Parental behavior and newborn attachment in birds: life history traits and endocrine responses

Abstract

In birds, parental care and attachment period differ widely depending on the species (altricial or precocial), developmental strategies, and life history traits. In most bird species, parental care can be provided by both female and male individuals and includes specific stages such as nesting, laying, and hatching. During said periods, a series of neuroendocrine responses are triggered to motivate parental care and attachment. These behaviors are vital for offspring survival, development, social bonding, intergenerational learning, reproductive success, and ultimately, the overall fitness and evolution of bird populations in a variety of environments. Thus, this review aims to describe and analyze the behavioral and endocrine systems of parental care and newborn attachment in birds during each stage of the post-hatching period.

Keywords: altricial; corticosterone; imprinting; nesting; precocial; prolactin.

Figure 1

Phases of parental behavior in precocial and altricial birds. The depicted stages of parental behavior are characterized by the type of species (e.g., if birds build a nest or use pre-existing surfaces). As an example of a precocial bird, the hatchling of the domestic chick is born with functional sensory systems that facilitate its adaptation to the environment. In contrast, the African grey parrot –an example of altricial bird– has offspring with limited movement that requires longer rearing periods that is equivalent to increased parental care. The hormones involved are marked inside the orange rectangles, representing that its presence and increase modulates each stage. E2, estradiol; LH, luteinizing hormone; PRL, prolactin; T3, triiodothyronine.

Figure 2

Neuroendocrine regulation of parental behavior in male and female birds. PRL is considered the main hormone involved in parental behavior in birds. However, its concentration levels differ depending on the reproductive stage. For example, in non-laying female birds, the interaction of VIP with D2R inhibits PRL release, contrary to what happens during incubation, where the interaction of VIP with D1R stimulates PRL release. The increase in PRL is maintained after hatching and gradually decreases as the breeding cycle progresses. Mesotocin, the homologous of mammalian oxytocin, has a similar pattern to PRL in females, peaking at incubation and rearing, particularly in brain areas such as the SON, POM, and PVN. In the case of testosterone, male birds tend to have the highest levels during the mating and non-breeding season and decrease when entering the parental phase. DA, dopamine; D1R, dopamine 1 receptor; D2R, dopamine 2 receptors; POM, medial preoptic area; PRL, prolactin; PVN, paraventricular nucleus; SON, supraoptic nucleus; VIP, vasoactive intestinal peptide.

Figure 3

Hormonal influence on nest building and the first stages of egg-laying. Nest building, nesting behavior, and incubation of the eggs follow specific endocrine changes in all avian species. During nest building, the activation of the HPG axis and the consequent production of GnRH, LH, estrogen, P4, and testosterone participate to initiate this stage in both male and female individuals. Estradiol is particularly important for female courtship, nest-building, and the onset of incubation. For nesting and incubation, the main hormone involved is PRL, which concentrations increase to promote parental behavior before and after hatching. E2, estradiol; GnRH, gonadotropin-releasing hormone; HPG, hypothalamic–pituitary-gonadal; LH, luteinizing hormone; P4, progesterone, PRL, prolactin.