HSU Wildlife 365
(Humboldt State Univ. WILDLIFE 365)
LECTURE 18 – NESTS AND INCUBATION
； No birds bear live young. Instead, they prepare a nest to cradle their eggs and young. The risks associated with nest building and egg/young care are great; thus, evolutionary pressure has resulted in myriad solutions to diverse demands of insulation, crypsis, and energetic costs associated with nest-building and incubation.
II. Nest Architecture.
A. Nest types.
1. There are a truly phenomenal number of nest types out there.
And many still remain to be discovered. Marbled Murrelet
nests were discovered until 1974 -- in Santa Cruz. Only 10
described by 1987...now it's endangered.
2. Bird nests vary from: OVERHEAD
1. Floating mats
2. Open scrapes
3. Cavity nests
4. Stick nests
5. Colonial mud nests.
6. Open cup nests.
7. Hanging, woven nests.
B. Nest Materials. Likewise, a variety of nest materials are used.
1. Most birds use at least plant materials:
a. Sticks and twigs - common especially in larger arboreal
b. Reeds and grasses -- common especially in species that
c. Green vegetation -- especially important in controlling
2. Others do not.
a. Spiderwebs -- hummingbirds and others who suspend
c. Sand/rocks -- many shorebirds simply scrape a place on
the ground for their eggs.
3. Nets linings/"decorations"
a. Many species line their nests with unique materials to aid
in thermoregulation or crypsis.
b. Lichen, feathers, bark on outside of nest for crypsis.
c. Down, mammal hair, moss for insulation as nest lining.
C. Nest Building.
1. Who does it? -- Can be either or both sexes. In many
polygynous birds, only the female builds a nest. In many
monogamous birds, both species build. In some, such as the
Marsh Wren, males build many potential nests, among which
the female chooses. A major criterion for females in mate
selection is a male's nest-building skill.
2. How do they do it?
a. Most use primarily bill, but feet and body occasionally
used as well, especially for earth burrowers.
b. Bill most adapted for feeding, not as strongly influenced
by nest-building requirements. Nonetheless, a birds bill
is used variably as pliers, needle and thread, tweezers,
hammers, plasterer's tool, shovel, etc.
c. Weavers have the most advanced nest-building skills,
demonstrating an impressive use of knots OVERHEAD. 3. How do they know how to do it?
a. Most bird nests are so recognizable that an observer can
determine its builder to species with no birds present.
The nests are stereotyped, “if you’ve seen one Robin
b. Many birds raised in isolation in captivity can build nests
nearly identical to those in the wild, suggesting a major
c. Others clearly refine their nests based on experience.
First year Eurasian Jackdaws start by shoving everything
they can into a nest, but wind up selecting twig diameters
and lengths that are very similar to those of adults.
d. Some species imprint on the nest location and
architecture in which they were raised -- especially
D. Nest Safety. Predation on nests and their contents (eggs, chicks, and/or adults) can be very high, and it imposes a major constraint on avian fitness. Birds adopt nest building and attendance strategies that minimize predation risk via invisibility, inaccessibility, and impenetrability.
1. Invisibility. Many nests are cryptic -- when adults sit on
them they are nearly invisible. Ground nesting nightjars
provide perhaps the best examples of this. Ground nesters,
typically, are more at risk than tree nesters, since many
mammalian predators are confined to terrestrial niches. In
general, it is often hypothesized that predation rates are
higher in tropics than elsewhere, but whether this is a result of high daily predation rates or longer development (and thus exposure) due to limited food is unclear.
a. Building nests in trees helps, but there are also many
arboreal predators -- squirrels, snakes, and especially
other birds -- Jays, Crows, etc.
b. Many birds build on islands safe from predators, where
their numbers can reach astronomical heights. Horned
Coots pile up stones in the middle of lakes on which they
build reed nests -- they make their own islands.
3. Impregnability. Other birds build enclosed or pensile nests to discourage predators. Covered nests are less visible to predators sure, but often these nests cannot be penetrated by predators, which may be too large to squeeze through entrance holes. Also, pendulant nests effectively exclude snakes, which require supporting structures nearby to enter a nest.
E. Colonial Nesting.
1. Basics. About 13% of all birds nest colonially, including most seabirds. Evolution of colonial nesting behavior results from costs and benefits associated with (1) shortage of nesting sites that are safe from predators (2) abundant and/or unpredictable food that is distant from safe nest sites.
2. Benefits. -- reduced predation.
a. A safe spot for one pair is a safe spot for 1000 pair. b. A large group may be able to better detect and repel
predators that a single pair.
c. Predators may become satiated.
3. Benefits -- increased foraging efficiency.
a. Information Center Hypothesis -- example is Cliff
swallows bringing food at higher rates too large than to
small colonies. If food is patchy and unpredictable in its
distribution, birds in a colony may be able to follow
knowledgeable birds to feeding locations.
b. Also, if food is patchy but superabundant and predictable,
colonies may become established near them. These pops
are probably not food limited -- example of Red Crossbill
colonies near abundant areas of pine seeds, austral
seabirds in path of Humboldt Current.
a. Predation. Colonies may attract predators that would
otherwise pass by highly dispersed nests -- Gulls often
are attracted to other seabirds' nesting colonies. b. Intra-specific competition.
i. Nest material stealing.
ii. Could exhaust local food supplies.
c. Could encourage build up of ectoparasites that would
otherwise be held low. Cliff Swallows in large colonies
had higher parasite loads than small colonies -- trade-off
with 3a. Interesting to know growth rates of those chicks.
F. Nest microclimates. [NOT COVERED – not on exam]
1. Eggs have very narrow temperature tolerances -- so nests are
“designed” help maintain constant temperatures.
2. Many nests thicknesses vary with local climate, as small
deviations in thickness can greatly affect insulatory
properties. A 5 mm difference n hummingbird nests can
increase required energy inputs to maintain egg temps by
3. Deep burrows are very well insulated, but are poorly
ventilated -- many burrow nesters' activities in nests is
dictated more by CO build up than by temperature 2
III. Incubation. [NOT COVERED – not on exam]
A. Brood Patches.
1. To help keep eggs warm (optimal temp of most is 37-38;C),
most birds develop a patch of featherless skin on their bellies.
In some, the feathers drop involuntarily (hormone controlled),
but in others (e.g., waterfowl) they pluck their own feathers,
using the feathers in turn to line the nest.
2. In sexes in which both sexes typically incubate, both sexes
develop the patch. In many others, only the females develop
a brood patch, which can be useful in sexing birds in the hand
during the breeding season. However, usually both sexes are
capable of developing a brood patch, in case one sex dies
before fledging, the other can take over.
1. Keeping eggs warm.
a. The safe range is 35-41. Below 35 and development in
greatly slowed, below 26 and the embryo rarely recovers.
b. Most birds regulate temperature by alternating sessions of
incubation and recesses. OVERHEADS
c. Experiments altering nest temperature have resulted in
predicted changes of session/recess behaviors.
d. They continually rotate and turn eggs to keep them all
approximately the same temperature.
e. Other strange adaptations do exist -- moundbuilders
regulate temperature of eggs resting on top of rotting
2. Keeping eggs cool.
a. Birds nesting open sunny environments (tropical seabirds)
risk dangerously high egg temperatures.
b. They often sit to minimize heat gain; their black backs
absorbing heat and the wind whisking it away.
c. Many also periodically cover the eggs lightly with water
to increase evaporative cooling, or they sprinkle with
sand to provide insulation from the heat.
C. Incubation Periods.
1. Incubation periods range from 10-100 days. Defined as date
of last egg laid to last hatch date.
2. Corresponds, quite roughly, to body size. Small song birds
have the shortest incubation periods. Much variation
3. Woodpeckers have very short incubation periods for their
body size. Their young are especially altricial -- perhaps an
adaptation to minimize time until chicks can thermoregulate
(chicks better than eggs at thermoregulation) to enable adults
to spend time off nest procuring food for them. Probably
only possible because woodpeckers nests are so safe from
predators by being in new cavities.
4. Species with precocial young typically have longer
5. To some extent, there is a trade-off between adult and young
survival in incubation periods. The longer the chicks remain
quiet in eggs, the more likely they will survive. But, the
longer an adults sits vulnerably on the nest, the more likely
she/he is to be nailed by a predator. So you might expect
species with long lives to maximize lifetime reproductive
success (LRS) by opting for proportionately shorter
incubation periods. Problem with testing this idea is that
longer-lived species also tend to be larger bodied, which
elevates incubation period. It is not clear what determines
inter and intra-specific differences in incubation periods.