Parental care refers to the level of investment provided by the mother and the father to ensure development and survival of their offspring. In most birds, parents invest profoundly in their offspring as a mutual effort, making a majority of them socially monogamous for the duration of the breeding season. This happens regardless of whether there is a paternal uncertainty.
Origin Birds originated from earlier theropod dinosaurs and underwent body miniaturization over a 50 million year period. Changes in anatomy are rearrangement of body mass, adults retain juvenile traits including large brain mass and eyes despite a smaller snout (paedomorphism), and aerial abilities. (Michael S. Y. Lee, Andrea Cau, Darren Naish, and Gareth J. Dyke)Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds •Michael S. Y. Lee, •Andrea Cau, •Darren Naish, •and Gareth J. Dyke Science 1 August 2014: 345 (6196), 562-566. [DOI:10.1126/science.1252243]
The Archaeopteryx was the first fossil bird recorded with evolved feathers.How birds became birds •Michael J. Benton Science 1 August 2014: 345 (6196), 508-509. [DOI:10.1126/science.1257633] The forelimb in the Archaeopteryx could have been used for parental care of offspring because enlarged feathers were possibly used to shield offspring from the suns' rays and for flight. (Carey, J.R., and Adams. J (2001))Carey, J.R. and Adams, J. (2001). The predaptive role of parental care in the evolution of avian flight. Archaeopteryx 19: 97-108. Older potential avialans have since been identified, including Anchiornis, Xiaotingia, and Aurornis.
Kavanau (1987) was the first to find that unique bi-parental care seen in modern birds probably evolved from extinct birds. They developed the ability to provide protection, escorting, nurturing and egg guarding abilities for their young. Evolution of homeothermy and flight most likely occurred in bi-parental birds with precocial chicks. Kavanau said extant birds (David J. Varricchio)Avian Paternal Care Had Dinosaur Origin •David J. Varricchio, •Jason R. Moore, •Gregory M. Erickson, •Mark A. Norell, •Frankie D. Jackson, •and John J. Borkowski Science 19 December 2008: 322 (5909), 1826-1828. [DOI:10.1126/science.1163245 evolved and learned flight through evolution to access ground nests faster. (Kavanau)Kavanau JL, 1987. Lovebirds, cockatiels, budgerigars: behavior and evolution. Los Angeles: Science Software Systems.
Van Rhijn (1984, 1990), Handford and Mares (1985), and Elzanowski (1985) were the first to announce the earliest form of parental care as being mono-parental male care. van Rhijn J, 1984. Phylogenetical constraints in the evolution of parental care strategies in birds. Neth J Zool 34:103-122.Elżanowski A, 1985. The evolution of parental care in birds with reference to fossil embryos. In: Acta XVIII Congressus Internationalis Ornithologici, Moscow, 1982, vol. 1 (Ilyichev VD, Gavrilov VM, eds). Moscow: Nauka; 178–183.Handford P, Mares MA, 1985. The mating systems of ratites and tinamous: an evolutionary perspective. Biol J Linn Soc 25:77-104.
Wesolowsi (1994) contradicted Kavanau's reasoning by saying flight evolved due to parental care not reproduction as previously thought. While flight was being enhanced in evolutionary stages, lack of parental care meant that the increasing number of large eggs required a higher level of investment. This created young that were able to take flight shortly after hatching which is known as precocial, in the form of unassisted paternal (male only) care. The next stage of evolution replaced this with bi-parental care (with a few exceptions). Ligon (1999) suggested with Vehrencamp (2000) that male incubation existed first and later gave way to shared and finally female only incubation.
A possible evolutionary timeline by Kavanau suggests that basal theropod dinosaurs evolved into birds that evolved unique bi-parental care, after which avian birds evolved homeothermy and flight.
Burley and Johnson (2002), Tullberg et al. (2002), Prum (2002), and Varricchio et al. (1999) questioned the male evolutionary shift from no care to male care. They proposed like Kavanau's model that parental care came first leading to bi-parental care in extant birds.
The origin of parental care in birds is still a controversial topic today. (Tomasz Wesolowski)Tomasz Wesolowski The origin of parental care in birds: a reassessment Behavioral Ecology (2004) 15 (3): 520-523 doi:10.1093/beheco/arh039
Different modes of parental care Bi-parental care Bi-parental care is the most common form in birds, especially in passerines. A mating pair equally contributes to feeding and guarding the offspring. It occurs in approximately 85% of bird species.Andrew Cockburn Proc. R. Soc. B: 2006 273 1375-1383; DOI: 10.1098/rspb.2005.3458. Published 7 June 2006 The hatchling benefits from the mutual care at the cost of the parents' future reproductive success. Each parent tries to find a mate who will not desert the nest and has high qualities that showcase their parental skills (e.g. ornamental cues). The good parent hypothesis states that birds can invest more energy towards their own survival rate by choosing an ideal mate.
Evolution in potential mates to advertise their parental strengths through ornamental cues (e.g. a yellow chest patch in Iberian rock sparrows) are based on the differential allocation hypothesis. This hypothesis states that the bigger the ornamental cue a mate has, the more investment is put towards the offspring. As a result of bi-parental care, the offspring are usually stronger than birds which are only cared for by one parent in Iberian rock sparrows. (Vicente García-Navas)Rincón, Amanda García Del, Esperanza S. Ferrer, Hicham Fathi, and Vicente García-Navas. "Mating Strategies, Parental Investment and Mutual Ornamentation in Iberian Rock Sparrows (Petronia petronia)." Behaviour150.14 (2013): 1641-663. Web.
Maternal vs paternal care In bi-parental care, the male provides food and the female is a caretaker. Both ensure the survival of the offspring. The female may care for her young by covering them to keep them warm, shielding them from the sun or from rain and guarding them from predation. The male may also feed the female, who in turn regurgitates the food to the chicks. In female red-eyed vireos the roles are reversed. Nonbreeding adults or juveniles in acorn woodpeckers contribute the care through collaboration with the parents. (Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye)Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye. (Polyandry in Northern Jacanas)
Mono-parental care Male only care occurs in only 1% of bird species (approximately 90 species). Female only care occurs in 8% of species (approximately 772 species). (Andrew Cockburn)Andrew Cockburn Proc. R. Soc. B: 2006 273 1375-1383; DOI: 10.1098/rspb.2005.3458. Published 7 June 2006 A hypothesis states that the parent that invests less reproductive effort in comparison to its mate, will have a higher chance of deserting because it loses less if successful offspring are not produced. However, in some birds (such as the snail kite found in South America, the Caribbean and Florida), the male and the female sometimes compete over which one will desert the nest regardless of which one has invested more into the reproductive effort. (Robert Trivers (1972))STEVEN R. BEISSINGER, Anim. Behav., Mate desertion and reproductive effort in the snail kite 1987, 35, 1504-1519
Parental care in polyandrous species
Polyandry care occurs in roughly 9% of bird species (approximately 852 species). (Andrew Cockburn) The two forms of polyandry are sequential and simultaneous polyandry. Sequential polyandry refers to the mating strategy females use in certain situations. First, they will mate with one male and raise the offspring for a short period of time. Then they will mate with another male and care for that clutch resulting in more genetic diversity and quantity of the offspring per season. Females never incubate offspring alone unless the male has been killed. Some examples of birds which practice sequential polyandry include spotted sandpipers and red-necked phalaropes. Temminck's stint, little stint, mountain plover, and sanderling share in common that the females lays a clutch of eggs and the male incubates them. Second clutches are incubated by the female herself.
In polyandry, one female mates with multiple males (and one male only mates with one female) and is a unique mating system which occurs in less than 1% of all bird species. Parental roles are reversed and cause males to provide most of the care given to offspring. Parental roles also cause a reverse in phenotypic differentiation (genetics) resulting in more colorful and larger females compared with males.
Two main types of polyandry exist: simultaneous polyandry and sequential polyandry. An even rarer subtype called cooperative simultaneous polyandry also exists in some species.
In simultaneous polyandry, the female will dominate a certain territory which contains several small nests with two or more males, who take care of the offspring. Parental roles are unique, since females compete for the males, who do most of the parental care work. Northern jacanas practice simultaneous polyandry.
The females will mate with the males in her territory, often on the same day. In return, the females will help defend the territory. No copulation occurs during the incubation period and during the first six weeks after the offspring are hatched. If the eggs are lost, the female will mate with the males once again. This can lead to competition in some species.
In cooperative simultaneous polyandry, multiple males mate with a single female and a clutch of mixed eggs (belonging to multiple males) is cared for by the whole group. Species that exhibit this behavior include certain types of Acorn woodpeckers and Harris's hawks.
In sequential polyandry (the most common form of polyandry) a female mates with a male and lays her eggs. This female then departs, leaving the male to care for the clutch while she repeats the process with another male. Species which exhibit this behavior include certain types of red and red-necked phalaropes, and spotted sandpipers which breed in South America. (Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye) Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye. (Polyandry in Northern Jacanas)
Factors affecting parental care Ecological conditions The male to female ratio has an effect on the type and amount of care provided. With an increase in available mates in some birds (such as the rock sparrow), female desertion rate increases leading to more mono-parental care. When female rock sparrows were exposed to an abundant number of male mates approximately 50% of the females deserted their first nest when the hatchlings were on average 14.3 days old. The fathers successfully took over all parental duties (Pilastro).
Female birds can determine the sex of their chicks Female birds are able to produce more of a certain sex of birds that are more likely to survive under extreme conditions. In birds, the females' egg determines the sex of the offspring, not the male's sperm. In zebra finches, a study showed the effect of food on sex ratio production. For females, egg production is a metabolically exhausting and nourishment draining process. It was found that the sex of an egg right after it has been laid and the amount of nutrients made available to a growing embryo can be determined as well. Bigger eggs mean bigger young that have a higher survivability rate.Alison N. Rutstein, •Lucy Gilbert, •Peter J. B. Slater, •and Jeff A. Graves Sex-specific patterns of yolk androgen allocation depend on maternal diet in the zebra finch. Behavioral Ecology (Jan./Feb. 2005) 16 (1): 62-69 first published online July 28, 2004doi:10.1093/beheco/arh123