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Chick Rearing and Cooperative Breeding -- chick rearing depends a lot on egg size and incubation period -- larger eggs, longer incubation, chick more.

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Presentation on theme: "Chick Rearing and Cooperative Breeding -- chick rearing depends a lot on egg size and incubation period -- larger eggs, longer incubation, chick more."— Presentation transcript:


2 Chick Rearing and Cooperative Breeding -- chick rearing depends a lot on egg size and incubation period -- larger eggs, longer incubation, chick more developed at hatch -- in general, larger birds produce smaller clutches with large eggs; small birds the reverse

3 Some exceptions do occur: -- passerines lay proportionally larger eggs per body mass than other birds -- kiwi has largest egg per body mass at 25% -- takes female 33 days to produce egg, 75-80 to incubate -- chick hatches precocial, feeds on its own

4 precocial eggs will have 40-50% yolk by weight, 2-3X larger than altricial eggs of bird with same body mass

5 Factors that control egg size include: 1.Food availability -- a stable food source selects for larger eggs and precocial young -- food variable or scarce, small eggs and altricial young Royal Tern cormorant

6 2. Predation -- altricial young are helpless, take longer to raise -- constant feeding trips can draw attention of predators -- longer time at one spot, better chance of being found In tropics, lots of predators but only 15% of birds have precocial young while 35% are altricial -- high risk in losing nest to predation selects against large investment in large eggs -- better to have small eggs, easily replaced if lost to predation, than large eggs with large energetic loss -- in temperate regions, more precocial eggs as food is more seasonal, less predation, and cooler temperatures

7 3. Clutch size -- a bird can produce many small eggs but only 1-2 large -- cannot produce many large eggs -- in general, birds should be selected to produce the maximum number of eggs possible for prevailing conditions Food Limitation Hypothesis -- first proposed by David Lack (1911-1973) -- well known British ecologist, wrote many books including Ecological Isolation in Birds Population Studies in Birds, Darwins Finches

8 Tests of Lacks hypothesis had mixed results: -- some birds fit the model, others did not -- some species could raise more young, but dont -- e.g., gulls with eggs added to nest raise all young -- but…pay the price in future survival

9 There are natural ways for gulls and other birds to avoid expending too much energy for breeding

10 Third Chick Disadvantage

11 Brandts Cormorants can lay 5-6 eggs, but vary amount each year based on winds, upwelling -- use proximate factors to adjust timing of nest construction and clutch size -- will abandon eggs or young if food supply drops -- selects for limiting reproductive effort each year, high survivorship of parents, and high lifetime reproductive success

12 Siblicide also may occur as a means to reduce clutch size -- can be obligate or facultative -- behavior stimulated by hunger -- obligate in egrets, south polar skua

13 Preconditions for siblicide behavior to evolve: 1.clutches of two or more young food supplies, occasional or consistent 3.small amounts of food fed directly to chicks by adults, setting up fights between chicks 4. weapons are needed (hooked bill) 5. small nests and territories, no escape 6. hatching asynchrony so that first chick is larger

14 Chick Growth -- typical pattern is that growth is slow at first, then goes rapidly until close to adult weight -- slow start especially for altricial chicks, cant thermoregulate yet and need brooding by parent -- once physiology is developed, chick growth rate increases -- many birds fledge, though, at weights above or below average adult weight


16 Aerial insectivores such as swallows may fledge above adult weight -- more muscle mass in wings while learning how to forage -- tissues then regress to normal size and weight Seabirds may have more water in tissues to help them grow at first Most chicks fledge below adult weight and continue growing while feeding on their own -- ground-nesting birds may have more growth in legs than wings at first, helps them leave nest and forage

17 Some seabirds may have large feet in chicks to help them swim -- murrelets leave nest after 1-2 days to forage at sea -- feet unusually large for body size, then body catches up -- common murres take chicks to sea when half grown -- male takes chick after they jump from cliffs

18 Chick growth rates and thermoregulation can be facilitated when they form creches -- chicks may gather at one nest in a colony to huddle together while parents out foraging -- run back to own nest when parents return (cormorants) -- also get creches outside of nests in common areas -- can be for huddling and thermoregulation (penguins) or for greater protection from predators (terns)

19 Brood sizes of 2 or more chicks can be advantageous to both chicks and adults for growth and energy savings -- titmice in England required less food per chick with larger broods -- with three chicks, feed 1.6 to 1.9 g per day -- with 13, ate only 0.6 to 0.8 g per day Heat exchange among chicks in larger brood kept need for more food to a minimum

20 Cooperative Breeding in Birds -- when more than just parents participate in feeding and raising young -- helpers at the nest, usually young from previous breeding -- once thought rare in birds, now known in > 220 species

21 How does helping behavior evolve? -- appears altruisticwhy help when the offspring are not yours? -- obvious advantage to the breeding pair with increased care and protection of young -- advantage to helpers varies in each system

22 The causes for the behavior may include: 1.non-adaptive hormonal stimulation -- birds want to breed, but cant -- end up helping to act out behavior stimulated by hormones -- breeders tolerate the helper 2.Inclusive fitness -- helpers are related in part to young in nest (same parents) so share genes 3.Risk of dispersal -- helpers are often young of previous year -- delay leaving natal territory due to risks of of dispersal

23 4. Experience -- helpers learn how to care for and raise young -- this experience will help them become more successful breeders later, especially first time 5. Access to mates -- some helpers may form pair bond to one of the breeding pair if the pair dissolves or one mate dies -- this would explain helpers not related to breeding pair Several long-term studies now address some of these points for evolution of helping behavior

24 Acorn Woodpeckers -- studied by Walt Koenig at Hastings Reserve, CA for over 25 yrs -- birds form clans of 10-15 related birds -- 1-3 breeding females per year -- males compete for mating with females, 1-4 may mate -- all eggs laid in one nest cavity

25 All clan members help incubate eggs, feed young Young helpers stay in clan to avoid dispersal Breeding pair, when dies, may be replaced by an unrelated pair outside the clan Large clans with many helpers fledge more chicks than small clans Helping behavior especially important in bad food years

26 Pygmy Nuthatch -- have social groups with one breeding pair and 1-3 helpers, often threesomes -- helpers usually male offspring from previous year -- helpers help feed young, mob predators

27 -- use tree cavities for nesting and for winter roosts -- nests with helpers have less predation -- helpers have access to winter social groups, mates

28 Study in Arizona in 1980s showed helpers did not make much difference in breeding success -- only one out of four years of study showed a benefit to breeding pair with helpers vs. those without -- real advantage to helper with access to winter roost cavities, increases survivorship in winter -- record count for winter roost: 167 birds!

29 Florida Scrub-jay -- studied for >25 yrs in Florida by Glen Woolfenden -- up to 55% of nests have helpers, usually male offspring from previous breeding -- groups have large territories, up to 9 ha, for one monogamous pair plus 1-6 prebreeding helpers

30 Scrub/oak/palmetto habitat -- poor habitat, so large territory and helping needed to be successful and survive -- this habitat unlike richer oak woodlands in western U.S. where jays do not have helpers

31 Most Florida scrub-jays delay breeding until 2-3 yrs old -- helpers are 1-2 yr old prebreeders -- by age 3, most are breeding on own territories: 98% female and 77% male of cohort are breeding by this age -- most helpers are males, may be able to obtain territory if breeding male dies Helpers act as sentinels to watch for predators while other group members feed -- will give alarm calls to warn others

32 Do helpers gain from their experience? study showed that: 1-yr helper2+ yr helper Survivorship 0.86 0.72 RS 2.03 1.74 These results indicate no advantage for survivorship if helping two or more years No increase in RS for helping longer either Woolfenden concluded that long-term helpers are poor quality males, less fit than those that help one year and then get territory. Less fit helpers begin wandering after age 2-3: Wandering Inept Male Prebreeders

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