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.
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!
III. 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!