Download Brood Reduction 1

Brood Reduction
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One of the most unusual problems faced by avian parents is having to raise chicks of other
species
Many species of birds are brood parasites
Rather than build their own nests, they simply lay their eggs in the nests of others
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We’ve already mentioned egg dumping, the facultative intraspecific form of brood parasitism
Ostriches, doves, grebes, gulls, and many songbirds practice egg dumping
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If the egg is dumped before the host begins to lay, she may simply toss it out before she lays
her own
But if the egg is added after her own clutch has started, the host cannot tell the difference and
raises the additional egg as if it were her own
Egg dumpers may also remove a host egg and replace it with one of her own
Obligate brood parasites are better known
Interspecific form of brood parasitism, has evolved independently at least seven times
The European Cuckoo (Cuculidae), honey-guides (Indicitoridae), and the cowbirds (Icteridae)
are examples
North American cuckoos (Yellow-billed and Black-billed) usually parasitize each other (!)
Cowbirds are the most important brood parasites
> South America = Shiny Cowbird
> North America = Bronzed Cowbird in the West and the Brown-headed Cowbird in the East
Brown-headed Cowbirds are very successful
Of the 220 species they have tried to parasitize, they have successfully parasitized at least 144
host species
A cowbird female typically lays 2-5 eggs per week, up to 30-40 eggs per season
Interval between these clutches is short!
Brown-headed Cowbird is the only passerine whose ovaries and oviducts remain fully
engorged and functional after the clutch is laid
Only 3% of their eggs mature into adults, but their populations continue to grow, and they are
steadily increasing their range
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Over their two years of egg laying, they can lay up to 80 eggs
With a 3% survival rate they can each leave 2.4 adults behind
Replaces pairs at the rate of 1.2 pairs per year, enough to double the population size every 8
years
Many strategies for persuading the parents of other species to raise your young
European Cuckoo is known to parasitize at least 125 species of passerines
The sight of its host actually stimulates it to begin laying
Lay their eggs quickly, as little as 15 sec.
Eggs are also thicker than those of the host, and more likely to crack them open when dropped
on top of them
Different cuckoo females specialize in parasitizing different host species
Their eggs mimic those of their host, and this similarity is obviously genetic
But the females appear to mate with random males, so how can such egg mimicry be
maintained?
In southern England cuckoos commonly parasitize several species of warblers, pipits and
wagtails
The host eggs are very different from one another, so the cuckoo’s egg has a very generalized
pattern that doesn’t closely correspond to any particular species
The young of the cuckoo have an interesting innate reflex
Any touch to the back of the baby bird triggers a pushing reflex with the back, which acts to
force eggs or nest mates up and over the edge of the nest!
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Other species kill the host’s young directly, rather than evict them from the nest
Baby honeyguides have specialized hooks on their beaks which they use to maim and kill
their nest mates
The hooks drop off when the young are about two weeks old
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Another strategy is to monopolize parental feedings by sheer size
Brood parasites select smaller species
This sets up a size dichotomy
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Larger young of the brood parasite grow more rapidly, can easily reach higher and beg louder
than their smaller nest mates
They dominate feeding visits and starve their foster siblings to death
So brood parasites can
> Evict eggs/nestlings
> Kill nestlings directly
> Monopolize parental feedings
Many host species have evolved countermeasures against brood parasites
They may simply desert any nest containing a strange egg
Or they may evict any strange object that appears in the nest
Perhaps the most unusual host/parasite relationship is that between the Giant Cowbird and its
hosts, the Chestnut-headed Oropendola, Wagler’s Oropendola, and the Yellow-rumped
Cacique (Smith 1968)
Nestlings of these species are often attacked by botfly maggots, which can kill 90% of the
nestlings
Giant Cowbird babies pluck these maggots from the hosts’ bodies and eat them
Only 8% of the nestlings die in fly-infested nests when cowbirds are present
So they are tolerated when fly infestations are a problem
But colonies placed near bee or wasp nests, (insects that attack botflies) do not need cowbirds
to help them out, and throw cowbird eggs out of the nest
So the Giant Cowbird has evolved two egg types:
> Mimetic egg, which it places in nests of birds immune from botflies
> Plain egg, in nests subject to botfly attacks!
This strange coevolution is a rare benefit from parasitism
Most songbird populations are decimated by cowbird parasitism
Damage estimates:
> Red-Eyed Vireos 40-70% of nests
> Eastern Phoebe 20%
> Song Sparrow 40%
> Kirtland’s Warbler 55-75%
Host devotes all of its energy to raising the larger and more aggressive parasitic chick, who
soon becomes the sole survivor of the parasitized brood
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How did this strategy evolve?
Maybe from the loss of synchronization of nest building and incubation
Loss of the nest is a common occurrence for birds
Females may have ended up with no nest for their eggs
Opportunistically began egg dumping in the completed nests of other birds of their own
species
Intraspecific parasitism --> interspecific
Or perhaps some females wanted an insurance egg in another nest in case their own nest was
predated…
The rewards are fairly high for this behavior
Not only is the parasite relieved of the burden of raising her own young, she can use the
additional energy to lay even more eggs
Birds are prone to brood parasitism for a number of evolutionary reasons
There has been little selective pressure on birds being able to discriminate their own eggs
from those of others
This sounds a little surprising - eems rather fundamental to be able to recognize your own
offspring
But the eggs of birds rarely leave the nest on their own, so the rule of thumb has always been in my nest = mine
Probably not been much pressure on birds to recognize their own nestlings for the same
reason
The hard-wired response prevails - if it’s in my nest, then feed it!
No selective pressure on discriminating between species begging calls
Maybe weakening this hard-wired response is far more damaging to the continuation of the
species than occasional attacks by parasites
Northern Cardinal shows how deeply ingrained these reflexes can be
Probably lost its brood, observed feeding a pond full of goldfish for several days
The goldfish were used to being hand fed, responded to the Cardinal by sticking their heads
up with their mouths open
She obliged by feeding them worms!
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In many species of birds, a significant portion of mortality is directly due to the action of close
kin
Infanticide = any behavior by parents that contribute directly to the death of a chick
Siblicide = any behavior by siblings or half-sibs that contribute directly to the death of a
sibling or half-sib
Parents rarely intervene
Mock et al . (1990) - characteristics of siblicidal bird species:
> Competition for food
> Provision of food in small units
> Possession of suitable weaponry
> Competitive disparities among nestlings
> Spatial confinement in the nest
Many large predatory birds commonly practice siblicide (raptors, pelicans, herons, etc.)
Their formidable weapons are very effective in the close confines of the nest
Competitive disparities between nestlings result from asynchronous hatching, common
characteristic of siblicidal species
Hatching asynchrony results when eggs are incubated as soon as they are laid
Allows some nestlings to gain a significant weight and height advantage
David Lack - one or more density-dependent mortality factors must operate to link constant
clutch size with variable adult mortality
Lack thought that the quality and quantity of food brought to the nest was the critical factor in
maintaining nestlings
Asynchronous hatching in raptors led to starvation of the youngest and weakest chick if food
supplies were reduced or uncertain
If Lack's hypothesis that brood reduction depends on food supply is true, asynchronous broods
should give older chicks an advantage
> Larger, should have higher success rate begging food from parents
> Larger, stronger, should be better equipped to survive periods of food shortage
Braun and Hunt (1983) - Black-legged Kittiwakes (Rissa tridactyla) older siblings frequently
ejected the younger chicks, usually following severe aggressive attacks
Initial size differences between first and second chicks were rapidly accentuated by the first
chick dominating parental feeding
Brood reduction was highest during periods of food shortage, supporting Lack's hypothesis
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Hahn 1981 manipulated nests of the Laughing Gull (Larus atricilla)
Fledging success was measured in 48 normal (asynchronous) broods, and in 13 artificially
synchronized broods
Results supported Lack’s brood reduction hypothesis
> Asynchronous broods had a higher fledging rate than synchronous broods
> Sibling size differences, based on hatch order, corresponded directly with feeding success
Several alternate hypotheses to Lack's brood-reduction hypothesis:
> predation hypothesis
> insurance hypothesis
> peak-load reduction hypothesis
The predation hypothesis seeks to explain asynchrony as an adaptation where nest predation is
heavy
By speeding up the fledging process for at least some young, the impact of predation might be
minimized
The insurance hypothesis pertains to raptors and other species with small clutches and long
nesting periods
The second or later offspring are an insurance policy against the possible loss of the first-born
This differs from Lack's hypothesis
Later offspring are an insurance against accidental sibling loss, rather than against uncertain
food supplies
The peak-load reduction hypothesis states that parents spread out offspring so that their
individual peak demands for food will not coincide as they grow
Support for the predation hypothesis comes from study on the Common Tern (Sterna hirundo)
Bollinger (et al 1990) manipulated normally asynchronous broods to produce synchronous
broods
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Survival in synchronous broods was better than in asynchronous broods, contrary to Lack's
brood reduction hypothesis
C-chicks invariably had lower survival rates
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Common Terns are subject to very high levels of egg predation
Early incubation may be more effective than nest guarding in preventing egg predation
Early incubation also sets up hatching asynchrony and siblicide
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Dorward (1962) proposed an insurance egg hypothesis to explain obligate siblicide in the
Masked Booby (Sula dactylatra)
The reduction in parental fitness caused by investment in the doomed chicks is less than the
probable gain if the first-hatched chick dies before siblicide occurs
A recent review of the Sulidae (gannets and boobies) found that two booby species with
relatively low hatching success (51% to 61%) produced an insurance egg
Four booby species with high hatching success (at least 85%) produced single-egg broods
(Anderson 1990)
Anderson (1990) also found from a field study of the Masked Booby that 19.2% of second
chicks survived when the first chick died
The insurance egg hypothesis has also been supported for white pelicans (Pelecanus
erythrorhynchos)
Second white pelican chick usually dies from siblicidal attacks and food deprivation
In 20% of pelican nests observed by Cash and Evans (1986), the second chick survived while
the first died
Perhaps the second egg initially acts as an insurance policy
If both chicks survive, the brood can later be adjusted to fit a reduced food supply