Eggs

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Eggs
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Egg - Final Form
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Egg - Final Form
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Eggs - Large vs. Small Birds

• Larger birds produce larger eggs
• Relative size of egg declines as bird size
  increases
• E.g., small birds egg may be 15 of weight,
  large birds lt5

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Egg Shape

• Shape related to shape of birds pelvis
• Rounded to elongated

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Egg Shape

• Shape related to nesting habitat
• Pointed eggs roll in circle

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Egg Color

• Originally white
• Natural selection has favored colored eggs
• Inconspicuous

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Egg Color

• White eggs found in
• cavity nesters,
• those that cover eggs when they leave,
• or sit on them immediately after laying first egg

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Egg Color

• Many solid colors, lots of spotting
• Usually species-specific colors, patterns

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Egg Laying

• Laid at time so that hatching occurs when food
  for young will be abundant
• Owls - winter
• Insect-eaters - spring

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Egg Laying

• Most common laying time - morning
• Common to lay one egg/day until clutch complete

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Egg Laying

• One egg/day - most songbirds, many ducks,
  woodpeckers
• Many larger birds have greater time period
  between eggs

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Clutch Size

• Much variation
• Patterns among birds highly variable
• Larger birds may have fewer eggs than small
  birds, but not hard and fast rule

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Clutch Size

• General latitude pattern within same or closely
  related species
• More eggs further north
• Better resources
• Longer days to find food
• Fewer predators

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Replacement Clutches

• Most birds capable of renesting if initial
  nesting attempt fails (except large birds with
  long incubation periods)
• Repeat courtship, copulation, and so on

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Multiple Clutches

• Many birds raise multiple broods during a single
  season
• Eastern Bluebird - 2 or 3/season
• Tree Swallow - only 1/season

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Determinate Layers

• Stop laying eggs after fixed number produced
• If 1 or more destroyed, it cannot be quickly
  replaced

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Indeterminate Layers

• Keep laying until they have enough, even if some
  are destroyed
• Destroyed ones can be replaced quickly

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Incubation

• Two basic strategies
• 1) Start immediately
• 2) Wait

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Incubation

• Strategy 1 Start immediately after first egg is
  laid
• Loons, owls, hawks eagles, gulls, hummingbirds
• Long incubation period during bad weather

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Incubation

• This strategy produces hatching asynchrony
• Young hatch in same pattern as laid
• Large age/size differences among nestlings

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Incubation

• Strategy 2 Wait
• Start incubation after last egg of clutch is laid
• Ducks, geese, Galliformes, most songbirds
• Short incubation, better weather

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Incubation

• This strategy produces hatching synchrony
• All eggs hatch on same day
• All nestlings the same age/size

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Incubation Brood Patches

• Contact between body heat, eggs
• Single, bare skin area develops in most birds
  (songbirds, raptors, pigeons)
• Others may have 2 or more
• Some have none (ducks, geese, penguins)
• Create one by pulling out down feathers or have
  pouch-like flap of belly skin

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Incubation Brood Patches

• Present in sex that incubates, or both if both
  participate
• Woodpeckers - male/night, female/day
• Doves/pigeons - male/day, female/night
• House Sparrow - male 10 AM-Noon only, female the
  remainder

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Incubation Brood Patches

• Some species, female only
• Stay all night, leave periodically to feed
• 60-80 nest attentiveness during day

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Incubation Temperature

• Usually 35C or higher
• Higher temp. produces faster development, shorter
  incubation period

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Incubation Period

• Incubation period highly variable, no general
  trend
• Larger birds tend to have longer incubation
• Precocial species longer than altricial
• Cavity nesters longer than open nesters
• Examples bald eagle (both incubate) 35-46 days
• Robin (female) 11-14 days
• Bluebird (female) 12-14 days
• House wren (female) 13-14 days

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Hatching

• Egg loses weight (evaporation, water loss)
• Egg-tooth - top of beak tip
• Hatching muscle - back of neck

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Hatching

• Used to chip circular hole out of large end
• Pipping process
• Chick spins around several times until it finally
  chips out

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Hatching

• Synchronized hatching may result from click
  communication among chicks in eggs
• Accelerates/slows down development so all hatch
  at same time

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Brood Parasitism

• Origin?
• Nest loss, using old nests, steal nests

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Brood Parasitism

• European Cuckoo
• Time laying in other nests so other birds gone
• Egg color, pattern matches host egg
• Varies by region

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Brood Parasitism

• Shorter incubation period than host
• Push out other eggs, nestlings
• Aggressive begging

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Brood Parasitism

• N. Amer. examples
• Brown-headed cowbird (obligate)
• Redhead duck (strong tendency)

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Brood Parasitism
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Precocial Young

• Eyes open
• Down-covered
• Leaves nest within day or two
• Semi-precocial - stay at nest, fed by parents
  (e.g., gulls)

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Altricial Young

• Eyes closed
• Naked
• Unable to leave nest (fed by parent)
• Semi-altricial - down-covered, stay in nest
• Eyes open - hawks
• Eyes closed - owls

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Types of Young
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Brooding

• Altricial young must be brooded by parent to
  warm, dry them
• Requires several days to develop endothermy
• Direct all energy into growth

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Brooding

• Brooding reduced as young get older
• Ceases within a few days

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Feeding

• Show precocial young the food, they feed
  themselves
• Altricial young must be fed for varying times

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Feeding

• Monogamy - both parents feed
• High-protein food - insects, animals (whole or
  part, regurgitated), crop milk

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Feeding

• Parents may have nest helpers
• Older young, or unpaired birds waiting for
  opportunity to nest

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Stimulus for Young to Beg

• Begging begins in response to
• Air currents
• Vibrations of nest
• Touch

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Stimulus for Adults to Feed

• Give food to young in response to
• Cries, twitters
• Head waving
• Gaping mouths (yellow, orange, red, targets)

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Trips for Food

• Big foods - few trips per day
• Small foods - hundreds or thousands of trips per
  day
• Great parental energy expenditure
• Weight loss, mortality

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Sanitation

• Dispose of egg shells
• Predator protection
• Eat, feed to young, dispose of

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Sanitation

• Fecal sacs
• Covered by mucous membrane
• Eat (undigested), or dispose of to keep nest
  clean

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Nestling Period

• Period of time from hatching to fledging
• Great variation based on bird size, nesting
  habitat

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Nestling Period

• Longer in species with long incubation period
• Longer in larger species
• Longer in cavity nesters

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Nestling Period

• Great-horned Owl
• Incubation 4-5 weeks
• Nestling 4-5 weeks

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Nestling Period

• House Sparrow
• Incubation 11-14 days
• Nestling 15 days

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Nestling Period

• Downy Woodpecker
• Incubation 12 days
• Nestling 20-22 days

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Nestling Period

• Pileated Woodpecker
• Incubation 18 days
• Nestling 26-28 days

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Nestling Period

• Ends when young leave the nest
• Still follow parents, may still beg for food
• Eventually driven off by parents

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Altricial vs. Precocial

• Altricial start life behind precocial
• Altricial catch up quickly and pass precocial
• Eating machines
• Altricial reach adult size more quickly