Birds – Part VI

VERTEBRATE ZOOLOGY (VZ Lecture28 – Fall 2012 Althoff - reference PJH Chapter 17)

Bill Horn

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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 hypothesisb) Predation hypothesisc) 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 precoccial)

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

• Altricial

• ______________— eyes open, covered withdown, able to walk, remain at nest (gulls &terns)

• ______________—covered with down, unable to leave nest, fed by parents1. eyes open: herons and hawks2. eyes closed: owls

Altricial vs. Precocial

• Eggsamount of yolk in eggsamount of yolk remaining

@ hatching

• Chickssize of eyes & braindevelopment of musclessize of gutrate of growth after

hatching

Greater for Greater for

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 long-

distance.

Fig. 17-34, p481 PJH

Short-tailed shearwater

30,000 km (18,640 miles ) annual migration

Note:

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