Animal Diversity Web

Ge­o­graphic Range

African grey par­rots (Psit­ta­cus eritha­cus) span the for­est belt of cen­tral and West Africa in­clud­ing the oceanic is­land of Príncipe (Gulf of Guinea). In West­ern Africa, they are found in coastal coun­tries such as Sierra Leone, Ghana, and the Ivory Coast. The two known sub­species of African Grey Par­rots have vary­ing ranges. Psit­ta­cus eritha­cus erithi­cus in­hab­its a range ex­tend­ing from Kenya to the east­ern bor­der of the Ivory Coast and in­clud­ing the in­su­lar pop­u­la­tions. Psit­ta­cus eritha­cus tim­neh has a range from the east­ern bor­der of Ivory Coast to Guinea-Bis­sau. (Melo and O'Ryan, 2007)

• Biogeographic Regions
• ethiopian
  • native

Habi­tat

The habi­tat of African grey par­rots is usu­ally moist low­land forests, al­though they are found up to 2,200 m al­ti­tude in the east­ern parts of the range. They are com­monly ob­served at for­est edges, clear­ings, gallery forests, man­groves, wooded sa­van­nahs, cul­ti­vated areas, and gar­dens. African grey par­rots often visit open land ad­ja­cent to wood­lands, they roost in trees over water and may pre­fer roost­ing on is­lands in rivers. These par­rots make their nests in tree holes, some­times choos­ing lo­ca­tions aban­doned by birds like wood­peck­ers. In West Africa, the species makes sea­sonal move­ments out of the dri­est parts of the range in the dry sea­son. ("Psit­ta­cus eritha­cus, Lin­naeeus, 1758", 2008; Athan and Deter, 2000; Melo and O'Ryan, 2007)

• Habitat Regions
• tropical
• terrestrial

• Terrestrial Biomes
• savanna or grassland
• forest
• scrub forest

• Other Habitat Features
• suburban
• agricultural
• riparian

Phys­i­cal De­scrip­tion

The plumage of Psit­ta­cus eritha­cus is var­i­ous shades of grey with very dis­tinc­tive red tail feath­ers. African grey par­rots typ­i­cally mea­sure 33 cm from head to tail and weigh up to 407g. They have an av­er­age wingspan of 46-52 cm. (Athan and Deter, 2000; Melo and O'Ryan, 2007)

Psit­ta­cus e. eritha­cus> is re­ferred to as the nom­i­nate race and is light grey. In­di­vid­u­als of this sub­species have dis­tinct red tails and solid black beaks. These birds have bare white face patches and some­times bright, usu­ally pale, sil­very yel­low eyes. Many of the grey con­tour feath­ers are edged with white. This gives them a smooth, lacy ap­pear­ance. They may be some­what sex­u­ally di­mor­phic. (Athan and Deter, 2000; Melo and O'Ryan, 2007)

Psit­ta­cus e. tim­neh in­di­vid­u­als are smaller and darker with a ma­roon, brown­ish wash over the red tail. They have black-tipped, dark pink­ish max­illa and solid black mandibles. Their iris has more of a sil­ver ap­pear­ance rather than yel­low (Athan and Deter, 2000; Melo and O'Ryan, 2007)

• Other Physical Features
• endothermic
• homoiothermic
• bilateral symmetry

• Sexual Dimorphism
• sexes colored or patterned differently

• Average mass

  407 g

  14.34 oz

• Average length

  33 cm

  12.99 in

• Range wingspan

  18 to 20 cm

  7.09 to 7.87 in

Re­pro­duc­tion

African grey par­rots are very so­cial birds. Breed­ing oc­curs in loose colonies with each pair oc­cu­py­ing its own tree. In­di­vid­u­als se­lect mates care­fully and have a life­long monog­a­mous bond that be­gins at sex­u­ally ma­tu­rity, at three to five years of age. Few de­tails are known about courtship in the wild, but dis­play flights around nest holes have been ob­served and recorded. Males feed mates (courtship feed­ing) and both sing soft mo­not­o­nous notes. At this time the fe­male will sleep in the nest cav­ity while the male guards it. In cap­tiv­ity, males feed fe­males after cop­u­la­tion events and both sexes par­tic­i­pate in a mat­ing dance in which they droop their wings. ("African Grey Par­rots", 2006; "Psit­ta­cus eritha­cus (African Grey Par­rot, Congo African Grey Par­rot, Grey Par­rot)", 2008; Athan, 1999; Pep­per­berg, 2001)

• Mating System
• monogamous

The breed­ing sea­son varies by lo­cal­ity, but ap­pears to co­in­cide with the dry sea­son. African grey par­rots breed once to twice a year. Fe­males lay three to five roundish eggs, one each at in­ter­vals of two to five days. Fe­males in­cu­bate the eggs while being fed en­tirely by the male. In­cu­ba­tion takes ap­prox­i­mately thirty days and the young emerge from the nest at twelve weeks old. ("African Grey Par­rots", 2006; "Psit­ta­cus eritha­cus (African Grey Par­rot, Congo African Grey Par­rot, Grey Par­rot)", 2008; Athan, 1999; Pep­per­berg, 2001)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual
• oviparous

• Breeding interval

  African Grey Parrots breed once to twice a year.

• Breeding season

  Reproduction appears to coincide with the dry season.

• Range eggs per season

  3 to 5

• Average time to hatching

  30 days

• Average fledging age

  12 weeks

• Range time to independence

  2 to 3 years

• Average time to independence

  3 years

• Range age at sexual or reproductive maturity (female)

  3 to 5 years

• Range age at sexual or reproductive maturity (male)

  3 to 5 years

After the young emerge from the nest, both par­ents feed, raise, and pro­tect them. Both par­ents care for their clutch of young until they reach in­de­pen­dence. ("African Grey Par­rots", 2006; "Psit­ta­cus eritha­cus (African Grey Par­rot, Congo African Grey Par­rot, Grey Par­rot)", 2008; Athan, 1999; Pep­per­berg, 2001)

• Parental Investment
• altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-independence
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• extended period of juvenile learning

Lifes­pan/Longevity

In cap­tive and wild par­rots the av­er­age lifes­pan is be­tween 40 and 50 years. In cap­tiv­ity, African grey par­rots have a mean lifes­pan of 45 years, but they can live up to 60 years. In the wild, the av­er­age lifes­pan is 22.7 years (n=120). ("African Grey Par­rots", 2006; "Psit­ta­cus eritha­cus, Lin­naeeus, 1758", 2008; Ryan, 2002)

• Range lifespan
  Status: captivity

  40 to 60 years

• Average lifespan
  Status: captivity

  45 years

• Average lifespan
  Status: wild

  22.7 years

• Typical lifespan
  Status: captivity

  40 to 50 years

Be­hav­ior

Wild African grey par­rots are very shy and rarely allow hu­mans to ap­proach them. They are highly so­cial and nest in large groups, al­though fam­ily groups oc­cupy their own nest­ing tree. They are often ob­served roost­ing in large, noisy flocks call­ing loudly dur­ing morn­ings and evenings and in flight. These flocks are com­posed of only African grey par­rots, un­like other par­rots that are often found in mixed flocks. Dur­ing the day, they break into smaller flocks and fly long dis­tances to for­age. They often roost in trees over water and are said to pre­fer roost­ing on is­lands in rivers. Young birds stay with their fam­ily groups for a long pe­riod of time, up to sev­eral years. They so­cial­ize with oth­ers of their age in nurs­ery trees, but re­main in their fam­ily group within the larger flock. Young African grey par­rots are cared for by older birds until they are ed­u­cated enough and old enough to be­come in­de­pen­dent flock mem­bers. Young ex­hibit ap­pease­ment be­hav­iors to­wards older mem­bers. As they ma­ture, birds be­come more ag­gres­sive with con­specifics. African grey par­rots in the wild must learn a com­plex set of skills. They need to learn how to sep­a­rate de­sir­able food plants from toxic plants, how to de­fend ter­ri­tory, how to rec­og­nize and avoid preda­tors, how to find safe water, and how to re­join their fam­i­lies when sep­a­rated. Also, they must learn how to de­velop role-ap­pro­pri­ate be­hav­iors such as com­pet­ing and de­fend­ing nest sites and rais­ing off­spring. Com­pe­ti­tion for nest holes dur­ing mat­ing sea­son makes the species ex­tremely ag­gres­sive. Be­cause African grey par­rots are par­tial ground feed­ers, there is a se­ries of be­hav­ioral events that occur be­fore land­ing and safe con­sump­tion takes place. Groups of par­rots gather at a bar­ren tree until it is com­pletely filled with hun­dreds of birds that par­take in preen­ing, climb­ing, vo­cal­iz­ing, and so­cial­iz­ing. Even­tu­ally the birds make their way down to the ground in waves with the en­tire group never being on the ground at the same time. Once on the ground, they are ex­tremely vig­i­lant, re­act­ing to any move­ment and/or sound. ("African Grey Par­rots", 2006; Athan and Deter, 2000; Athan, 1999; Galef, 2004; Luescher, 2006; Wright, 2002)

Stud­ies have found that African grey par­rots have com­plex cog­ni­tion and are con­sid­ered one of the most in­tel­li­gent species of an­i­mal. They test and form ideas about the world. In a study pre­formed by Irene Pep­per­berg (2007), African grey par­rots were tested on in­sight­ful be­hav­ior and im­i­ta­tive com­pe­tence. The re­sults showed that the two par­rots with lim­ited vo­cab­u­lary im­me­di­ately acted out the cor­rect phys­i­cal, in­sight­ful task. The par­rots that had re­ceived train­ing in ref­er­en­tial Eng­lish speech at­tempted in­stead to ma­nip­u­late their trainer. They en­gaged in de­lib­er­ate com­mu­ni­ca­tion as a prob­lem-solv­ing strat­egy, which is an ad­vanced stage of de­vel­op­ment, even for human in­fants. The most fa­mous African Grey Par­rot, Alex, ex­hib­ited cog­ni­tive ca­pac­i­ties com­pa­ra­ble to those of ma­rine mam­mals, apes, and 4 to 6 year old chil­dren. Some of Alex’s ac­com­plish­ments in­cluded the abil­ity to label 50 dif­fer­ent ob­jects, 7 col­ors, 5 shapes and quan­ti­ties up to and in­clud­ing six. He would com­bine these la­bels to iden­tify, re­quest, refuse, cat­e­go­rize, and quan­tify about 100 dif­fer­ent ob­jects. Alex also had func­tional use of phases and had a con­cept of cat­e­gory. African Grey Par­rots have be­hav­ior pat­terns that match some non-hu­man pri­mates and that par­al­lel com­bi­na­tory de­vel­op­ment. In an ex­per­i­ment per­formed by Galef (2004), African grey par­rots were ob­served to copy a va­ri­ety of ac­tions in­volv­ing six dif­fer­ent body parts and to as­so­ci­ate each ac­tion with its ap­pro­pri­ate label. This is a form of so­cial-learn­ing and im­i­ta­tion. (Galef, 2004; Pep­per­berg, 2001; Pep­per­berg, 2006; Pep­per­berg, 2007)

• Key Behaviors
• arboreal
• scansorial
• flies
• glides
• diurnal
• crepuscular
• motile
• migratory
• territorial
• social
• colonial

Com­mu­ni­ca­tion and Per­cep­tion

Wild African grey par­rot flocks fol­low a daily pat­tern of vo­cal­iza­tions. Usu­ally the flock is quiet from sun­set until the next dawn. At day break, the flock be­gins to vo­cal­ize be­fore set­ting out to for­age at dif­fer­ent lo­ca­tions through­out the day. At dusk, upon re­turn to the roost­ing site, there is a pe­riod of vo­cal­iza­tion. There are a va­ri­ety of dif­fer­ent types of calls and vo­cal­iza­tions, in­clud­ing alarm calls, con­tact calls, food beg­ging calls, and ag­o­nis­tic calls. Con­tact calls are of par­tic­u­lar im­por­tance be­cause they serve to iden­tify where other mem­bers of the flock are and help pro­mote flock co­he­sion. Alarm calls in­di­cate vary­ing lev­els of dis­tress, these calls are par­tic­u­larly loud and of a fre­quency that car­ries well in order to warn fel­low flock mem­bers. Young learn these vo­cal­iza­tions from par­ents and flock mates, so pet par­rots will not learn ap­pro­pri­ate vo­cal­iza­tions, but will show sim­i­lar pat­terns and use of calls. Bot­toni et al. (2003) found that African grey par­rots demon­strated com­plex cog­ni­tive com­pe­tence in un­der­stand­ing both the sim­i­lar­i­ties and dis­sim­i­lar­i­ties among mu­si­cal note fre­quen­cies and were able to mas­ter the mu­si­cal code. It was de­ter­mined that African grey par­rots must iso­late a sound from back­ground noise, im­i­tate it, cat­e­go­rize the acoustic stim­u­lus, en­code it into long term mem­ory, and mon­i­tor the out­put sound to match it with the in­ter­nal tem­plate. The fa­mous African grey par­rot, Alex, achieved a rudi­men­tary form of com­mu­ni­ca­tion, in­clud­ing con­tex­tual and con­cep­tual use of human speech. That re­search showed that African grey par­rots are ca­pa­ble of far more than sim­ply mim­ic­ing human speech. (Bot­toni, et al., 2003; Luescher, 2006; Pep­per­berg, 2000)

• Communication Channels
• visual
• acoustic

• Other Communication Modes
• mimicry
• choruses

• Perception Channels
• visual
• tactile
• acoustic
• chemical

Food Habits

African grey par­rots are her­bi­vores. In the wild, they feed pri­mar­ily on nuts and fruits, sup­ple­mented by leafy mat­ter, fruits, in­sects, bark, and flow­ers. African grey par­rots eat mostly com­mon fruits, such as oil-palm (Elaeis guinen­sis). (Athan and Deter, 2000; Faye, 2006)

• Primary Diet
• herbivore
  • frugivore
  • granivore
• omnivore

• Animal Foods
• insects

• Plant Foods
• leaves
• seeds, grains, and nuts
• fruit
• flowers

Pre­da­tion

Psit­ta­cus eritha­cus are ha­rassed and preyed on by palm-nut vul­tures (Gy­po­hierax an­golen­sis). Sev­eral species of hawks also prey on fledg­lings and adults. Mon­keys prey on eggs and young in nests. When feed­ing on the ground, African grey par­rots are vul­ner­a­ble to ter­res­trial preda­tors. ("African Grey Par­rots", 2006; Athan and Deter, 2000)

• Known Predators

  • palm-nut vultures (Gypohierax angolensis)
  • other raptors (Falconiformes)
  • monkeys (Cercopithecidae)

Ecosys­tem Roles

African grey par­rots may dis­perse the seeds of fruits they eat. They act as de­fin­i­tive hosts to both tape­worms and blood par­a­sites. ("African Grey Par­rots", 2006; Athan and Deter, 2000)

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

African grey par­rots are the sec­ond most heav­ily har­vested par­rot in the world. The trade be­tween 1980 and 1995 doc­u­mented an ex­cess of 500,000 birds caught in the wild. From 1994 to 2003, just fewer than 360,000 wild caught par­rots were re­port­edly ex­ported from their na­tive range. They are one of the most pop­u­lar avian pets in Eu­rope, the United States, and the Mid­dle East. In Principe, trap­pers heav­ily har­vest African grey par­rots for the in­ter­na­tional pet trade. ("Psit­ta­cus eritha­cus, Lin­naeeus, 1758", 2008; Fahlman, 2002; Juste, 1995; Melo and O'Ryan, 2007)

• Positive Impacts
• pet trade
• ecotourism
• research and education

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

There are no known ad­verse af­fects of Psit­ta­cus eritha­cus on hu­mans.

Con­ser­va­tion Sta­tus

Psit­ta­cus eritha­cus is con­sid­ered to be a near threat­ened species be­cause of a re­cent analy­sis sug­gest­ing that up to 21% of the global pop­u­la­tion may be har­vested an­nu­ally. The quota for African grey par­rots in the De­mo­c­ra­tic Re­pub­lic of the Congo is 5000, 4000 in Congo, and 250 in Gabon. Un­for­tu­nately, there is no law pro­hibit­ing cap­ture and trade of par­rots. These birds are im­pacted by habi­tat de­struc­tion, in­dis­crim­i­nate use of pes­ti­cides, and hunt­ing by local in­hab­i­tants. Trap­ping for the wild bird trade is a major cause of de­cline in wild African grey par­rots pop­u­la­tions. ("CITES species data­base", 2008; "The IUCN Red List of Threat­ened Species", 2008)

• IUCN Red List

  Near Threatened
  More information

• IUCN Red List

  Near Threatened
  More information

• US Migratory Bird Act

  No special status

• US Federal List

  No special status

• CITES

  Appendix II

• State of Michigan List

  No special status

Other Com­ments

In cap­tiv­ity, African grey par­rots often suf­fer med­ical prob­lems such as obe­sity, hy­povi­t­a­minosis A, hypocal­cemia, amino acid im­bal­ance, or mi­cromin­eral de­fi­cien­cies. Prob­lems are often as­so­ci­ated with poor diets com­posed of seeds and nuts with sup­ple­mented fruits. These food sources con­tain high lev­els of car­bo­hy­drates. African grey par­rots can also have cal­cium de­fi­cien­cies which can lead to seizures. They can suf­fer from res­pi­ra­tory ail­ments caused by fun­gal dis­eases, bac­te­r­ial in­fec­tions/pneu­mo­nia, and nu­tri­tional de­fi­cien­cies. Other health is­sues ob­served are; hy­per­k­er­a­totic swellings, ma­lig­nant tu­mors, tape­worm and blood par­a­sites. They are also sus­cep­ti­ble to Psittacine Beak and feather Dis­ease (PBFD). ("African Grey Par­rots", 2006; "Psit­ta­cus eritha­cus, Lin­naeeus, 1758", 2008; Ryan, 2002)

In cap­tiv­ity, African grey par­rots are prone to cer­tain be­hav­ioral prob­lems. For ex­am­ple, feather pick­ing in re­sponse to so­cial stress. Ter­ri­tory is­sues can also arise and are more com­mon in males than fe­males. (Athan and Deter, 2000; Faye, 2006)

Cap­tive African grey par­rots thrive on a diet of seeds, grain, pel­lets, and fruits and veg­eta­bles. It is sug­gested that pet own­ers sup­ple­ment the diet with cal­cium in the form of dark leafy greens, cal­cium-rich veg­eta­bles, fruits, oat­meal, al­monds, wal­nuts, hazel­nuts, and sesame seeds. Flax seed and sun­flower seed are also strongly rec­om­mended. Re­cent stud­ies have sug­gested that pel­leted diets are con­sid­ered nu­tri­tion­ally su­pe­rior to home­made diets and seed mix­tures and may re­duce the risk of ab­nor­mal grit con­sump­tion. (Athan and Deter, 2000; Faye, 2006)

Con­trib­u­tors

Tanya Dewey (ed­i­tor), An­i­mal Di­ver­sity Web.

Rachel Hol­man (au­thor), North­ern Michi­gan Uni­ver­sity, Alec R. Lind­say (ed­i­tor, in­struc­tor), North­ern Michi­gan Uni­ver­sity.

Ref­er­ences

2006. "African Grey Par­rots" (On-line). Ac­cessed April 10, 2008 at Eliteparrotsclub.​com/​articles/​species/​mediumsmall/​africangreyparrots.​html.

2008. "CITES species data­base" (On-line). CITES. Ac­cessed April 14, 2008 at http://​www.​cites.​org/​eng/​resources/​species.​html.

2008. "Psit­ta­cus eritha­cus (African Grey Par­rot, Congo African Grey Par­rot, Grey Par­rot)" (On-line). Zipcodezoo.​com. Ac­cessed March 20, 2008 at http://​zipcodezoo.​com/​Animals/​P/​Psittacus_​erithacus.​asp.

2008. "Psit­ta­cus eritha­cus, Lin­naeeus, 1758" (On-line pdf). Ac­cessed March 20, 2008 at http://​www.​cites.​org/​eng/​com/​AC/​22/​E22-10-2-Al.​pdf.

2008. "The IUCN Red List of Threat­ened Species" (On-line). Ac­cessed April 15, 2008 at http://​www.​iuncredlist.​org/​search/​details.​php/​47991/​clss.

Athan, M. 1999. Bar­ron's Guide to Com­pan­ion Par­rot Be­hav­ior. New York, New York: Bar­ron's Ed­u­ca­tional Se­ries.

Athan, M., D. Deter. 2000. The African Grey Par­rot Hand­book. Haup­pauge, NY: Bar­ron's Ed­u­ca­tional Se­ries. Ac­cessed March 20, 2008 at http://​books.​google.​com/​books?​id=qqrxmrS2bXQC.

Bot­toni, L., R. Massa, D. Boero. 2003. The Grey Par­rot (Psit­ta­cus eritha­cus) as a Mu­si­cian: an Ex­per­i­ment With the Tem­per­ate Scale. Ethol­ogy Ecol­ogy and Evo­lu­tion, 15: 133-141.

Fahlman, A. 2002. "African Drey Par­rot Con­ser­va­tion: a Fea­si­bil­ity eval­u­a­tion of De­vel­op­ing a Local Con­ser­va­tion Pro­gram in Pricipe" (On-line pdf). Ac­cessed April 10, 2008 at http://​www.​env-impact.​geo.​uu.​se/​84Fahlman.​pdf.

Faye, S. 2006. "African Greys aka Grey Par­rots, Gen­eral Info on the African Grey" (On-line). Avian­Web:Pet Bird Re­sources. Ac­cessed April 10, 2008 at http://​www.​avianweb.​com/​aricangreys.​htm.

Galef, B. 2004. So­cial Learn­ing and Im­i­ta­tion. Bi­ol­ogy of Be­hav­ior, 4: 261-269.

Hal­lan­der, J. 2001. "Flock Be­hav­ior: How it Af­fects our Com­pan­ion Par­rots" (On-line). The Grey Play Round­table, African Grey Info. Ac­cessed April 10, 2008 at africangreys.​com/​behavior/​flocks.​htm.

Juste, J. 1995. Trade in the Gray Par­rot (Psit­ta­cus eritha­cus) on the Is­land of Principe (SAO TOME and Principe, Cen­tral Africa): Ini­tial As­sess­ment of the Ac­tiv­ity on Its Im­pact. Bi­o­log­i­cal Con­ser­va­tion, 76: 101-104.

Luescher, A. 2006. Man­ual of Par­rot Be­hav­ior. New York, New York: Black­well Pub­lish­ing.

Melo, M., C. O'Ryan. 2007. Ge­netic dif­fer­en­ti­a­tion be­tween Principe Is­land and main­land pop­u­la­tions of the grey par­rot (Psit­ta­cus eritha­cus), and im­pli­ca­tions for con­ser­va­tion. Mol­e­c­u­lar Ecol­ogy, 16: 1673-1685.

Pep­per­berg, I. 2000. The Alex Stud­ies, Cog­ni­tive and Com­mu­nica­tive Abil­i­ties of Grey Par­rots.b. Boston, Mass­a­chu­setts: Har­vard Uni­ver­sity Press.

Pep­per­berg, I. 1991. A Com­mu­nica­tive Ap­proach to An­i­mal Cog­ni­tion: A Study of Con­cep­tual Abil­i­ties of an African Grey Par­rot. Pp. 153-186 in C Ris­tau, ed. Cog­ni­tive Ethol­ogy, the Minds of Other An­i­mals, Es­says in Honor of Don­ald R. Grif­fin. Mah­wah, New Jer­sey: Lawrence Erl­baum As­so­ci­ates.

Pep­per­berg, I. 2007. When Train­ing En­gen­ders Fail­ure to Im­i­tate in Grey Par­rots (Psit­ta­cus eritha­cus). Pro­ceed­ings of the 4th In­ter­na­tional Sym­po­sium on Im­i­ta­tion in An­i­mals and Ar­ti­facts: 0-10.

Pep­per­berg, I. 2006. cog­ni­tive and Com­mu­nica­tive Abil­i­ties of Grey Par­rots. Ap­plied An­i­mal Be­hav­ior Sci­ence, 100: 77-86.

Pep­per­berg, I. 2001. "Lessons from Cog­ni­tive Ethol­ogy: An­i­mal Mod­els for Etho­log­i­cal Com­put­ing" (On-line pdf). Ac­cessed April 10, 2008 at http://​www.​lehigh.​edu/​~mhbo/cogsZwe­bread­ings/Pepperberg23sep01.​pdf.

Ryan, T. 2002. Grit Im­paction in Two Neona­tal African Grey Par­rots (Psit­ta­cus eritha­cus eritha­cus). Avian Med­i­cine and Surgery, 16: 230-233.

Wright, M. 2002. "Un­der­stand­ing the Wild Na­ture of our Dreys" (On-line). Ac­cessed April 10, 2008 at http://​www.​lafeber.​com/​Laferber-Library/​Articles/​wright/​wildnature.​asp.