parents - as their healthy siblings.
• parent Barn Swallows, anxious to make the best use of limited food resources, use
gape color as a reliable signal of offspring quality.
Manipulative begging by parasitic cuckoo chicks
•
•
•
•
•
•
•
•
•
(Lotem 1998)
Common Cuckoo: obligate brood parasite, lays a single egg in nests of several passerine
species.
after hatching cuckoo ejects the host eggs or young from the nest and is raised alone.
a single cuckoo chick raised by a small passerine, such as the Reed Warbler, is fed at the
same rate as, and for a longer period than, a brood of four host young.
One suggestion to explain the success of the cuckoo chick in eliciting parental care was that
its large size, bright gape and intense begging provided the parents with a super normal
stimulus or with an image of an especially high quality offspring.
However, Davies et al. (1998) showed that large size alone was insufficient to stimulate
adequate provisioning. When they replaced a Reed Warbler brood with a single European
Blackbird chick (Turdus merula) or a Song Thrush (T. philomelos), which are similar in size to
the cuckoo chick, the rate of food delivery was significantly smaller than to a single cuckoo
chick and similar to a single Reed Warbler chick.
Further exploration suggested that the stimulus used by cuckoo chicks to elicit host care is
their unusual begging call, which, to human ears, sounds remarkably like the begging calls of
a whole brood of Reed Warblers.
on a sonogram cuckoo begging calls and those of a brood of Reed Warblers very similar.
In contrast, blackbird and thrush chicks have calling rates of only about one call per second,
which could explain their inability to elicit the same provisioning rate as a single cuckoo,
despite being the same size.
cuckoo chick needs vocal trickery to compensate for the fact that it presents a visual
stimulus of just
one gape and the cuckoo's way of deceiving its host is to pretend to be a group of several
offspring rather than appearing as a single high quality one.
A 17-day-old cuckoo nestling in a Reed Warbler nest.
(Photo by Tomas Grim; http://www.zoologie.upol.cz/osoby/Grim/obr_kukacka.htm)
•
•
•
Sonagrams (2.5 sec) of the begging calls of
(a) a single Reed Warbler,
(b) a brood of four Reed Warblers,
(c) a single cuckoo nestling, and
(d) a single European Blackbird nestling.
Owl chicks with impeccable manners
See a Barn Owl video courtesy of the BBC
• scientists assume nestlings call to attract the attention of parents.
• in some species, chicks call even when parents aren't around; nestlings possibly
communicating with each other.
• Roulin et al. (2000) chose two siblings at random from broods of Barn Owls (Tyto
alba) and gave one of the chicks in each brood dead mice to eat during the day.
• They found that the hungry nestling cried more often during the following night
than the chick that had eaten.
• once the hungry chick had been fed, its sibling started to beg more.
• In another experiment, the more siblings in a nest, the less the chicks called.
• Assume that chicks don't beg when they have little chance of getting food.
• If one nestling is more hungry, the value of the food for it is higher; it will fight
physically for the prey.
• not worthwhile for less hungry nestling to compete.
• chicks monitor each other's hunger levels by the intensity of cries; less hungry birds
back down, saving energy and waiting their turn.
• nobody previously considered that nestlings might communicate in the absence of
parents; possible that nestlings of other species behave in the same way.
Adult provisioning and fledgling plumage
•
•
•
•
•
•
•
•
•
Ultraviolet (UV) reflectance has been implicated in mate selection.
some bird species plumage of young varies in UV reflectance in the nest, long before mate
choice and sexual selection
Most birds molt juvenile body plumage before reaching sexual maturity; some conspicuous
traits of the juvenile body plumage may have evolved by natural selection, possibly via
predation or parental preference.
This hypothesis is largely untested and predicts a differential allocation of food between
fledging and total independence, which is a time period of 2–3 weeks where offspring
mortality is also highest.
Tanner and Richner (2008) tested the prediction that parents use the individual variation in
UV reflectance among fledglings for differential food allocation.
They manipulated UV reflectance of the plumage of fledgling Great Tits (Parus major) by
treating chest and cheek feathers with a lotion that either did or did not contain UV blockers
and recorded food allocation
The visible spectrum was minimally affected.
Females fed UV-reflecting offspring preferentially, males had no preference.
This is the first evidence showing that UV reflectance in young birds has a signaling function
in parent–offspring communication and suggests that the UV traits evolved via parental
preference.
Food Choices
• Most parent birds feed their young the same food that they eat: insect eaters feed
their young insects, fish eaters bring fish, seed eaters bring seeds
• seed-eaters and fruit-eaters may also provide their young with insects (which
contain more protein) (Skutch 1976).
• Parents also vary the size of food items, typically bringing smaller items to very
young (and small) nestlings and larger items to larger, older young.
• The food given to young birds contains all the moisture they need, and parents do
not bring them water to drink (Skutch 1976).
• Notable exceptions are the sandgrouse (e.g., the Black-bellied Sandgrouse to the
right) --- parents (particularly males) have specially modified abdominal feathers
with great water holding capacity.
• In fact, these feathers have structural modifications (the barbules are not hooked
together) that make them three to four times more absorbent than synthetic
sponges (del Hoyo 1987).
• Adults may travel great distances to water, soak up the water in the abdominal
feathers, then fly back to the nest so their young can drink the water from their
plumage (Skutch 1976).
• Female ruby-throated hummingbird feeding
http://www.youtube.com/watch?v=QvnNl74Gxt8
More spiders = smarter young 1
• Early nutrition shapes life history; parents should provide a diet to
optimize nutrients.
• In a number of passerines, there is an often observed, but
unexplained, peak in spider provisioning during chick development.
• Arnold et al. (2007) showed that proportion of spiders for nestling
Blue Tits (Cyanistes caeruleus) varies significantly with the age of
chicks but is unrelated to timing of breeding or spider availability.
• parental prey selection supplies nestlings with high levels of taurine
at younger ages.
• This amino acid known to be both vital and limiting for mammalian
development; found in high concentrations in placenta and milk.
• Based on the known roles of taurine in mammalian brain
development and function, Arnold et al. (2007) then asked whether
by supplying taurine-rich spiders, avian parents influence the stress
responsiveness and cognitive function of their offspring.
More spiders = smarter young 2
• wild Blue Tit nestlings were provided with either taurine supplement
or control treatment once daily from the ages of 2-14 days.
• pairs of size- and sex-matched siblings captured for testing.
• Juveniles that received additional taurine as neonates took
significantly greater risks when investigating novel objects.
• Taurine birds were more successful at a spatial learning task.
• individuals that succeeded at a spatial learning task had shown
intermediate levels of risk taking.
• Non-learners were very risk-averse control birds (no supplement)
• Early diet therefore has downstream impacts on behavioural
characteristics that could affect fitness via foraging and competitive
performance.
• Fine-scale prey selection is a mechanism by which parents can
manipulate the behavioral phenotype of offspring.
Life-history aspects of family strife 1
• (Forbes and Mock 2000) -- Breeding birds have evolved life-history
traits that tend to maximize lifetime reproductive success.
• Within this broad pattern, many variations possible because all traits
are co-evolved with numerous others in complex ways.
• Clutch-size, for example, is frequently lower than the number of
young that parents are capable of supporting by working at top
capacity, especially in long-lived species.
• Nevertheless, studies of species with fatal competition among
nestmates (i.e., siblicide) have shown that parents routinely create
one offspring more than they normally will raise, as if counting on
brood-reduction to trim family size after hatching.
Life-history aspects of family strife 2
• Three general and mutually compatible parental incentives for initial
over-production have been identified:
1. Initial over-production allows parents to capitalize when
unpredictable upswings in environmental conditions increase the
number of high-quality young that can be brought to independence
at affordable levels of effort— the Resource-tracking Hypothesis
2. Initial over-production may allow parents to rear the full
complement of young when various accidents befall a member of
the core brood— the Replacement Offspring Hypothesis.
3. In many taxa marginal offspring may be able to provide various
services to members of the core brood, perhaps as a helper, a meal,
or simply as a blanket— the Sib Facilitation Hypothesis
Life-history aspects of family strife 3
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In fact, these three categories of potential value for marginal
offspring can be mutually compatible:
1. by creating an extra egg, parents may simultaneously improve
the thermal environment for small nestlings (each having a
better surface-to-volume ratio in cool conditions),
2. obtain a handy insurance policy against early loss of a core chick
3. be prepared for the occasional good-food year.
The older, larger chick pushes its younger
sibling toward the edge of a Brown Booby
nest (Source: www.sciencenews.org/articles
/20030215/fob7.asp)
Brood-reduction
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In obligate brood-reducing species, hatching asynchrony is typically highly exaggerated,
such that first-hatched nestlings enjoy an almost insuperable competitive edge.
In siblicidal birds (where brood reduction is substantially caused by sibling aggression),
including various pelicans, eagles, boobies, egrets, and cranes, the marginal nestling is
typically bludgeoned to death at an early age whenever the full brood hatches.
Where nest-mate asymmetries are less extreme, the executions tend to be more
protracted and less certain.
In many facultatively siblicidal birds such as Blue-footed Boobies (Sula nebouxii) and
Cattle Egrets (Bubulcus ibis), the extra nestling is often maintained for days or even
weeks, during which it simultaneously embodies insurance value (enjoying enhanced
survival if eventually predeceased by a senior nestmate) and potential of a larger
number of offspring if ecological conditions prove generous.
The parentally determined initial competitive asymmetries thus modulate the costs and
likelihood of brood reduction and the duration of insurance coverage. --- Forbes and
Mock (2000).
Nazca booby siblicide http://www.youtube.com/watch?v=p1JUu9bUDIw
Defending Young
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Predators of various types will prey on nestlings, and adults in most species
exhibit defense when potential predators approach a nest.
Defense may involve calling, approaching predator, or even striking the
potential predator.
Levels of defense by adult birds are typically greater during the nestling
period than during incubation.
This may be because the reproductive value of young to parents increases
as they age.
The effectiveness of parental defense varies among species (e.g., birds with
'weapons' like raptors represent a greater threat than birds without such
'weapons') and with the type of predator.
For example, a Black-capped Vireo has little chance of successfully
defending nestlings from a Broad-winged Hawk (see video of Broad-winged
Hawk predation of Black-capped Vireo nest).
Female harrier http://www.youtube.com/watch?v=kQC_7YEA7x4
Gray catbird vs. black rat snake http://www.youtube.com/watch?v=VCX-cx83qhE
Nest defense as parental care
• Intensity of nest defense against a human intruder was recorded for male
and female Northern Hobbies.
• Sergio and Bogliani (2001) found that, except during incubation, intensity of
nest defense by females was higher than by males.
• For both sexes, defense intensity increased from incubation to fledging,
within the nestling stage, and from fledging to the first 10 days of the
postfledging period.
• Intensity was correlated with brood size in females, but not males.
• The intensity of defense from incubation to nestling stages, and even within
the nestling period, mirrored the increase in survival probabilities of the
offspring.
• That is in agreement with predictions of parental investment theory, based
on the growing reproductive value of the young, due to an increase in
expected fitness benefits for the adults.
• Female intensity of defense was positively correlated with brood size.
• Large broods have greater reproductive value for the parent than smaller
broods, and that may select for higher levels of optimal defense.
• Average intensity of nest defense (±1 SE) by 42 male Northern
Hobbies (open square) and 42 female Northern Hobbies (closed
circle) during 42 nest defense trials.
• Intensity of defense against a potential human predator was
estimated through an ordinal aggressiveness score ranging from 1
(flies away and disappears) to 8 (stoops closely at the intruder).
Fledging
(Michaud and Leonard 2000)
• Parental care benefits parents through growth and survival of young.
• feeding offspring is energetically costly, and protecting them from predators
increases the risk of injury or death to parents.
• Therefore, parents are expected to care for offspring until the costs of care
outweigh the benefits.
• Because offspring may benefit from care that extends beyond the parental
optimum, the length of the dependent period may be a source of conflict
for parents and young (Trivers 1974).
• In altricial birds, conflict could also occur over leaving the nest, or fledging.
• Parents may benefit by decreasing length of nestling period and conserving
energy for future reproductive attempts and/or migration, while offspring
may benefit by extending nestling period and decreasing thermoregulatory
and activity costs.
• timing of fledging may not necessarily be a source of conflict.
• For instance, parents and young may all benefit from earlier fledging when
the risk of nest predation is high.
Timing of fledging
• Before determining whether conflict over the timing occurs, understand factors
• At least three factors are potentially relevant:
1.
2.
3.
nestling development
parental behavior
nestmate interactions.
• The condition of nestlings at the time of fledging influences postfledging survival
and predicts a relationship between nestling development and fledging; studies
found a relationship between features such as wing length or body mass and
fledging age.
• Parents also could influence the timing of fledging;
• a decrease in feeding rate in period leading up to fledging could encourage nestlings to leave
nest and approach parents for food.
• parents might lure young from the nest by perching nearby with food or by calling to
nestlings.
• competition among nestmates could play a role in the fledging process.
• In several species, large broods fledge before small broods, apparently because of
an increase in competition for food and space in larger broods.
• Nestmate interactions also could affect individual nestlings differently and thus
influence fledging order within broods.
• For instance, in Marsh Tits (Parus palustris), smaller nestlings initiate fledging under
low food conditions, presumably to reduce competition from larger nestmates
(Lemel 1989).
Videos
• Young harpy eagle fledging – 80% down page
http://people.eku.edu/ritchisong/parentalcare.html
• Blue tit fledging http://www.youtube.com/watch?v=371HRP95vvU
• Young emperor penguins 85% down page
http://people.eku.edu/ritchisong/parentalcare.html
Adjustment of pre-fledging mass
loss by nestling swifts preparing
for flight
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Nestling birds often maintain nutritional reserves to ensure continual growth during
interruptions in parental provisioning.
However, mass-dependent flight costs require the loss of excess mass before fledging.
Wright et al. (2006) tested whether individual variable mass loss prior to fledging is
controlled through facultative adjustments by nestlings, or whether it reflects physiologically
inflexible developmental schedules.
They found that in the face of natural and experimental variation in nestling body mass and
wing length, Common Swifts (Apus apus) always achieve very similar wing loadings (body
mass per wing area) prior to fledging, presumably this represents the optimum for flight.
Experimental weights (approx. 5% body mass) temporarily attached to nestlings caused
additional reductions in mass; final wing loadings still matched those of control siblings.
reductions in nestling wing length (approx. 5% trimmed from feather tips) resulted in similar
additional mass reductions, allowing wing loadings at fledging to approach control levels.
Nestlings may assess body mass relative to wing area via wing flapping and special 'pushups' (on the tips of extended wings) performed in the nest.
Thus, by facultatively adjusting body mass, but not wing growth, nestling swifts are always
able to fledge with aerodynamically appropriate wing loadings.
Convergence of wing loading values (body mass per
wing area) during the last two weeks prior to
fledging (at day = 0) for unmanipulated control
nestling swifts from broods of two and three.
Extended parental care in an altricial
bird
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In many altricial birds, fledglings disperse when they are no longer fed, this marks end
of parental care.
In some species, young remain in close association with parents after nutritional
independence.
Because juveniles are inferior foragers at this stage, they might benefit from parental
assistance in locating good feeding sites, but this possibility remains largely unexplored.
Radford and Ridley (2006) showed parents and helpers in Pied Babbler (Turdoides
bicolor) societies use a recruitment call to direct nutritionally independent, but
inexperienced, foragers to particular food patches.
Observations and a playback experiment indicated that adult babblers use a “purr” call
to recruit group members to a foraging patch.
Creation of experimental foraging patches supported observations that individuals tend
to call when they are foraging on abundant, divisible food sources and when group
contains independent fledglings (youngsters who are no longer fed directly).
Fledglings responded to calls more than adults, who frequently encountered aggression
from caller, and fledglings gained significant foraging benefits.
This is the first study to demonstrate that altricial birds may use recruitment calls to
extend parental care past the period of direct provisioning.