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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 19 – GROWTH AND DEVELOPMENT
I.
Introduction.
 Upon hatching, a chick need to develop quickly to independence to avoid
predation and prepare for the coming seasons, which, for many species, include
demanding migrations.
 The selfish interests of the chick inevitably conflict with those of its parents as
well as its siblings. Remember, an individual bird is ALWAYS working to
maximize its own fitness, that is, the amount of genetic material it passes on to the
subsequent generations.
 Thus, it is in the chick’s best interest for its parents to invest as much as they can
into the chick and still remain alive in order to be around to help the chick fledge.
But it is in the parents’ best interest to maximize their lifetime reproductive
success, which may entail “holding back” in a give reproductive event to
maximize their likelihood of reproducing again in the future.
 This idea of a conflict of interest will come up again in our discussion of bird
growth and parental investment…a little later on. I mention it now to keep it in
the back of your mind – chicks want all they can get, parents want to maximize
reproduction over their entire life.
II.
Early development
A. Hatching
1. Hatching is physically demanding for the feeble chick. It first
punctures the air space in the egg (the allantois), then it begins
“bumping” the egg with its bill (head it tucked up in larger end of
the egg). The bill is equipped with an “egg tooth” a calcified
projection that aids in cracking he shell.
2. This bumping creates tiny fracture sin the she’ll, and the chick
rotates (counterclockwise) for 1-4 days to create a sort of “ring of
fractures” through which it finally breaks free. The egg tooth
then is either reabsorbed (in songbirds) or falls (in most other
birds).
B. Synchronous vs. asynchronous hatching
1. Some clutches hatch synchronously, others asynchronously.
Most birds lay a single egg per day, and usually they are
similarly provisioned. Thus, they all have similar “cook times.”
So if the parent begins to incubate as soon as the first egg is laid,
the hatching times will reflect differences in laying times. If
she/he delays incubation until the clutch is complete, the eggs
should hatch more or less synchronously.
III.
2. Advantages of asynchronous hatching lie in the it minimizes the
time required for the first eggs laid to hatch and fledge –
important if predation pressure is high. It also staggers peak
feeding demands of the chicks, an important release of parental
investment in environments where food supply is low.
3. Advantages of synchronous hatching arise if the parent moves
the brood after hatching to some other are for safety or feeding,
as in many precocial species. In the mallard there are peculiar
adaptations to ensure tight synchrony (see text). Amazing!
Altricial vs. precocial development.
A. Patterns. The most conspicuous feature of chick growth is the dichotomy
of altricial and precocial modes of development.
1. Comparison of altricial and precocial development (see overhead
tables from text).
2. Recognize that this is a gradient – many intermediate modes.
3. Altricial chicks have large viscera that reflect their needs for fast
growth – they grow faster than precocial chicks do because they
are fed large nutrient-rich food items by the efficient parents.
Precocial chicks feed themselves.
4. Only certain food types even allow precocial development.
Terrestrial invertebrates and seeds can be relatively easily found
and procured by young birds, permitting precocial development.
Altricial species tend to rely on foods that require flight/skill/or
experience to obtain, or are otherwise difficult for young,
inexperienced birds to locate and capture.
B. We can examine altricial vs. precocial development as trade-offs, and
these trade-offs are tipped in favor of one mode over the other depending
on the environment of a particular species. What are the trade-offs:
1. For altricial development:
a. The major benefit is that this mode is a very rapid growth
rates, and that minimizes the time until chicks can grow to
independence (which of course is helpful to minimize
young mortality). Another benefit (beyond the scope of
our discussion however) is that altricial development
allows the brain to grow in size well after hatching, which
ultimately results in probable higher intelligence that is
allowed under precocial development.
b. BUT…this fast growth rate come only at the price of (1)
very high food demands, which necessitates the young
staying in the nest to be fed by parents, and that (2) extends
the highly vulnerable “nestling period.” [Note: the terms
nestling and fledgling are often used to describe baby birds.
To be strict, nestling refers to baby birds in a nest, fledgling
refers to bird out of the nest; but you will see that these are
not always used as consistently as perhaps they should be –
I am guilty of loose use of these terms. A more rigid
distinction of terms IS followed for naming the groups of
eggs and baby birds associated with a nest. The group of
eggs in a nest is called the “clutch”; the group of baby birds
(before and after leaving the nest) is called the “brood.”]
2. For precocial development:
a. The major benefits of precocial development are (1) a
slower growth rate that minimizes food demands and (2) a
rapid evacuation of the nest, which minimizes the
vulnerable nestling period.
b. BUT…this slower growth results in a longer time to
independence (which of course can enhance the young
mortality).
C. The balance of these costs and benefits (the trade-offs) is influenced by a
bird’s environment. There are 3 variables that influence this balance:
1. Food supply.
a. If food is difficult to obtain (e.g., requires flight or
experience), then altricial development is favored.
b. If food is relatively easy to obtain, precocial development is
possible, and in some cases, favored (depending on the
other variables).
c. If food is in high supply, then altricial development is
favored (because one of the “costs” of altricial development
is high food demands, but if food is abundant then this
costs is not very strong, and is likely outweighed by the
benefits of altr.).
d. If food supply is low, then precocial development is
favored (benefit of low food demands is strong).
2. Nest accessibility.
a. If nests are inaccessible, then altricial development is
favored (because the “cost” is an extended nestling period
is not so strong).
b. If nests are very accessible, then precocial development is
favored (benefit of repaid nest evacuation is very strong).
3. Nature of young bird predation.
a. If nest predation is a less of a limiting factor than predation
of freely moving young birds, then altricial development is
favored (don’t “want” chicks to leave nest until they are
full grown and ready to evade predators).
b. If nest predation is more of a limiting factor than predation
of freely moving young birds, then precocial development
is favored (“want” to get chicks out of nests as soon as
possible).
D. In sum, we see some patterns that favor the two modes of development:
1. Altricial development is most favored for:
a. species whose foods require flight and/or experience
b. species whose foods are abundant (predation > starvation),
and
c. species whose nests are relatively less vulnerable to
predation.
d. Who dies that sound like? Songbirds and arboreal/cavity
nesters!
2. Precocial development is most favored for:
a. species whose foods can be obtained by inexperienced,
flightless young,
b. species whose foods are scarce (predation < starvation),
and
c. species whose nests are vulnerable to predation.
d. Who does that sound like? Game birds and ground nesters!