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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 12 - ANNUAL CYCLE
Birds operate in world punctuated by cycles of stress and opportunity. The
major activities or birds -- reproduction, molt, and migration -- are guided by
internal clocks so that they coincide with appropriate seasons. In general,
seasonal changes in day length - called photoperiod - control gonadal
development and reproductive activities. Ultimate factors such as food
supplies and climate influence the evolution of activity timing.
I.
II.
The annual cycle.
A. Cycles vary considerably between species and regions.:
1. Temperate resident (e.g., a chickadee) ON BOARD
2. Tropical resident (e.g., a jacamar) ON BOARD
3. Temperate-Tropical Migrant (e.g., a migrant warbler) ON
BOARD
4. The important concept of repro-survival trade-off…..ON
BOARD
B. Temperate can be either N or S
a. birds breeding in southern hemisphere do so during the
warm productive months there; Sept. - Dec. The
shearwaters we saw on pelagic trip breed off of Chile and
off New Zealand, they "winter" (our summer) off Pacific
coast.
b. Birds breeding in tropics usually do so over longer periods
of time than do birds breeding in either N. or S. temperate
areas.
C. Varies considerably within species and regions.
1. White-crowned sparrows: OVERHEAD Gambel's migrates
but Nuttall's doesn't.
2. Hermit and Swainson's Thrushes co-exist in summer in Pacific
NW forests; Hermits winter along Calif. coast, but Swainson's
thrushes (with whom they are sympatric in summer) winter in
Central America.
The rhythms.
A. Circadian - having a periodicity of about 24 hr; used to regulate
behavioral patterns such as feeding, sleeping, singing, etc.
1. Not always strictly 24 hrs. kept in constant dim light,
Chaffinches in captivity begin to "wake up" an hour earlier
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III.
IV.
each day, suggesting that their circadian rhythm is close to 23
hours. OVERHEAD
2. Discrepancies between a bird's circadian rhythm and a 24 hr
cycle are adjusted, or entrained, by external cues, such as
sunrise, called Zeitgebers. Indeed, sunrise and sunset have
profound influences on all animals' lives, including our own behaviorally, psychologically, physiologically.
3. But why the discrepancy? Not understood, but leading
hypothesis is that animals need to be flexible for day length
can change with global climate changes. A rhythm shorter
than 24 hrs precludes them from getting “left behind” in a
warming climate, while a cooling climate would delay sunrise
and allow the zeitgeber to adjust the animal’s activity. It’s an
evolutionarily conservative strategy, just as it is conservative
to set your alarm earlier than you actually need to get
up…..”just in case.”
B. Circannual - having a periodicity of about 1 year; used to regulate
timing of molt, breeding, and migration. -- these cycles probably
also vary in some species from precise 12 month cycles and are
entrained by changing day length, but this is poorly studied.
Anatomy and Integration of Environmental Cues.
A. Photoperiod. William Rowan did much of this work with captive
birds (e.g., juncos) in the 1920's and 30's. He found that by
carefully adjusting birds' exposure to light and dark in the lab, he
could get juncos in winter to increase gonad size, and her could
get crows to orient to the north for what should have been winter
migration. The photoperiod cycles operated even when birds'
eyes were blocked or removed, so a neurological pathway was
clearly involved.
B. Anatomy of the whole thing.
1. Photoreceptors in the brain, the hypothalamus to be exact, are
sensitive to light that penetrates the skull and brain.
2. Once stimulated, these photoreceptors signal to the pituitary to
synthesize and release Follicle Stimulating and Leutenizing
Hormone, that affect gonad growth and development. FSH 
gamete formation and LH regulates secretion of test. and
estro. from gonads
3. The pathway for light to influence molt is less wellunderstood, but it in some way involves the thyroid and its
hormones. Reproductive hormones, androgens and estrogens,
generally inhibit molt, which is usually too energetically
demanding to undertake while breeding (except in a few
species).
Breeding Seasons
A. Most birds breed annually, but there is considerable variation.
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V.
1. Even in birds that do breed "annually," some do so by
attempting only one brood, others produce multiple broods
(but undertake the general breeding activity only once per
year).
2. Others actually have multiple reproductive periods per year.
Some tropical species nest continually.
3. Sooty terns on Ascension Island (tropical Atlantic) nest every
9.6 month. That is, high food availability allows them to
reproduce continually, but it takes time to raise young and
molt.
4. Many bird species reproduce variably during a year depending
on local environmental conditions. Many tropical and desert
breeding species begin breeding facultatively after rains begin.
B. Timing of breeding. In birds that do exhibit discrete breeding
seasons, various factors influence the timing of breeding.
1. Rainfall a. Budgerigars in Australia wait, up to several years, for
onset of rains, at which time they go like crazy until rains
cease.
b. Many others species, most species in Northern Neotropics,
initiate breeding at end of dry season, so that when the
chicks hatch the rains have begun and food is plentiful.
This indicates that provisioning food for young is more
demanding that obtaining enough food for egg production.
c. Birds on Cocos Island, off Costa Rica, breed in dry
season. Food is plentiful year-round, but the birds can
thermoregulate better when its dry.
2. Temperature - American Robins breed across a broad
latitudinal range over which the onset of egg-laying is equally
variable. Robins begin nesting 3 days earlier for every degree
of latitude and progressively cooler temperatures.
3. Other factors a. The link between rain and/or temp and breeding activity
probably evolved so that birds could time their
reproduction to coincide with abundant food. These
patterns have evolved when future food is predictable
based current temperature and/or rainfall. But sometimes
food is not so predictable, and birds have evolved to
respond to food itself.
b. Pinyon Jays feed extensively and opportunistically (spring
summer fall) on seeds of pinyon pine, which has patterns
of abundance and shortage that are difficult to predict.
Experiments in lab have indicated that just looking at
green pinyon pine cones can stimulate gonadal
development.
Molt
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VI.
A. Complete molt is extremely energetically demanding. Feathers
weigh up to about 40% of total lean dry body mass; to replace
them all is very demanding.
B. Because it is so costly, birds usually relegate molt to periods of
the annual cycle when food availability is high but when other
energetic demands are low (i.e., not during migration and
reproduction). Various patterns exist:
1. The simplest pattern is (for N. temperate spp. With 2 annual
molts):
breedingcomplete molt (prebasic)fall
migrationwinterincomplete molt (body feathers only;
prealternate) breeding
2. Here's another, common in arctic nesters (which are under
extreme time constraints in their short arctic breeding season):
They initiate prebasic molt before breeding, arrest during
incubation, and resume molt at staging (migration gathering
areas) or wintering areas. ON BOARD
3. Here's another, the one for ducks:
The males do their prebasic molt shortly after breeding and
hold it (basic plumage) for just a few weeks (the eclipse
plumage), then undergoing prealternate molt back to familiar
drake plumage in fall, so that they can attract mates in winter.
Females (hens) do the standard sequence; they don't have to
impress anybody.
4. Here's another, one that is common in species that winter in
relatively benign regions (migrant landbirds in tropics,
shorebirds wintering in food-rich areas):
They initiate prebasic molt before migration, arrest it during,
then resume it on wintering quarters, leisurely replacing
feathers over the entire winter.
5. Here's another one, a weird one:
Female hornbills get "entombed" in their nests during
breeding; they rapidly drop all feathers and replace them then
-- might as well as long as male can feed her enough.
6. Here's another weird one:
Common White-terns have all white plumage. Much feather
strength is derived from pigments, which this hapless species
lacks. So her poor feathers degrade quickly, she replaces
them continually.
That wraps up timing and physiology of annual cycle with respect to
2 activities of: molt and reproduction. Next lecture and the one after
we go over one of the most fascinating and beautiful patterns in all of
avian biology: migration.