Parental Care IV: Hatching to Fledging

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/parentalcare.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.

superprecocial Malleefowl (Leipoa ocellata) chick(Source: http://abc.net.au/science/scribblygum/October2000/gallery.htm) • 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 ducklingsPhoto 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):

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 post-hatching 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 • 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). Used with permission of Takashi Koike Source: http://www.fsinet.or.jp/~bird/bird/greattit/kara96.html Great tit fecal sac

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: • 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 • 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 American avocet

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, Philepittacastenea, 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 • many young birds beg for food by making lots of noise & opening their beaks wide to reveal brightly colored 'gapes'. • Among Barn Swallows (Hirundorustica) 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. cspottiswoode.free.fr/Anders/Research.htm

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 (Turdusmerula) 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 • 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. See a Barn Owl video courtesy of the BBC

Adult provisioning and fledgling plumage • Ultraviolet (UV) reflectance has been implicated in mate selection. • some bird species plumage of young varies in UVreflectance in the nest, long before mate choiceand sexual selection • Most birds molt juvenilebody plumage before reaching sexual maturity; someconspicuous traits of the juvenile body plumage may haveevolved by natural selection, possibly via predation or parentalpreference. • This hypothesis is largely untested and predictsa differential allocation of food between fledging and totalindependence, which is a time period of 2–3 weeks whereoffspring mortality is also highest. • Tanner and Richner (2008) tested the predictionthat parents use the individual variation in UV reflectanceamong fledglings for differential food allocation. • They manipulatedUV reflectance of the plumage of fledgling Great Tits (Parusmajor) by treating chest and cheek feathers with a lotion thateither did or did not contain UV blockers and recordedfood allocation • The visible spectrum was minimally affected. • Females fed UV-reflecting offspringpreferentially, males had no preference. • This is the firstevidence showing that UV reflectance in young birds has a signaling function in parent–offspringcommunication and suggests that the UV traits evolved via parentalpreference.

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: • 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 • 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. • 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 • In fact, these three categories of potential value for marginal offspring can be mutually compatible: • 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), • obtain a handy insurance policy against early loss of a core chick • be prepared for the occasional good-food year.

Brood-reduction The older, larger chick pushes its younger sibling toward the edge of a Brown Booby nest (Source: www.sciencenews.org/articles/20030215/fob7.asp) • 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 facultativelysiblicidal birds such as Blue-footed Boobies (Sulanebouxii) 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 siblicidehttp://www.youtube.com/watch?v=p1JUu9bUDIw

Parental Care IV: Hatching to Fledging