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VERTEBRATE ZOOLOGY
(VZ Lecture28 – Fall 2012 Althoff - reference PJH Chapter 17)
Birds – Part VI
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Bill Horn
Clutch Size
• How many eggs should a bird produce?
• Optimal clutch size should = maximum number of
young (chicks) the parents can bring to the
fledgling size that ___________ their chances of
survival
• Major hypotheses that address this question of
clutch size are:
a) Trade-off hypothesis
b) Predation hypothesis
c) Seasonality hypothesis
Trade-off Hypothesis
• Focuses on behaviors that maximize the
_____________ production (i.e., reproductive
success) of an individual bird.
• Another way to say this…
…if a bird is long-lived (i.e, >2-3 years),
it is better to abandon a nest or
produce fewer young per year if
there is an obvious risk of being predated
(the adult, that is, not the nest) staying put.
• Many passerines and shorebirds “operate” under
this strategy. Many live to be 5-15 years or older
• Over their lifetime, may produce >10-20 fledglings
• Easier to feed a ____ young during a nesting cycle
Predation Hypothesis
• Basically is saying “put all your eggs in _____
basket.”
• If there is no more likelihood of predation of a
nest with 12-15 eggs, for example, than a nest
with 3-4 eggs, then “go for it.”
• Many upland gamebirds (wild turkey, quail, prairie
chickens, etc.) have this strategy.
• More frequently, species exhibiting this strategy
have ________ young. (snowy owls exception)
• Risks: a) larger clutch size, more trips to the
nests to feed (if not
precocial)
b) more scent
c) more noise (if not
Seasonality Hypothesis
• Variations in clutch size associated with either
a) geographic comparisons (north vs.
south) b) habitat comparisons (wet vs. dry)
• Tied closely to amount / available of food…as
well as competition there might be for that food
from other species nesting at the same time.
• For example, if rainy season brings on lush
vegetation and/or abundance of insects, being
able to feed young (or hide young) might be
easier…so clutch size could larger.
• If more competition, then one would expect
smaller clutch size
Spectrum of Maturity of Young @ hatching
• Precocial: eyes open, covered with feathers,
leave nest after one or two days
a) some are independent of parents at
hatching (megapods)
b) follow parents but find their own
food (ducks & shorebirds)
c) follow parents and are fed by
them (grebes, rails)
vs.
• Altricial: eyes closed, little or no down, unable
to leave nest, fed by parents (passerines)
Spectrum of Maturity of Young @ hatching
• Precocial
• ______________— eyes open, covered with
down, able to walk, remain at nest (gulls &
terns)
• ______________—covered with down,
unable to leave nest, fed by parents
1. eyes open: herons and hawks
2. eyes closed: owls
• Altricial
Altricial vs. Precocial
• Eggs
Greater for
amount of yolk in eggs
amount of yolk remaining Greater for
@ hatching
• Chicks
size of eyes & brain
development of muscles
size of gut
rate of growth after
hatching
Greater for
Greater for
Greater for
Greater for
Migration
• Most pronounced and/or best known for birds
• Provides a characterization of a vertebrate trait
for being able to move long distances (not only
birds, but some mammal species, some
fishes…and some reptiles)
• Response to changing (seasonal)
a) _______________
b) _______________
• Requires
a) endurance
b) ability to navigate
• Some are short-distance migrants, some longdistance.
Short-tailed shearwater
Note:
Fig. 17-34, p481 PJH
30,000 km (18,640 miles ) annual migration
Physiological Preparation for Migration
• Fat becomes principal energy store
• Birds undergo a period of heavy feeding that results in
pre-migratory fattening:
a) fat deposits made in body cavity
b) fat deposits made in subcutaneous
tissue
* * this can an increase of up to ______ the
‘regular’ amount present….resulting in
increase of 20-50% of the non-fat body mass
• Known as Zugdispostion (migratory prep)
• Then Zugstimmung (migratory “mood”)
• Caged birds, prevented from migrating, exhibit
__________________
Orientation and Navigation
• Two general categories proposed for how birds
orient during migration:
a) long-distance migration is extension of
tendency to explore territory beyond the
the home range, learning to recognize
____________ as one goes along
b) ability to home through unfamiliar territory
using ________ navigation system relying
on possibly 1) sun (pigeons experiments)
2) polarized light
3) ultraviolet light
4) magnetism (pigeons)
5) stars
…and there may be __________ systems