The three major events in the avian annual cycle
Reproduction
Molt
Migration
Reproduction
- good food, mat present
Molt
- mate lost or young independent
- food available but not sufficient for breeding
- no mate or mate not in breeding condition
Resident annual cycle
- Plan expensive activities for times of higher food availability (breeding, motling, dispersal, parental care)
- dry season around december has less food available, so no activities here
Migratory species annual cycle
- Molt occurs and then Zugunruhe (body mass increases here)
There is a genetically based polymorphism for migratory behavior, including early molt, premigratory fattening, and migratory restlessness, in the two forms of this species
A molt index of 1 = beginning or end of the molt
A molt index of 5 = heavy molt, includes most of the feather coat
Environmental cues
- Activated by internal (usually hormonal) responses that trigger the transition to the next stage
- molt, breeding, migration, etc.
Increasing vs decreasing day length
- increasing: migration, prealternate molt in females, nesting behavior, breeding grounds arrival and usage
- decreasing: molt, courtship acitivites, migration, pair formation, arrival on wintering grounds
Circadium rhythms
Roughly 24 hours
Adjusted, or entrained, by external cues, such as sunrise called Zeitgebers (“time givers”)
- example: Common chaffinch
Understand what happens to a bird’s circadian rhythm when light/dark cycles are manipulated
- Under constant dim illumination (LL), the cycle drifts one hour of clock time unless it is synchronized by an external stimulus, such as regular 24-hour light–dark (LD) cycles (but still rough following)
role of pineal gland and the suprachiasmatic nucleus in regulating day/night cycles?
Coordinates rhythms across the entire body
Active during the day and inhibits melatonin biosynthesis in the pineal gland
Circannual cycle
- Roughly 1 year
Entrained by changing day length
- example: european starling -Gonads change with seasonal photoperiod. Testis size in male European Starlings (Sturnus vulgaris) naturally fluctuates in response to seasonal day length shifts (pale line). When experimentally exposed to a constant day length of 12 hours of light and 12 hours of darkness, this response changes (dark line) to running freely on a circannual internal clock. The annual cycle is depicted in calendar months, beginning with June.
How can long-term experimental manipulation of day length (photoperiod) can alter a bird’s ability to reproduce?
- example is Dark-eyed Junco
- Longer day lengths cause the testes of captive Dark-eyed Juncos to increase prematurely to full size in January (lower left) and again in April (lower right) instead of in May and June, as in wild juncos (previous slide)
- Mean temperature is the average air temperature in that month
Photoperiodic response in male Japanese Quail
- Over a period of 40 weeks intact males and males without a pineal gland responded to the photoperiod cycles by entering into reproductive condition on long day cycles and exiting on short days
Blind and pinealectomized males did not cycle
external coincidence model
- External light triggers a physiological reaction
- Day length is measured by the increased amount of time that daylight periods coincide with the photosensitive phase of the circadian rhythm (oscillation peaks)
HPG and HPA endocrine axes
- Regulate hormone production by the gonads and by the cortex of the adrenal gland
HPG
- Hypothalamus, Pituitary, Gonadal
- Increase (GnRH) or decrease (GnIH) the production of follicle‐stimulating hormone (FSH) and luteinizing hormone (LH) by the anterior pituitary
- Regulate the production of sex steroid hormones such as estradiol and testosterone
HPA
- Hypothalamus, Pituitary, Adrenal
- Act on the anterior pituitary to increase production of adrenocorticotropic hormone (ACTH)
- Acts on the cortex of the adrenal gland to increase production of corticosterone (the avian version of human cortisol)
HPA/HPG affect example in white crowned sparrows
- In males, testes and cloacal protuberances begin to enlarge prior to arrival at the breeding grounds, once reproductive hormones begin to increase seasonally
- In females, seasonal ovarian growth correlates with increasing estrogens and luteinizing hormone.
Understand the variation in the hormonal control of reproductive behavior as it relates to mating system
- song sparrow
- Male testosterone levels vary by breeding stage and mating system
- Song Sparrows (Melospiza melodia) are monogamous and have two peaks of testosterone around prebreeding and mating periods each year
- Yellow‐headed Blackbirds and Red‐winged Blackbirds are polygynous and have high testosterone from prebreeding until the incubation and chick-rearing stage without any dip in high hormone levels.
Parts of endocrine system releasing hormones
- hypothalamus
- pituitary gland
- anterior pituitary gland
- posterior pituitary gland
- pineal body
hypothalamus
- releasing factors that act as a control on the anterior pituitary gland
example: oxytocin
Anterior pituitary gland
- Thyroid Stimulating Hormone – stimulates the thyroid gland.
Adrenocorticotrophic Hormone – stimulates the adrenal cortex.
Sex hormones – stimulates the sex glands:
Luteinizing Hormone
Follicle Stimulating Hormone
Melanin Stimulating Hormone – function in birds is unknown.
Growth Hormone – stimulates growth of the animal.
Posterior Pituitary
-produces arginine vasotocin
- stores oxytocin
Pineal body
produces melatonin
