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Brood Reduction • • • 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 • • We’ve already mentioned egg dumping, the facultative intraspecific form of brood parasitism Ostriches, doves, grebes, gulls, and many songbirds practice egg dumping • • • • • • • • • • • • • • 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 • • • • • • • • • • • • • • • • • 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! • 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 • • • Another strategy is to monopolize parental feedings by sheer size Brood parasites select smaller species This sets up a size dichotomy • • • • • • • • • • • • • • • • • 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 • • • • • • • • • • • • • • • • • • • • • 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! • • • • • • • • • • • • • • • • • 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 • • • • • • • • • • • • • 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 • • Survival in synchronous broods was better than in asynchronous broods, contrary to Lack's brood reduction hypothesis C-chicks invariably had lower survival rates • • • 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 • • • • • • • • • • 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