Ecology of Brood Parasites, Term Paper Example

Some birds have a unique system of rearing young.  They forgo the pleasure themselves, and instead deposit their eggs in the nest of an unsuspecting host bird, which then raises the young intruders as if they were her own.  This is called brood parasitism.  “About 1% of all bird species are brood parasites, including the honey guides (Indicatoridae), nearly half of the 130 species of cuckoos (Cuculidae), two genres of finches (Vidua and Anomalospiza, Ploceidae), five cowbirds (Icteridae), and a duck (Heteronetta atricapilla, Anatidae).” (Payne 1).  Brood parasitism is relatively rare in nature and birds by far parasitize more than any other type of vertebrate that shows prenatal care.  Despite the amount of known bird species that are brood parasites, it is still uncertain as to why some species become brood parasites.  There are many theories as to how specific host nests are picked and the evolutionary paths that led to the choice of hosts by brood parasites like the cuckoo and the brown headed cowbird.  However, researchers have been “unable to find any consistent environmental, ecological or behavioral correlates of this mating system.” (Bennett 151).  There has been much research and study done regarding the ecology of brood parasitic birds giving science a clearer picture as to the evolution of brood parasitism and the co-evolution between the parasitic bird and its host.  Future research will reveal unanswered questions regarding evolutionary adaptations that make the parasitic birds more successful and their hosts more resilient to brood parasitism.

There are two different types of brood parasites.  Either the mother lays her eggs into the nest depending on the host to rear the young by feeding them and teaching them to fly, or the egg is simply incubated by the host and when the hatchling emerges it is able to care for itself right away.  Most parasitic birds are altricial; their nestlings depend on the host for food.  The parasitic duck, however, obtains only protection and warmth from the host and feeds and cares for itself shortly after hatching.” (Payne 1) Bennett and Owens (151) attempted to understand the relationship between parasitic bird species in order to determine what the common factor was that turned bird species into brood parasites.  However, they were not able to find “any consistent environmental, ecological or behavioral correlates of this mating system.  It is also not correlated with the other common form of egg-dumping; intraspecific brood parasitism.” (Bennett 151)  They did, however, find a genetic link in that brood parasitism “always occurs in families that contain cooperative breeders, or in families that are the sister-lineage to a family that contains cooperative breeders.” (Bennett 151)

Brood parasitism evolved as a response to some factor that made breeding more difficult.  “The principal life history trade-off involved in brood parasitism is that by foregoing parental care, brood parasites greatly increase their annual fecundity.” (Smith 1)  It could have been that at one point in the past of its evolution, a bird species encountered a situation where food or breeding space was scarce.  By laying eggs in multiple host nests at during a breeding time frame, the mother ensures that at least some of her offspring will make it.  “Most workers are agreed that wild Brown-headed Cowbirds lay eggs on 70-80% of days during a two-to-four-month breeding season, for a total of at least 40 and perhaps as many as 100 eggs per year.” (Smith 2)  This ability greatly increases the species chances for survival when compared to a bird who lays only a single nest of eggs per year and who must ensure the survival of each of her nestlings in order to make the breeding successful.

Brood parasites coincide their breeding period with that of their favored host species.  Payne (80) studied the ecology of the Brown-headed Cowbird in California in 1973 and found that as the nesting season began, the cowbird would wait until late April and early May to lay their eggs as the hosts that nest earlier tend to be “poor fosters and have never been recorded to rear a young cowbird successfully.” (Payne 97)

One of the most mysterious aspects about brood parasitism is the parasites ability to recognize its foster parents as hosts and not confuse themselves as a member of the host species.  In most “mammal and bird species, individuals do not recognize conspecifics from birth, but rather learn species characteristics during early associations with their parents or siblings.  Such “imprinting” involves the formation of an internal representation of salient phenotypic attributes, i.e. a recognition “template.” Later on, juveniles recognize individuals whose phenotypes match those mental images closely enough as conspecifics.” (Hauber 1041).  Brood parasite young have a unique ability, then, to emulate and copy the host; by mimicking sounds, yet still develop a unique understanding of their role as a different species.

The mechanics of this have perplexed science.  There have been several possible explanations put forth as to the solution to this apparent paradox.  According to Hauber (1041), one theory is that the recognition template is inherited by the young genetically.   Another theory is that the behavior is passed on due to a combination of genetic predisposition and environmental cues.  Hauber proposed a new hypothesis of how the recognition template is passed down among generations of brood parasites.  “The idea is that the recognition process is initiated by exposure to some unique species-specific attribute, such as a vocalization, behavior or other physical characteristic.  This cue will have to identify its bearer as a conspecific unambiguously – essentially it will be a “password”. Similar to movement in classical imprinting, this password will trigger learning by the naïve individual of other aspects of the phenotype of the password giver, thereby enabling it to form a recognition template that incorporates multiple cues.”

Hauber tested his hypothesis by studying a particular vocalization of the brown-headed cowbird. The cowbird was chosen because it had been recognized that juvenile cowbirds “begin associating with conspecifics soon after leaving their foster parents.”  It was concluded that there was a vocalization that the cowbirds made, a social “chattering” that did indeed appear to trigger responses in young cowbirds.

Another mystery that surrounds the brood parasite is its ability to find and recognize the host species nests.  “Although a number of hypotheses have been proposed for how the European cuckoo, Cuculu canorus females may find hosts belonging to their foster parents’ species, clear evidence is lacking for any of them.” (Teuschl 1425)  Teuschl, Taborsky and Taborsky put forth the hypothesis that habitat-imprinting may play an important role the ability of cuckoos to recognize their host species.  Cuckoos, they hypothesize, are able to remember the environment, including nest specifics, from the time that they spent in a foster nest.  Cuckoos are generalists and parasite a wide variety of nests.  “Cuckoo eggs have been found in nests of more than 100 different species.” (Teuschl 1425)  However, each female cuckoo will parasite only one specific host species and lay eggs in only those nests.  This behavior suggests that the female remembers the specific host species that raised her.  The researchers proposed that the young cuckoos imprint their birth environment.  The cuckoos would then “establish home ranges in areas consisting mainly of a habitat that resembles the habitat they had experienced as nestlings.  When they parasitize the hosts available in this habitat, there in an enhanced chance of encountering nests of the host species by which they were reared.” (Teuschl 1426)  The researchers found that cuckoos did show a significant amount of habitat preference for the habitat that most closely resembled the one in which they had been raised. “Early experience in a particular environment determines subsequent habitat preferences in several bird species and can even reduce or change genetic preferences for specific habitats.” (Teuschl 1430)

Genetic deposition has been thought to exist between the brood parasite and their host.  One example that has been found is in the case of the golden eye duck (Bucephala clangula).  Some members of the species lay their eggs in the nests of members of their own species. The adaptive behavior enhances the ability of the mother to lay eggs that will be successful.  In a study (Andersoon and Ahlund 2000) genetic link between parasite mothers and host mother golden eye was established using protein fingerprint.  Parasite mothers will choose hosts that are related to her often as close as close as first cousins.  “Relatedness to the host is higher in nests where a parasite lays several eggs than in those where she lays only one. Returning young females parasitize their birth nest mates (social mothers or sisters, who are usually also their genetic mothers and sisters) more often than expected by chance. Such adult relatives are also observed together in the field more often than expected and for longer periods than other females. Relatedness and kin discrimination, which can be achieved by recognition of birth nest mates, therefore play a role in these tactics and probably influence their success.” (Andersson and Ahlund 13188).

Species of birds that are brood parasites have evolved several methods for increasing the survival of their eggs in the host nests.  Some species of birds use selective breeding to develop egg mimicry. With egg mimicry, the parasitic bird will try to increase the chances of its eggs being accepted by the host by trying to match their eggs phenotype to that of the host.  The more closely the parasites eggs resemble the eggs of the host, the less likely the chance that the host will expel the foreign eggs from the nest. One example of a brood parasite that uses egg mimicry is the cuckoo.   The phenotype of the egg is believed to pass through the female cuckoos. “Cuckoo eggs have been found in nests of more than 100 different species. Eleven main host species and a similar number of secondary ones have been identified.  Egg colors and patterns differ greatly between these host species, and the cuckoo eggs vary accordingly.” (Teuschl, Taborsky, and Taborsky 1425).  They found during that egg mimicry would be more successful for the cuckoos when habitat imprinting was used by the cuckoos in order to identify their hosts.  If the cuckoos continued to return to use the foster parent species that had raised them, their chances for survival would be greater.  If they cuckoo was hatched and cared for so that it made it to adulthood, it is likely that the egg it hatched from closely resembled the eggs of the foster parent.  Thus, habitat imprinting may also play an important role in the genetic predisposition for the eggs to be successful due to mimicry in the nests of the host species.

Further DNA evidence was found by Gibbs et al linking egg mimicry to the female cuckoos in association with habitat imprinting. “Females of each race lay a distinctive egg type that tends to match the host’s eggs.  The puzzle is how the gentes remain distinct.” (Gibbs et al 183)  They discovered that “gentes are restricted to female lineages, with cross mating by males maintaining the common cuckoo genetically as one species.”  This evidence supports the hypothesis of environmental imprinting by young cuckoos.  In addition, the evidence presented by Gibbs et al suggests that the cuckoos have evolved using more than one host.  “Our results also support the idea that common cuckoos often switched hosts during evolution, and that some gentes may have multiple, independent origins, due to colonization by separate lineages.” (Gibbs et al 183)

Some brood parasites will remove some of the eggs from the nest of their hosts in order to increase the chances of survival for their eggs.  In the brown-headed cowbird, for example, it had been observed that a female will remove at least one egg from the host nest.  It has been shown by the experiments of Peer and Bollinger that “by removing an egg of a large host, the female cowbird increased the likelihood that her egg will hatch and decreases the length of time required for the egg to hatch.  Host egg removal also helps to ensure that the parasitic egg will hatch first and increases the disparity between the hatchling of the parasite and the host’s nestlings.” (Smith 189)

Vandalization of nests is also common with species that practice brood parasitism.  Some brood parasites will puncture the eggs of host nests in order to increase their own eggs chances for survival.  This occurs in the shiny cowbird of Puerto Rico.  A study by Nakamura and Cruz (Smith 178) was undertaken to study the observation that there had been an increase in the occurrence of egg puncture by the cowbird on host species. “The frequency of egg puncture increased from 2.8% to 29% in 1982-1987.  Egg puncture was limited to the nests of parasitized species and occurred in seven of the eight species parasitized.  Host and cowbird eggs were punctured at similar frequencies.”  The cowbird uses the sabotage method for two reasons.  One is to eliminate competition from other cowbirds.  When an egg is damaged, the host will normally kick the damaged egg out.  Sometimes the host will abandon the nest all together.  If this happens, the host will be forced to lay a new clutch, enabling the cowbird an opportunity to lay an egg with the new clutch.  There are other reasons hypothesized for the behavior. “Since host eggs are punctured more frequently than cowbird eggs, this behavior may reduce competition for the cowbird young in the host nest.”  (Smith 178) Whether or not egg punching is a positive adaptive behavior remains to be seen.  It appears to be both destructive to the host and the parasite, which may make the behavior a more negative reaction to an overpopulation of cowbirds in the area.  When competition for appropriate nests to parasitize becomes high and stressed, the cowbird may resort to egg punching in an attempt to find a suitable host for their own eggs.

As the brood parasite adapts its behavior and genetics to make it a more successful parasite, so too the hosts adapt to the brood parasite, in an attempt to make them more successful in avoiding brood parasitism.  This can be shown using the example of the relationship between the cuckoos and one of their favorite hosts, the reed warblers. This study (Davies and Brooke 262) showed that the reed warbler will reject the cuckoo eggs about 19% of the time even though the cuckoo does display egg mimicry to the warbler eggs.  “Parasitized clutches suffered less predation than un parasitized clutches, suggesting that the cuckoo itself was the major predator, plundering nests too advanced for parasitism so that the hosts would re-lay.” (Davies and Brooke 262)  Their study did reveal that egg mimicry that was poor would result in the eggs being more likely to be rejected.  However, once the host has decided the eggs are good, she will normally raise the young no matter what they look like once they are hatched.  The success of brood parasitism seems to rest on the eggs being accepted on the host, not once the young are hatched.

Research has revealed that host species that have evolved egg phenotypes in order to more efficiently defend their nests against brood parasitism.  The African village weaverbird (Ploceus cucullatus) has eggs that are highly consistent within clutches but highly variable between individuals.  The African weavers are parasitized both by each other and by the diekerik cuckoo.  By laying clutches of eggs that are highly consistent, the weaver is better able to spot the intruder egg.  As well, it is harder for a cuckoo or other parasite to evolve eggs to mimic a specific race of host within the species.  Lahti (18057) also found further evidence that the consistent egg clutches were an adaptation to brood parasitism.  The weaver populations were introduced over a century ago to the islands of Mauritius and Hispaniola.  These islands are devoid of egg-mimicking brood parasites. “In these two populations, between-individual variation and within-clutch consistency in egg appearance have both decreased, as has the incidence of spotting, relative to the source populations in Africa.” (Lahti 18057)

However, despite all that is known and observed about the relation between the brood, parasites and the hosts, data shows that this relationship can change quickly due to evolutionary pressures or environmental changes.  In the example of the cuckoo and the reed warbler, Brooke and Davies (873) showed that when historical data was analyzed, there was a rapid change in the amount of nests that were being parasitized by the cuckoo.  Over the past 40 years, the cuckoo suddenly stopped parasitizing the nest of species that it had historically.  The cuckoo’s use of the reed warbler, at the same time, more than doubled.  This change in host use did not necessarily benefit the cuckoos, since the number of breeding females dropped significantly over the time period.  Also, the egg mimicry of the cuckoo did not change significantly over the time period, which suggests that this tactic takes time in order to develop successfully.

Host species are constantly evolving new techniques to escape parasitism from brood parasites because of the fact that brood parasitism usually reduces hosts’ reproductive success.  This means that in order for the host to maintain its own reproductive success, it must develop new strategies to deter efforts by brood parasites to invade their clutches.  A study published in 2009 (Sato, Tokue, Noske, Mikami, and Ueda 67) “revealed a previously unknown anti-parasitism behavior in the large-billed Gerygone, which is a host species of the little bronze-cuckoo, a host evicting brood parasite.  The host forcibly pulled resisting nestlings out of their nests and dumped them.”  This is a unique feature since other hosts have never been observed to dump out nestlings, only eggs.  The Gerygone may be displaying a new evolutionary technique for dealing with brood parasites that may affect its being used as a host for the cuckoo in the future.

One of the things that make brood parasites so successful is their ability to adapt to a wide range of environmental pressures.  Species that are brood parasites tend to be flexible in their mating timing as well as their use of space. The brown-headed cowbird in particular has been studied using radio tracking to understand its use of territory and space in relationship to its hosts.  “The Brown-headed Cowbirds in the Sierra Nevada’s of California breed and feed in almost totally disjunct areas that reflect optima for finding host nests and food, respectively.” (Rothstein and Verner 1984).  The cowbirds would spend the mornings in the area in which their hosts lived, in order to gain access to nests in which to lay eggs.  Cowbirds, then, typically lay eggs in the mornings and feed in the afternoon.

There are many unanswered regarding the brood parasites and how they choose and maintain their host relationships for optimal breeding success.  There is much variation in the likelihood that a species will be used as a host, or “parasitized.” Some species are used as hosts by several species or interspecific brood parasite across their breeding range, whereas other species breed sympatrically with brood parasites but have never been recorded as hosts.  Some of this variation has already been explained with respect to variation between hosts in the suitability of their nest site for parasitism.  It has also been revealed that some species that are not currently parasitized probably were once parasitized at a high rate but have developed anti-parasitism techniques.” (Bennett 152).

In order to better understand the evolutionary pathways that have resulted in brood parasitism much more research will need to be undertaken.  In this particular mating nesting strategy, there appears to be much sudden adaptation and evolution regarding the relationship between the parasites and their host.  The recent study undertaken by Sato et al. in 2009 revealed a totally new strategy for anti-parasitism that had never before been witnessed by hosts of the cuckoo.  This shows that adaptation is still occurring and the brood parasites must continue to adapt as well in order that their particular breeding strategy remains effective.  Studies that determine how hosts nest use by the parasites fluxuates over time will reveal the adaptations that brood parasites are making in their choice of hosts.  Egg mimicry, an important part of the success of a brood parasite egg being accepted by the host, is also little understood and needs further research.  Even though egg mimicry seems to be such an important part of the success of the brood parasites egg being accepted it seems that most of the brood parasites do not have eggs that closely resemble the host’s eggs.  It could be that brood parasites constantly have to adapt to new hosts, at least in evolutionary terms, and that the genetic phenotype of egg mimicry takes longer to perfect, so it does not “catch up” with the behavior.  Perhaps the parasite-host relationship doesn’t even last as long as it takes for the phenotype of egg mimicry to be perfected.

Brood parasitism is an interesting breeding strategy that appears to work well for birds in general giving the high percentage of bird species that practice it. Understanding the ecology of these birds is important for two reasons.  One is purely scientific and is to add to the collective knowledge of our environment.  The other is for conservation purposes.  Bird parasites, like the brown-headed cowbird, stress host populations and limit their reproductive success.  In the current age when most species are being stressed by environmental degradation, some of the host species are becoming stressed on two levels, one by the cowbirds, and on the other side by habitat destruction caused by human activity.  In order to preserve the host species as well as the brood parasites, the relationship between the two and the breeding strategy must be understood fully and completely.  Nature exists in a delicate balance that is always in a state of flux.  Understanding how this works is the key to preserving the natural world.

References

Andersson, M., Ahlund, M.  “Host –Parasite Relatedness Shown By Protein Fingerprinting in a Brood Parasitic Bird.” Proceedings of the National Academy of Sciences of the United States of America. 97.24 (2000): 13188-13193.

Bennett, Peter M., Owens, Ian P.F. Evolutionary Ecology of Birds: Life Histories, Mating Systems and Extinction New York: Oxford University Press, 2002.

Brooke, M. de L., Davies, N.B. “Recent Changes in Host Usage by Cuckoos Cuculus canorus in Britian.” Journal of Animal Ecology 56 (1987): 873-883.

Davies, N.B., Brook, M. de L “Cuckos Versus Reed Warblers: Adaptations and Counteradaptations”. Animal Behavoir 36.1 (1988): 262-284.

Gibbs, H. Lisle, Sorenson, Michael D., Marchetti, Karen, Brooke, M. de L., Davies, N.B., Nakamura, Hiroshi. “Genetic Evidence for Female Host-Specific Races of the Common Cuckoo.” Nature 107 (2000): 183-186.

Hamilton, William J., Orians, Gordon H. “Evolution of Brood Parasitism in Altricial Birds.” The Condor 67.5 (1965): 361-382.

Hauber, M.E., Russo, S.A., Sherman, P.W.  “A Password for Species Recognition in a Brood-Parasitic Bird” The Proceedings of the Royal Society. 268 (2001): 1041-1048.

Kleven, Oddmund, Moksnes, A., Roskaft, Eivin, Honza, Marcel.  “Host Species Affects the Growth Rate of Cuckoo (Cuculus canorus) Chicks.” Behavioral Ecology and Soiciobiology. 47.1-2 (1999): 41-46.

Lahti, David C. “Evolution of Bird Eggs in the Absence of Cuckoo Parasitism.” Proceedings of the National Academy of Sciences of the United States of America. 102.50 (2005): 18057-18062.

Payne, RB.  “The Breeding Seaon of a Parasitic Bird, the Brown-Headed Cowbird, In CentralCalifornia.”  The Condor 75.1 (1973): 80-99.

Payne, R.B. “The Ecology of Brood Parasitism in Birds.” Annual Review of Ecology and Systematics”. 8 (1977): 1-28.

Rothstein, Stephen I., Verner, Jared, Steven, Ernest.  “Radio-Tracking Confirms a Unique Diurnal Pattern of Spatial Occurrence in the Parasitic Brown-Headed Cowbird.” Ecology 65.1 (1984): 77-78.

Sato, Nozomu, J., Tokue, Kihoko, Noske, Richard A., Mikami, Osamu K., Ueda, Keisuke. “Evicting Cuckoo Nestlings from the Nest: A New Anti-Parasitism Behaviour.” Biology Letters 6.1 (2009), 67-69.

Smith, James N.M. Ecology and Management of Cowbirds and their Hosts Austin: University of Texas Press, 2000.

Teuschl, Y., Taborsky, B., Taborsky, M. “How Do Cuckoos Find Their Hosts?  The Role of Habitat Imprinting.” Animal Behavoir  56(1998), 1425-1433.