bird migration at ibadan, nigeria

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This article was downloaded by: [Arizona State University] On: 27 May 2014, At: 22:24 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Ostrich: Journal of African Ornithology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tost20 BIRD MIGRATION AT IBADAN, NIGERIA J. H. Elgood Published online: 06 Oct 2010. To cite this article: J. H. Elgood (1959) BIRD MIGRATION AT IBADAN, NIGERIA, Ostrich: Journal of African Ornithology, 30:S1, 306-316, DOI: 10.1080/00306525.1959.9633339 To link to this article: http://dx.doi.org/10.1080/00306525.1959.9633339 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms- and-conditions

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Page 1: BIRD MIGRATION AT IBADAN, NIGERIA

This article was downloaded by: [Arizona State University]On: 27 May 2014, At: 22:24Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Ostrich: Journal of African OrnithologyPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tost20

BIRD MIGRATION AT IBADAN, NIGERIAJ. H. ElgoodPublished online: 06 Oct 2010.

To cite this article: J. H. Elgood (1959) BIRD MIGRATION AT IBADAN, NIGERIA, Ostrich: Journal ofAfrican Ornithology, 30:S1, 306-316, DOI: 10.1080/00306525.1959.9633339

To link to this article: http://dx.doi.org/10.1080/00306525.1959.9633339

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: BIRD MIGRATION AT IBADAN, NIGERIA

BIRD MIGRATION AT IBADAN, NIGERIA

J. H. Elgood

Despite the vast literature on bird migration, there has been relatively littledata from West Africa, and more especially Nigeria, so it is hoped that this papermay help to fill in some gaps in the total picture.

In the first place it is necessary to draw a distinction between two main typesof migratory bird that are to be found anywhere within the African continent. Firstthere are the Palaearctic migrants that spend the northern winter within the Aethiopeanregion and secondly there are those birds which remain within Africa (or numbersof them do), but perform regular migratory movements within the continent, andbreed within its limits.

The general picture of the movements of the Palaearctic migrants is well estab­lished and has been the subject of an important review by Moreau (Ibis. 1952).Nevertheless it is hoped that data from Ibadan may prove of some interest. Withregard to the Intra-African migrants there is still so much that is obscure it is feltthat any contribution, however small, may be of value.

Ibadan is some 70 miles inland from the Guinea Coast, more or less northof Lagos (Latitude 71 0 N., Longitude 4 0 E.). The actual place in which most of theobservations have been made lies about five miles to the north of the city of Ibadanand comprises the area stretching from the grounds of the University College ofIbadan to the local reservoir, and thus includes a number of distinct habitat types.The College stands on gently undulating ground, much of which was formerly secondaryforest, but with considerable areas devoted to native farming. Where this farmingis widely practised the vegetation tends toward a type of savannah, largely as theresult of burning the bush. There is thus a good deal of argument as to the correctdescription of the vegetation. Quite the best that I know was given by ProfessorG. P. Wells when he came to Ibadan last year-he called it "Lush Bush". Someof it approaches true forest conditions and other parts are of a savannah type.

That the vegetation is of this intermediate type is clearly reflected by theavifauna it supports. Thus within the precincts of University College may be foundsuch birds as Chapin has quoted as typical forest species in the adjacent BelgianCongo. Noteworthy amongst these are Turacus persa, Centropus leucogaster,Eurystomus gularis, Trachylaemus purpuraius, Nicotor chloris, Oriolus nigripennis,and species of Malimbus such as nitens and rubricollis. On the other hand one alsoencounters such typical savannah species as Phoeniculus erythrorhynchus, Lybiusvieilloti, Cisticola lateralis, Bradornis pallidus, Corvinella corvinus and Prionopsplumatus.*

It is almost certain that none of the local vegetation is natural so that theterms secondary forest and derived savannah are probably the best to use for thetwo main kinds. But in addition there are also other habitat types within the areaunder consideration that are more obviously due to human activities. In Ibadantownship there is a truly urban area with which a few bird species are associated.

* The names used are those given by Bannerman.

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1959 ELGOOD: MIGRATION AT IBADAN 307

Secondly there is a large suburban area, and much of the site of University Collegemust be so classified, with large gardens and open spaces chiefly planted with exoticshrubs and trees. Then there are considerable plantations farmed to a great extentwith the aid of modern agricultural machinery and contrasting sharply with the areasfarmed according to native practice, in that the ground is completely cleared of theoriginal vegetation. Again some areas have been devoted to experiments in forestry,in which pure stands of such timber trees as teak are being grown. Last, but byno means least from the point of view of bird life, there is the reservoir offeringshelter to a considerable number of aquatic birds, both resident and migratory. Againstthis background of differing habitats it is not surprising that the area has a veryrich avifauna, well over 200 species having already been identified and their numberis still growing. But it is the habitats of human origin that support the Palaearcticmigrants.

Before discussing migratory movements of the birds it is next necessary todiscuss the climatic cycle. If Africa as a whole is considered, it is found that itsseasons are governed by the swing of the sun to and fro across the Equator. Thereis, of course, much variation from year to year but the general pattern of the seasonsdoes conform to the theoretical. Fig. 1 shows the actuall rainfall, on a half-monthlybasis, at Ibadan during the year 1954. It conforms to the pattern of a rainy seasonfrom March to November, broken by a "little dry" period in July and August,the remaining months being essentially dry season. It should also be mentioned thatthe rain peaks are seasons of fairly violent storms, often called tornadoes, and thatthe little dry season is characterized by a mainly overcast sky so that it might wellbe called the cool season. On the whole there is little variation in temperature, eitherdiurnally or seasonally, remaining between 20° C. and 30° C. most of the time.though in the dry season the cold Harmattan wind. dust laden from the Sahara. mayreach as far south as Ibadan and has been known to bring the temperature downin the small hours of the morning to below 10° C. During what has been calledthe cool season the temperature may not exceed 24 0 C. for days on end and may bebelow 150 C. at night.

RAINFALL. Ibadan, 1954precipitation in mm.

200,------------------ --.

150t-----

JOOt----__

50r.-----

FEB MAR APR-MAY JUN JUL AUG SEP OCT NOV DECFig. 1

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308 ELGOOD: MIGRAnON AT IBADAN OSTRICH SUP. 3

It is felt that little mention need be made of the local geology as the areais sufficiently uniform in this respect to have little effect. Although outcroppingsof the basement complex do occur, none are large enough to afford a special habitatand such species as Cisticola emini and Cercomela iamiliaris, typical of suchground, have not been found at Ibadan. The soil varies locally from a heavy lateriticclay to being quite sandy, the former supporting the more forest type of vegetationand the latter the more savannah type as already mentioned.

PALAEARCTIC MIGRANTS

Reference has already been made to Moreau's important paper on the placeof the Aethiopean region as a wintering ground for Palaearctic migrants. In this heshows that about 40% of all Palaearctic breeding species are migratory and that mostof them travel south of the Sahara. He analyses the passerine migrants into fivecategories classified according to the wintering area. Table 1 shows these categories,

TABLE 1

PASSERINE PALAEARCTIC MIGRANTS AT IBADAN CLASSIFIED AS TO WINTERINGAREA ACCORDING TO MOREAU'S SCHEME

Total No. of species ObservedMain Wintering Area at

Moreau Bannerman Ibadan

1. North of 10' N · . .. .. ., .. ·. 23 12 02. South to near Equator

(a) Largely in Palaearctic · . .. . . .. · . 3 3 0(b) Entirely S. of Sahara · . .. .. ·. " 1 1 1

3. From Sahara to Equator ·. .. .. . , ·. · . 12 10 24. Exclusively S. of Equator .. ·. .. ., ·. .. 8 2 05. Throughout Africa .. " .. " . . · . .. 15 15 8

Totals . , ·. .. .. .. . . .. 62 43 11

the numbers of species of each type as quoted by Moreau, the numbers of speciesof each type listed by Bannerman as occurring in West Africa and the numbers ofeach category observed at Ibadan. It will be realized that the first category cannotoccur at Ibadan, unless as a freak wanderer. The differences between Moreau'snumbers and those of Bannerman indicate the number of species that prefer theeastern side of Africa, many of them being Asiatic rather than European birds.hardly likely to turn up in West Africa. .The one "category 2" species to occurat Ibadan is Lanius senator. The two third-category species are Ficedula hypoleucaand Luscinia megarhyncha; while those of the fifth group include Budytes flavus,Anthus trivialis, Oenanthe oenanthe, Saxicola rubetra, Muscicapa striata, Phylloscopustrochilus and Hirundo rustica.

Thus some dozen passerines are known to winter at Ibadan out of about 30likely species. Not a few of the "missing" species are warblers that are difficultto identify in the field when not in song. It will thus be realized that the Ibadanarea makes a considerable contribution to the over-wintering of the Palaearcticpasserines, It must also be emphasized that some of these migrant species are repre­sented by very large numbers of individuals and this at a time when the availablefood. which must be shared with the resident populations, is minimal. For it is

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1959 ELGOOD: MIGRATION AT IBADAN 309

in the dry season, when plant growth is most slack, and therefore also the insectpopulations lowest, that there is this influx of migrants from Europe.

Table 2 shows the full list of Palaearctic migrants observed at Ibadan andalso indicates the frequency with which each species has been observed and whereseen often enough the period of stay in the area.

TABLE 2

PALAEARCTlC MIGRANTS SEEN AT IBADAN

Species

Charadrius dubius ..Charadrius hiaticulaActitis hypoleucos ..Tringa ocrophusTringa glareolaTringa erythropus ..Numenius arquatusFalco t. tinnunculusOtus scops ..Micropus apusCoracias garrulus ..Budytes flavusAnthus triavialis/cervinusMuscicapa striata ..Ficedula hypoleuca/albicollisOenanthe oenantheSaxicola rubetraPhoenicurus phoenicurus ..Luscinia megarhynchosPhylloscopus trochilusPhylloscopus sibilatrixHirundo rusticaLanius senator

Frequency

SeveralSeveral

InnumerableSeveralMany

Once onlyOnce only

SeveralOnce only

Many3 Times

InnumerableOnce (4 birds)

ManySeveral

Once onlyInnumerable

SeveralInnumerableInnumerable

SeveralInnumerableInnumerable

Period ofObservation

Only in April.Only in November

1/9 to 5/54/12 to 25/2

Only in April31/12/5525/8/54

1/2 to 27/329/2/56

4/10 to 19/513/11 to 6/42/10 to 9/5

13/11/5530/9 to 24/420/11 to 7/4

16/11/559/10 to 30/4

Only in January9/11 to 29/39/9 to 29/3

Mostly Nov. & April2/10 to 26/49/12 to 19/4

I do not propose to go through the whole list but one or two species warrantspecial mention. I was most surprised to observe' a single Curlew Numenius arquatusone wet afternoon in August 1954. It flew around the College playing fields, makingits unmistakable call, and also settled for full observation. It has been suggestedthat this species may remain in Africa to breed, since stragglers are found in southernAfrica during the months of northern summer. This may have been such a bird.

Another interesting record is that of the European Scops Owl Otus scops.While breakfasting on my verandah on 29 February 1956, I became aware of a smallowl perched in a Caesalpinia bush within a few feet of my head and I had nodifficulty in collecting it. The skin was sent to the British Museum and Mr. Macdonaldconfirmed my opinion that it was in fact the European species and not Otussenegalensis. Bannerman quotes no records of Otus scops south of a line from aboutTimbuctoo to Lake Chad, so that this Ibadan record appears to extend considerablysouthward, the known winter range' of this species in West Africa.

There is one other species that I should like to mention, though it has notbeen seen at Ibadan, and that is Delichon. urbica. Records of this species fromWest Africa are sparse, so that it is worth mentioning that I once met it in Nigeria.at llorin, about 100 miles north of Ibadan. A flock of about 20 birds, in companywith rather more Hirundo rustica were seen circling in the early evening on 22

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310 ELGOOD: MIGRATION AT IBADAN OSTRICH SUP. 3

March 1956. (Incidentally I had journeyed from Ibadan to Ilorin that day, and forwell over 50 miles Micropus apus was seen on the wing in large numbers, so thata vast number of migrating birds must have been encountered in all.)

Passing from references to individual species there are one or two generalizationsworthy of remark. Many writers on bird migration, notably Landsborough Thomson,have emphasized the remarkable constancy of the dates of arrival and departure ofmigrant species. It is as though the calendar date rather than any internal physio­logical process or external environmental factor determines their movements. In thecase of those species met with sufficiently frequently it has been found that theirarrival and departure dates exhibit this constancy at Ibadan, Table 3 shows arrivaland departure dates for three species over a period of seven years. The dates forBudytes fiavus and Saxicola rubetra are based on sight records while those for Lusciniamegarhyncha are a little less reliable being based on hearing the song. Of course

TABLE 3

Species

Winter Butytes flavus Saxico1a rubetra Luscinia megarhynchos

Seen from to Seen from to Heard from to

1950-51 .. - 29/4 - 27/4 - -1951-52 .. 19/10 3/5 21/10 8/4 11/11 11/31952-53 .. 11/10 28/4 14/10 22/4 14/11 17/31953-54 .. 4/10 3/5 13/10 12/4 13/11 3/31954-55 .. 3/10 6/5 9/10 14/4 Leave 12/31955-56 .. 4/10 9/5 16/10 30/4 20/11 29/31956-57 .. 2/10 7/5 22/10 15/4 9/11 29/3

the most striking constancy is shown by Budytes fiavus which is a very conspicuousspecies in its winter haunts. Its arrival and departure for the past four years aremost remarkable. I think the somewhat aberrant figures for the earlier years to bedue to the fact that the site of University College was not then fully developed andthat the birds arriving in the area would not necessarily make for the open spacesof the College playing fields as they have done in more recent years.

It is outside the scope of this paper to comment on the behaviour of Palaearcticmigrants in their winter quarters and only one or two brief remarks will be made.Some species seem to be silent, while others are quite vocal. Lanius senator hasnever been known to utter a sound while Luscinia megarhyncha seems to sing mostof the time though its song is a shadow of the full spring song in its breeding haunts.

Again some species show marked territorial behaviour, being found time andagain in the same place and displaying with vigour when another of their speciescomes near. This applies especially to Saxicola rubetra, and Luscinia seems to remainin the same thicket throughout the winter months. On the other hand such speciesas the waders, swallows and wagtails remain in large wandering flocks, though thelast named seem to have a fairly circumscribed area within which they remain.

This last point raises a query with regard to their departure. Species that remainin flocks will obviously stimulate one another with increasing excitement prior todeparture. But what of those species that are essentially solitary during the wintermonths? I have found no evidence whatever of their being gathered up into migratory

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1959 ELGOOD: MIGRATION AT IBADAN 311

flocks by members of their species wintering further south. What does precipitate theirdeparture? Weather factors at Ibadan are almost as variable from year to year as theyproverbially are in Europe. The rate of change of day length is scarcely noticeableas the total change between December and June is only about one hour. I shall returnto these problems later.

AFRICAN MIGRANTS

TABLE 4

AFRICAN OR WIDE RANGING SPECIES THAT ARE MIGRANT AT IBADAN

Species

Aquatic SpeciesPhalacrocorax africanusAnhinga rufaEgretta garzettaPandion haliaetusPorphyrula alIeniRostratula benghalensisCorythornis cristata

Dry Season MigrantsArdea cinereaBubulcus ibisArdeola ralloidesMilvus migrans parasitusAccipiter badiusAfroxyechus forbesi ..Clamator glandarius ..Scotornis cIimacurusHalcyon leucocephalaAerops albicoIIisCampephaga phoeniciaCinnyris venustusPachyphantes pachyrhynchus

Wet Season MigrantsAccipiter melanoleucusPoicephalus senegalusMerops malimbicusLybius vieiIIotiTchagra senegalaEmberiza cabanisiColiuspasser macrourus

Well Established Migrant Rare at IbadanSphenorynchus abdimii

Frequency

ManyMany

SeveralSeveralSeveralManyMany

ManyInnumerable

ManyInnumerableInnumerable

ManyManyManyMany

InnumerableManyMany

Several

SeveralSeveralManyMany

SeveralSeveralMany

Once only(13 birds)

Period ofObservation

May to DecemberFebruary to November

March to AprilMay to SeptemberApril to September

April to JuneJanuary to June

October to MayOctober to May

November to AprilSeptember to June

October to MaySeptember to April

January to MayNovember to April

November to FebruaryOctober to May

November to AprilAugust to MarchJanuary to July

May to AugustJune to NovemberApril to September

February to SeptemberJune to SeptemberMarch to August

May to November

April (16/4/53)

I now wish to consider the movements of essentially African, or else very wideranging species. Table 4 gives a list of these species. For convenience they have beengrouped under three main heads. The first are essentially aquatic, most of which areknown to exhibit local migratory movements, from one body of water to another (oftenacross the Equator), as one river dries, or perhaps more often its banks tend to overflowso that no shore line is available for feeding. In no case in this group is the evidenceconclusive that the species is truly migratory and they are only included in the hope thatsome day the information may help to fit into a larger picture. The second group

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312 ELGOOU: MIGRATION AT IBADAN OSTRICH SUP. 3

are headed dry season migrants and for all but the last two species there can be nodoubt that they are dry season birds at Ibadan. Ardea cinerea is a case of some interestas this species is certainly represented in West Africa throughout the year, but no birdshave ever been seen at Ibadan between June and September. Whether those individualsthat winter at Ibadan are migrants from Palaearctica is not clear but there can be nodoubt that they move away. Most of the birds of this group move northward fromIbadan to breed with the arrival of the rains. But there are some interesting exceptions.The two raptors, Milvus migrans and Accipiter badius, breed in the Ibadan area andmove away subsequently. It is by no means certain where they go but both are foundin Northern Nigeria at the time they are away from Ibadan, so one is tempted to believethey move north. If this is so, it is remarkable that birds in the northern hemisphereshould breed at the southern end of their annual migration. Another exception isClamator glandarius that visits Ibadan only from January to May and in so doing ispresent for the peak of the breeding season of the Pied Crow Corvus albus known to bea host of this parasitic species. It may be observed that all three of these birds arejust as common in southern Africa and there is much to be learned of the movementsperformed by them.

The plover Ajroxyechus [orbesi affords another rather interesting case as it hasbeen established by Mr. L. H. Brown that this species is almost a "vertical migrant"since it repairs in the rains to the tops of inselbergs to breed. Since there are none ofthese rocky hills very near to Ibadan, the birds do have to travel northward to reachthem.

One last case worth special mention from this group is Halcyon leucocephala.In general this species is known to winter in the coastal parts of Nigeria but to repairnorthward to breed in the rainy season. But recently, in late May of this year, I founda bird of this species feeding young at a point some 60 miles due west of Ibadan, butin savannah country, indicating a change in habitat rather than a change in latitudeas being the essential feature of this species' movements.

Not only is the list of "Wet Season Migrants" to Ibadan a short one, but fornone of them is the evidence fully convincing so once more they are presented fordebate rather than anything else. The case. for Merops malimbicus is the best estab­lished as this species can hardly be missed when present on account of its characteristiccall. It has only been recorded between April and September. Coliuspasser macrourusis a common bird from May to November, but as it has an eclipse plumage quitedifferent from that of the breeding male, it is just possible that I have failed to recognizeit at other times of the year.

I have classed Sphenorhynchus abdimii by itself as this is a bird well known forits movements within Africa. Ibadan, however, seems to lie away from its normalmigratory path, but as a flock of 13 of these birds was once encountered it seemedworthy of mention.

One general feature of this list is the almost complete absence of passerines, theonly passerine species which is undoubtedly migrant being Campephaga phoenicia. Forthe other species it is merely a case of putting on record that there is some evidencefor migratory behaviour and hoping that future work at Ibadan, or elsewhere, will con­firm or refute this evidence.

Data has already been produced to show the constancy of arrival and departuredates for the Palaearctic migrants and it is a remarkable fact that much the same sortof constancy is shown by those forms which remain within Africa. With such speciesan occasional straggler may be met with, but if these are ignored then the sort ofresults given in Table 5 are obtained. Quite the most impressive figures here are the

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TABLE 5

ARRIVAL AND DEPARTURE DATES OF THREE AFRICAN MIGRANTS AT lBADAN

Species

Bubulcus ibis Milvus m. parasitus Aerops albicollisYear

1st Rec, Last Rec. 1st Rec. Last Rec. 1st Rec. Last Rec.

1950-51 .. - 16(5 - - 24(10 22(51951-52 ·. 11(11 16(5 - 8(6 19(10 6(61952-53 ·. 8(11 10(5 3(10 3(6 9(10 Il(51953-54 .. 18(10 17(5 3(10 27(5 19(10 24/51954-55 · . Leave 19/5 19/9 31/5 5/10 4/51955-56 .. 22(10 22(5 2(10 14(6 3/10 28(51956-57 .. 24(10 21/5 30/9 7/6 16(10 16(5

dates of departure of the Cattle Egret Bubulcus ibis. Since this species leaves to breedfurther north it is not surprising that it leaves with regularity, and since it is a veryconspicuous species, one observer can get useful data of its movements.

I should like at this point to make special mention of Falco tinnunculus. It hasbeen considered to be a dry season migrant to West Africa, and there is little doubtthat individuals belonging to the European sub-species, F. t, tinnunculus, visit the Ibadanarea during the winter months, especially in February and March. But during the rainyseason much darker birds, which are almost certainly F. t. rufescens, are to be found atIbadan. I have one of these darker. birds in captivity and hope to have the sub-speciesconfirmed when it dies but as it is still flourishing it seemed a pity not to await con­firmation of its identity. This species was captured in August of last year and otherslike it have been seen from June to September.

There are considerable numbers of species which various writers on African birdshave stated to be migrant at least in some parts of their range and attention is nowdirected to some of these. Accipiter bad-ius is one such and there can be no doubt thatthis bird is migratory at Ibadan. It is a common, noisy and otherwise conspicuousspecies from late September to about the middle, of May, breeding early in the year.But it leaves the area for the remaining short period for an unknown destination.

In common with most other bird observers, it has been my practice to keep dailylists of species observed and heard. In an effort to give these daily lists some sortof numerical comparative value I have used a scale of 1 to 5 to give some measure offrequency of observation. Thus 1 indicates a single observation, 2 a few observations,3 several, 4 many and 5 very many. From such figures it has been possible to deducean index for each month of the year and by comparing the monthly indices evidencefor or against migratory behaviour may be obtained.

Table 6 gives indices arrived at in this way for a typical resident speciesPycnonotus barbatus, a typical migratory species Budytes flavus, and three of thesespecies which have been thought to be migratory, namely, Eurystomus aier, Avicedacuculoides and Lampromorpha caprius. It will be seen at once that at least some indivi­duals of all three species are to be found at Ibadan throughout the year.

It should be mentioned that the indices for Lampromorpha caprius are derivedfrom sound records, the other two from sight records. Eurystomus afer breeds fromMarch to May and is then an aggressive species and as such more conspicuous, so thatthis alone could be responsible for the higher indices for those months, rather than

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TABLE 6

MEAN MONTHLY INDICES FOR CERTAIN SPECIES AT IBADAN

Mean Monthly IndexSpecies -------------------------

J. F. M. A. M. J. J. A. S. O. N. D.------------------------

Pycnonotus barbatus Res. 65·9 57·5 51'7 52·0 57·1 57·1 49·0 51·8 52·7 65·2 55·2 62·5Eurystomus afer ? 8'8 18·1 21·2 24'6 28'5 19'5 12'7 16·1 12·0 9·3 13·7 10·3Aviceda cuculoides ? 2·6 2·2 1·8 2'3 3'2 6'9 4·0 2·7 1·9 1·3 2·5 2·5Lampromorpha

caprius .. ? 18'5 23·4 22'5 16'4 14·7 21'0 18·1 16·9 19·5 15·0 1·6 4·5Budytes flavus .. Mig. 42'3 43'1 40'2 46·2 3·7 0·0 0·0 0·0 0·4 53·8 74·0 69·0

any influx of individual birds to swell the resident population. It should be mentionedthat the actual value of the so-called indices give some measure of the relative common­ness of the different species, so that the low figures for A viceda cuculoides simply meanthat this is not very common. On the other hand, there is certainly no pattern of changein the index to suggest that the numbers are seasonally increased by the arrival ofmigrants to swell the resident population. Again the low values for the sound indicesof Lampromorpha caprius in November and December may well be due to the speciesbeing rather more silent at the beginning of the dry season than to any movement awayfrom the area at this time. Certainly the early dry season sees a cessation 0 ... breedingactivity on the part of the various species of weavers that act as hosts to this cuckoo.

MIGRATORY MECHANISMS

I now propose to wander from the field of recorded data to that of speculation,to consider briefly the unsolved problem of the mechanisms that release migratoryactivity. To consider, in fact, what light is thrown on the problem by studying birdmigration at a station of low latitude such as Ibadan.

Much recent work has been shown that changing day length is a potent factor inprecipitating bird migration in sub-tropical and temperate latitudes, but it is fairlyobvious that this factor alone can have little effect at such places as Ibadan where thetotal change in day length between June and December only amounts to approximatelyone hour and the rate of change at the periods of maximal migratory activity isnegligible. There is, of course, a general change in weather over these periods ofmigratory activity but the variation in weather differs so much from year to year inmarked contrast to the relatively constant dates for arrival and departure for even theintra-African migrants. It seems unlikely therefore that the weather plays any directpart in inducing migration. Again so far as departure from Ibadan of both the Palae­arctic migrants and the largest group of the African migrants (the dry season visitors)is concerned, they do so at a time when food is most abundant. They leave soon afterthe start of the rains when vegetation is growing rapidly and insect populations areincreasing correspondingly. It is clear therefore that a shortage of food cannot beresponsible for their departure.

On the other hand, it is certain that changes in the gonads accompany this changein behaviour which leads to migration. For the majority of the recognized migrants atIbadan, departure is departure to a more northerly breeding ground, with increasingsex hormone secretion. But this is not the case for Milvus migrans, nor for Accipiterbadius, both of which depart after breeding. It is likely that degenerative changes in the

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1959 ELGOOD: MIGRATION AT IBADAN 315

gonads may occur in these species, but the:re can hardly be a food shortage for them.Why they should migrate seems a mystery. One can only cloak one's ignorance bysaying that these species have an inherent internal rhythm. A rhythm which is gearedto the seasons in a general way but having such precision that it is capable of initiatingmigratory movement with remarkably constant regularity despite the variability fromyear to year of most of the more obvious environmental factors showing seasonalfluctuation.

Experiments are being initiated at Ibadan to obtain information, by ringingsuch Palaearctic migrants as Budytes flavus. It is hoped to be able to demonstratethat what at the moment is little more than a hunch is in fact true. This hunchis that migrants are as faithful to their winter quarters as they are known to beto their breeding grounds. The evidence for this so far available is affordedby those species showing territorial behaviour in their winter quarters. Nightingalesare present in the same thicket year after year and Whinchats too reappear fromyear to year in exactly the same territories, so as to suggest that it is the sameindividuals returning to the same winter ground.

Again with Bubulcus ibis the evidence is even stronger. A flock of thesebirds is to be seen daily during the dry season at the farm of University College.The flock crosses Ibadan town night and morning to its roost at Moor Plantationto which it seems to remain faithful from year to year. This is of pzrticular interestas, until recently, Moor Plantation was the only place within the area where a herdof cattle might be constantly found. The starting of a herd at University Collegein 1950 has offered excellent feeding conditions by day for these birds, but the flockremains faithful to the old roost at Moor Plantation, presumably as it was the originalroosting place for this species in the area. There can be little doubt that they arethe same individuals, or many of them are, returning year after year to the samewintering Iground.

It may well be said that bird migration studies present an inexhaustible field.the data for the intratropical species still being negligible. It may well be an outcomeof this Congress that greater and wider spread ringing together with a better knowledgeof the work being done in other parts of this continent may contribute substantiallyto a furtherance of our knowledge of African bird migration.

DISCUSSION

L. H. BROWN: I think, from my own Nigerian records, that variation in the intensityof the harmattan affects the dates of movement of intra-African migrants, at any rate at Ilorin,This was especially so in respect of Halcyon leucocephala and Milvus migrans. I agree withMr. Elgood that the Cattle Egret is a migrant. Merops malimbicus breeds on the banksof the Niger but disappears during the flood season. to some unknown place. A careful studyof the weather conditions might throw much light on migration in Africa. In Kenya, theincidence of the rains has a reversible effect on the Dabchick; and in exceptional rains thePurple Gallinule may move upwards of 100 miles from its normal haunts to newly filled, smallswamps.

G. J. BROEKHUYSEN: We have made similar observations in the Cape; wintering birdscome back to the same spots year after year. For instance, the Bar-tailed Godwit is rarein the south-west Cape but up to 120 are found every summer at Langebaan, probably thesame birds. Our figures for migrants are very similar to Mr. Elgood's although we are inan area where no passage migration takes place. Can you suggest any explanation?

J. H. ELGOOD: Ibadan is almost on the south coast, so most of the birds remain there.The Spotted Flycatcher, however, is clearly a passage migrant and the European Swallow ismuch more numerous at migration times. I suggest that the explanation is the shape .of Africa,which puts Ibadan and the south-west Cape in a similar position from the point of view ofsouth-bound migrants.

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316 ELGOOD: MIGRATION AT IBADAN OSTRICH SUP. 3

J. M. WINTERBOTTOM: The European Swallow at Cape Town shows a double peak. InMr. Elgood's figures I thought I detected a double peak for the Yellow Wagtail.

J. CHAPIN: In central equatorial Africa, the great rain forest is an effective barrier andmany rnigants stop at its northern edge. Others go round; but most of the birds reachingSouth Africa probably come down the east coast. Relatively few birds make the long hopover the forest.

G. J. BROEKHUYSEN: Have you any records of the Cape Gannet?J. H. ELGOOD: No, I have never seen it; but I do not live on the coast.G. J. BROEKHUYSEN: We have ring returns from as far along the coast as Port Harcourt.J. CHAPIN: The northernmost record is a museum specimen from Batounga.c. J. SKEAD: In what numbers does the Cattle Egret occur? And are these numbers

constant during their stay, and from year to year?J. H. ELGOOD: Over the last seven years, the numbers have been fairly constant at 70-100.J. CHAPIN: This bird ought to receive the concentrated attention of all bird ringers and

ornithologists, especially those interested in migration. Round Kiwu they occur all throughthe year in off-season plumage and roost regularly in a definite area. From October to April,none come up the 1,500 ft. to the dairy-farm, but from 20 April to October, they come upregularly. I suggest that there are two groups of these birds, one ranging more widely thanthe other. The wide-ranging birds perhaps come from the south-probably Northern Rhodesiaand Kenya; and the others from the north--the Sudan perhaps. It is an interesting point thatthe Cattle Egret seems to be less migratory at the ends of its range than in the middle.

G. J. BROEKHUYSEN: Our South African birds breed in August-October. A bird ringedat Johannesburg has been recovered in Northern Rhodesia and one ringed at Rondevlei inPortuguese East Africa, so birds from the edge of the range do move about.

c. J. SKEAD: Another Johannesburg bird has been taken at Kiwu.J. CHAPIN: The breeding season in the Cape is quite different from that at Johannesburg

and thence northwards.J. VINCENT: In Natal, Cattle Egrets breed later than in the Cape. We have ringed

thousands but have had no returns. In the Natal highlands, however, the egrets disappear inthe dry season and we have had returns of these birds, but only of adults and all from northof where they were ringed.

w. P. STANFORD: Mr. Plowes had no returns from thousands of Quelea ringed near Umtali,only from adults.

J. M. WINTERBOTTOM: Swallows do return to the places where they were hatched.c. J. SKEAD: A swift in King William's Town bred in the same nest in which it had

been reared.

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