As day length increases, photic stimulation of the hypothalamus results in the secretion of Gonadotropin releasing hormone (GnRH below). When activated by GnRH, the anterior pituitary secretes two gonadotropin hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH acts on sperm-producing structures in the testes, while LH acts on the interstitial cells of the testes causing them to secrete the steroid hormone testosterone. The pituitary gland monitors the amount of testosterone in the blood, thus creating a negative feedback loop to maintain hormone levels within a set range (Akins and Burns 2001).
Ambient visual cues, such as daylight, activate photosensitive loci in the brain both indirectly, through the eyes, and directly, through the skull. The hypothalamus of the bird brain contains special cells that are sensitive to extremely low light levels, intensities comparable to the amount of light that can penetrate brain tissue (Akins and Burns 2001).
Displays at Nest Ledge. Prominent Perching: perches in conspicuous position near nest ledge May be accompanied by Advertisement Wail. Exhibited more frequently by male, especially before female arrives. Believed to function in mate attraction early in season and also signals site ownership. First indication of pair development is Mutual Perching or Roosting, where male and female perch quietly together . Progresses from perching on same cliff at some distance to perching side by side. In established pairs, may be accompanied by Peeping, mutual preening, nibbling at toes or bill of mate, or Billing, in which one bird turns its head upside-down and engages bill of the other. Activity on nest ledge increases as courtship progresses.
Ambient visual cues, such as daylight, activate photosensitive loci in the brain both indirectly, through the eyes, and directly, through the skull. The hypothalamus of the bird brain contains special cells that are sensitive to extremely low light levels, intensities comparable to the amount of light that can penetrate brain tissue (Akins and Burns 2001).
Displays at Nest Ledge. Prominent Perching: perches in conspicuous position near nest ledge May be accompanied by Advertisement Wail. Exhibited more frequently by male, especially before female arrives. Believed to function in mate attraction early in season and also signals site ownership. First indication of pair development is Mutual Perching or Roosting, where male and female perch quietly together . Progresses from perching on same cliff at some distance to perching side by side. In established pairs, may be accompanied by Peeping, mutual preening, nibbling at toes or bill of mate, or Billing, in which one bird turns its head upside-down and engages bill of the other. Activity on nest ledge increases as courtship progresses.
