Download Parental Care 1

Parental Care
•
•
Thermal properties of nests are critically important
The thicker the nest, the better it retains heat, and the less time and energy is needed for
incubation
•
•
Eggs must be kept at a constant temperature of 37 - 38oC
Can’t be allowed to slip below 35oC, or rise above 40oC, or the embryo may die
•
•
White and Kinney (1974) reduced the thickness of Village Weaver nests
Increased the time females had to incubate, thus reducing foraging time
•
•
•
•
•
•
•
•
•
•
•
•
•
Extra 0.05 cm of thickness may seem trivial but it cut incubation time in hummingbird nests by
13% (Smith et al. 1974)
Nest placement also makes a big difference in nest temperature
Cactus Wrens in Arizona nested in the sun early in the season, but switched to shaded nests later
on (Ricklefs and Hainsworth 1969)
Drent’s (1975) study of Herring Gulls shows how egg temperatures slowly rise during the first
two weeks of incubation
Time the adult needs to sit on the nest (attentiveness) also increases during this period
Incubation is very costly, consuming up to 25% of a birds’ daily energy requirements
But the stable thermal environment of the nest actually reduces the energetic needs of the
incubating parents
Must take great care not to overheat eggs
Frequently turn the eggs, switch the positions of the eggs from the warmer interior to the cooler
edges of the nest
Birds that nest in deserts or hot beach sand often sprinkle water on their eggs or shade them with
their bodies
Incubation times vary from species to species
Woodpeckers hatch out in about 10 days, but it takes 80 days to make a kiwi
Precocial species generally have longer incubation times than altricial birds
•
•
•
•
•
•
Davis et al. (1984) got Belding’s Savannah Sparrows to incubate artificial eggs
Raising the temperature of artificial eggs decreased incubation time, while lowering egg
temperature increased incubation time
Eggs lose about 15% of their water content during incubation
Birds in drier climates therefore have shorter incubation times
•
Up to a point, water loss from the egg is adaptive
Air pocket left behind in the shell will supply the chick with its first breath as it struggles to hatch
out
If water loss exceeds 20%, however, the embryo may die
•
•
•
Males and females share incubation duties in 54% of all species
Females are the sole incubators in 25% of species, and males alone incubate in 6% of species
The other 15% of species are too weird to classify!
•
•
•
•
•
•
•
Larger birds, like penguins, have elaborate changeover rituals
Smaller birds generally spell one another discreetly to avoid calling a predator’s attention to the
nest
Birds have a special bare spot on their chest or abdomen called a brood patch
This area is free of feathers, and functions to convey body heat directly to the eggs
Many species pluck their own feathers out of their brood patch, and use these feathers to help line
the nest
The brood patch is very well supplied with blood vessels to maximize heat transfer to the eggs
Skin of the brood patch is tough enough to resist blistering or chafing from endless hours of
incubation (saddle sores)
•
•
•
As hatching approaches, males begin to supplement the females’ diet
Practicing for the great demands about to be imposed by hungry chicks
In a few species, like hornbills, males supply all the females needs during incubation
•
•
Precocial chicks start to talk to one another through the shell
Younger chicks click rapidly, letting older chicks know they need to slow down
•
•
Older chicks click slowly, letting younger chicks know it’s time to get going
The final stimulus is the jostling of the adjacent eggs as the lead chick begins to hatch
•
•
•
•
•
Parents sometimes intervene to help shatter the shell, especially in species like the ostrich, whose
massive eggs may require several days of effort to escape
But for the most part, it’s a lonely struggle in the dark
Baby birds are aided in their great escape by two structures that disappear soon after birth:
> egg tooth
> hatching muscle
Egg teeth are located on top of the bill near the tip
Tiny projection is a reptilian feature - you can see the same thing on the noses of baby turtles
fresh out of the egg
•
•
•
The hatching muscle is located on the back of the neck
It gives the weak and tiny chick the extra power it needs to break through the shell
After birth, it soon fades away
•
•
•
Chicks must first pierce the inner membrane at the blunt end of the egg with the egg tooth
Breaks into the air cell that forms as the water supply in the egg is reduced
It must then bump its head against the inside of the shell to gradually weaken it
•
After a day or two of steady bumping, the chick finally breaks through
•
•
Birds are endothermic, but can’t regulate body temperatures very effectively at birth
So small, they have a high surface area to volume ratio, so they lose a lot of heat
•
•
Precocial birds develop full control of thermoregulation much faster than altricial birds
Covering of downy feathers is a big help in the early stages of thermo-regulatory development
•
•
•
Takes about a week for the ability to thermoregulate to develop
During this time parents must continue to keep the chicks warm by brooding them
Sitting on eggs = incubating, sitting on chicks = brooding
•
•
At the same time that parents need to brood, they must also spend a large part of each day
gathering food for their hungry nestlings
Nashville Warblers show how parents must budget their time on and off the nest…
•
As the chicks age, brooding time decreases and feeding visits increase
•
•
The type of food birds bring back to the nest may be different from the bird’s normal diet
Baby birds need protein, and only insects can supply that protein in sufficiently concentrated
form
•
•
Even birds that are mainly frugivorous switch to an insect diet to feed their young
Most birds try to time their broods to coincide with the seasonal peak in insect abundance
•
•
Ruffed Grouse chicks eat 91% insects and 9% plant food when they are first fed
By August, insects drop to only 1% of their diet (Stewart 1956)
•
•
•
•
•
•
The Central American Quetzal is entirely frugivorous, but feeds its young a strict insect diet for
the first 10 days of life (Skutch 1945)
Altricial birds, like baby robins or mockingbirds, are born naked and helpless, just like us
Precocial birds like ducks and shorebirds, are born ready to leave the nest and follow their parents
to find food
There are many variations on this basic pattern
Passerines are entirely altricial, while herons and hawks are semialtricial
Hawks stay in the nest for a long time, although they are physically capable of leaving in a day or
two
•
•
Gulls and terns are semiprecocial
They can thermoregulate and move about, but prefer to stay in the nest and be fed
•
Megapodes are superprecocial, needing no whatsoever
•
•
•
•
•
•
•
•
•
Precocial birds have:
> longer incubation time
> larger egg size
> higher yolk content
Balanced by the reduced maintenance cost after hatching
Altricial birds have:
> less incubation time
> eggs are energetically less costly (lower yolk content, smaller)
Balanced by much higher maintenance cost after hatching
Ultimately, these strategies may be driven by food supply
Altricial chicks grow up to eat insects, other prey that has to be hunted and captured
This involves a great deal of learning about what is good to eat, where to find it and how to catch
it
Precocial chicks, on the other hand, feed on seeds and small invertebrates, which are easily found
and eaten
The precocial young of fish-eating birds, like loons and grebes, are an exception
•
•
Parents must still feed them until they have learned to catch fish for themselves
The trade-off here seems to be that the increased vulnerability of ground nesting birds to
predation
•
•
A nest on the ground full of noisy chicks is a tempting target
The sooner chicks leave the nest the better
•
•
Baby birds grow very rapidly, which dramatically increases the rate at which they must be fed
Altricial birds like starlings develop three to four times faster than precocial birds like Quail
•
•
•
•
•
•
•
•
•
•
•
Comparing the relative weight of their organs reveals the physiological tactics behind these
strategies
Precocial birds have larger brains, because they need to quickly establish their independence
Altricial birds devote more weight to digestive tissue, because they need to feed more heavily to
catch up during the first few weeks of life
Species-specific growth rates are affected by many proximate environmental factors
The abundance and distribution of food, and local temperature and rainfall can have a big impact
on early development
Common Swifts can take anywhere from 35 to 65 days to mature, depending on insect abundance
(Lack 1956)
They can also respond to a drop in insect abundance by entering a state of torpor, reduce their
energetic requirements
The number of chicks in the nest also affects growth rates
The more mouths there are to feed, the less food each one will get
Smaller chicks often starve
As we’ll see later on, many species practice brood reduction to bring brood size into line with
food availability