PARTITIONING OF NESTING SPACE AMONG OF THE BENGUELA UPWELLING REGION

DAviD C. DuFFY & GRAEME D. LA CocK

Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch, South , 7700.

Received February /985

SUMMARY

DuFFY, D. C. & LA CocK, G. D. 1985. Partitioning of nesting space among seabirds of the Benguela upwel­ ling region. Ostrich 56: 186-201. An examination of nesting habitats used by the four main of seabirds nesting on southern African islands (Jackass Penguin Spheniscus demersus, Cape capensis, P. neglectus and Cape Morus capensis) revealed relatively minor differences and extensive over­ laps between species, primarily in subcolony size, steepness of nesting substratum, and proximity to cliffs. A weak dominance hierarchy existed; could displace penguins, and penguins could displace cor­ morants. This hierarchy appeared to have little effect on partitioning of nesting space. Species successfully defended occupied sites in most cases of interspecific conflict, suggesting that site tenure by one species could prevent nesting by another. The creation of additional nesting space on Namibian nesting platforms did not increase harvests, suggesting that nesting space had not previously limited the total nesting population of Cape , the most abundant of the breeding species, in . While local shortages of nesting space may occur, populations of the four principal species of nesting seabirds in the Benguela upwelling region do not seem to have been limited by the availability of nesting space on islands.

INTRODUCTION pean settlement, space for nesting was limited. Af­ ter settlement, human disturbance such as hunting The breeding seabirds of the Benguela upwel­ and guano extraction reduced nesting space, re­ ling region off Namibia and are be­ sulting in smaller populations of nesting seabirds. lieved to be affected by commercial fishing activ­ Guano platforms constructed in the 1930s (Rand ities (e.g. Frost et al. 1976; Crawford & Shelton 1952; Berry 1975) led to increased nesting space 1978, 1981; Burger & Cooper 1984). Populations and populations. Most recently, intensive com­ of Jackass Penguins Spheniscus demersus have de­ mercial fishing has reduced populations. creased because of apparent competition with the If guano is to be used as an index of historical pelagic purse-seine industry (Frost et al. 1976; levels of stocks (Crawford & Shelton 1978) or Crawford 1981), while Cape Gannets Monls ca­ bird populations (Duffy & Siegfried 1985), the ef­ pensis have increased or maintained their popu­ fect of nesting space on resident guano-producing lations in some areas because they can exploit offal species requires examination. If species from demersal trawlers, but have decreased in are restricted to only a portion of the total poten­ other areas where trawlers are not active (Craw­ tially available nesting space by narrow nesting­ ford & Shelton 1981). Reviews of these seabird­ habitat tolerances, interspecific avian compe­ fisheries interactions have assumed implicitly that tition, exclusion by Cape Fur Seals Arctocephalus food is more important than other factors in deter­ pusillus (Shaughnessy 1984), or availability of mining relative and total populations of each spe­ islands within foraging range of food-rich areas, cies (e.g. Crawford & Shelton 1978, 1981; Furness guano would be a poor index of bird numbers and & Cooper 1982). Burger & Cooper (1984) sug­ thus of fish stocks. gested a more explicit model of population regula­ More specifically, this paper examines three as­ tion for Benguela seabirds. Initially, before Euro- pects of the nesting ecology of Benguela seabirds 186 1985 DuFFY & LA CocK: SEABIRD NEST PARTITIONING 187 using new field data and historical records: (a) whether available space limited bird numbers, 20c~ Cape Cross 1 Platforms based on guano records, (b) species' use of differ­ . Platfor Walvis ent nesting habitats and (c) species' interactions I Sandwich Harbour Bay over potential nesting space. Hollams Bird Is, NAMIBIA Mercury Is. Although 14 species of seabirds nest or have lchaboe Is. luderitz nested in the Benguela upwelling region (Brooke 1981a), we examined possible interactions be­ tween only four species: the Jackass Penguin

(mean mass 3 kg), Phalacrocorax lamberts Bay capensis (mean mass 1,2kg), Bank Cormorant P. Dassen I aldanha neglectus (mean mass 2kg), and Robben 1 • Cape Town Seal Is. (mean mass 2,6 kg) (masses from Furness & Dyer Is. Cooper (1982) and Cooper (1985)). The penguin, gannet, and Cape Cormorant are the most abun­ 15 20 25 dant nesting of the upwelling region ( ap­ proximately 30 %, 21 %, and 46 % of total breed­ FIGURE 1 ing seabirds respectively (Cooper eta/. (1984)), while the Bank Cormorant is locally abundant on Location of study sites used on the southern Afri­ two South African islands off Namibia (Cooper can coastline and locations mentioned in the text. 1981). Cape Gannets (breeding population ap­ proximately 128 000 pairs (Crawford eta/. 1983)), nest at six sites, all originally islands although one Namibia and South Africa. Three islands were ex­ is now connected to the mainland by a causeway amined for longer-term nesting patterns using (Crawford et al. 1983). Cape Cormorants (breed­ aerial photographs. The northernmost study site ing population approximately 277 000 pairs was (25 43S; 14 50E; 3 ha). It is (Cooper et al. 1982)) nest at 54 sites, which include rugged and steep, approximately 42 m above sea islands, artificial platforms, moored boats, main­ level, with extensive cliffs and sea caves. All four land cliffs, and an island in a coastal sewage works study species nest on the island as well as White­ (Cooper et al. 1982; Brooke & Loutit 1984). The breasted Cormorants P. carbo. Crowned Cormo­ size of the Jackass Penguin population is un­ rants P. coronatus were present but no nests were known. Counts of breeding individuals during seen. There is also a rapidly increasing colony of November (the month of least breeding activity on Cape Fur Seals. There is no terrestrial plant life or many islands (Cooper 1980; Duffy et al. 1984)) permanent human presence. A row of deserted produced a minimum breeding population size of buildings, two landing stages, a wooden nesting 67000 pairs (Shelton et al. 1984). Cooper eta!. platform, and stone retaining walls represent the (1984) estimated 171 710 pairs but considered this only human modifications. It is unclear whether to be inaccurate. Penguins nest at 27 sites, includ­ Mercury ever had a guano cap of any thickness. ing two on the mainland (Shelton et al. 1984). There is almost no intertidal zone and the southern Bank Cormorants (breeding population approxi­ and western sides of the islands are exposed to mately 9 025 pairs) nest at 45 sites, including strong wave action. islands and smaller inshore rocks (Cooper 1981, (2617S; 14 56E; 6,5 ha) has little 1982). vertical relief. On the northern half of the island, there is a flat plain which has been extended with METHODS wooden nesting-platforms. In the south, a hill rises to approximately 20m. The same species are Study sites present as on Mercury, although seals are confined Fieldwork was conducted during 1981-1983 at to the outer rocks. A complex of houses for the guano islands under the control of the Marine De­ resident staff is located at the northern tip. Two velopment Branch (Fig. 1), along the coasts of wooden docks, also used for nesting, extend from 188 DuFFY & LA CocK: SEABIRD NEST PARTITIONING OSTRICH 56 the eastern side of the island. A 2-m sea wall en­ different species occupy nesting space. We limited closes almost the entire island. Ramps provide our investigation to factors which appeared to be passage over the wall for the birds but appear to important in partitioning nesting space among spe­ exclude seals. There is no vegetation. An exten­ cies in a similar nesting community in Peru (Duffy sive intertidal zone and offshore reefs surround the 1983b ). island. The guano cap of up to 20m thick was re­ Assuming that a high island would allow species moved between 1843 and 1845 (Hutchinson 1950; the greatest opportunity to partition space by Shaughnessy 1984). choice of substratum steepness, we studied habitat Malagas (or Malgas) Island (33 03S; 17 56E) is use on Mercury Island, the highest of the Bengu­ located at the mouth of in South elan islands. In February 1981, all available space Africa. It has an area of 8,5 ha and height of 7 m, above the intertidal zone was covered by nesting or composed of low rocky ridges and gravel/guano roosting birds. To avoid disturbing nesting birds, flats. The four study species and Crowned Cormo­ we estimated the inclination of slopes ( approxi­ rants nest on the island. A few low retaining walls mately 0,15 m) under nests, and the overall incli­ and a path for guano collectors are the only ap­ nation of 2m slopes with the nest at the centre (cf. parent human alterations in the nesting area since Duffy 1983b), for 108 or 109 nests each of Jackass the guano cap was removed in 1845 (Burman & Penguin, Cape Gannet, and Cape and Bank Cor­ Levin 1974), though several buildings at the east­ morants. We started on the east side of the island ern end house a permanent staff. There is little or and sampled every fifth nest westward until ten no vegetation during the summer breeding season nests had been sampled or until that species no but low herbs occur in winter. The intertidal zone longer nested on the transect. Transects were is extensive but exposed to strong surf, except spaced so as to cover as much of the island as poss­ along the eastern coast. ible. We also recorded proximity to the nearest 1 m Other fieldwork was carried out at Possession drop-off as an indication of ledge nesting. We Island (27 01S; 15 12E; 20m height; 90ha), Bird measured the sizes of subcolonies which we de­ Island, Lambert's Bay, (32 05S; 18 24E; 7 m fined as groups of nests with internest distances no height; 3ha), Vondeling Island (33 09S; 17 58E; greater than 5 m and no intervening nests of other 10m height; 21 ha), Dassen Island (33 26 S; 18 species. 05E; 19m height; 222 ha) and Dyer Island (34 41 S; To study longer-term overlap in use of nesting 19 25E; 6 m height; 20 ha). All these have at least areas by species, we used aerial photographs of three of the four study species nesting. The islands Ichaboe (November 1967, November 1969, 20 are described by Rand (1963a, b). November 1978, 9-13 February 1981), Mercury (November 1956, November 1969, 28 November Space 1978, 6 August 1979), and Malagas Islands (10 Guano harvests within Namibia (data from Sea November 1967, November 1969, 11 December Fisheries Research Institute (SFRI)) were com­ 1978,4 November 1980, 12 December 1980) taken pared from 1898 to 1936 when only small platforms by the Sea Fisheries Research Institute (Crawford were in existence (about 0,16 ha, Rand 1952), and & Shelton 1981; Shelton et al. 1982). Maps of nest­ from 1937 to 1959 after guano began to be col­ ing areas were traced from each photograph. All lected from the more extensive platforms (8 ha). the species were identified on Malagas and Icha­ The Namibian population of Cape Pilchard Sardi­ boe but the two cormorants could not reliably be nops ocellata collapsed after 1959 (Crawford & distinguished on photographs of Mercury. We as­ Shelton 1978, 1981) so that food shortages might sumed that the distribution of birds reflected the have affected seabird populations and guano pro­ distribution of active or future nesting areas. We duction in subsequent years. compared the percentage overlap of area used by one species in any season studied with that used by Area a different species in any other nesting season; the Not all available space may be suitable for nest­ percentage of area used exclusively by a species; ing. To test this, we examined the physical charac­ and the percentage of each island's entire area teristics of nesting sites and the ways in which the used by each species during the period covered by 1985 DuFFY & LA CocK: SEABIRD NEST PARTITIONING 189 the photographs.

Species interactions As in Peru and Galapagos (Duffy 1983b, 1984), interspecific interactions were so rare that watches of specific pairs to measure frequency of occur­ rence of such interactions were unproductive. We 0 3 z scanned tens to hundreds of pairs at the borders of <( ::> contiguous courting or nesting groups of two spe­ (!) ' cies where either or both were defending nest sites until we observed an interspecific interaction in progress. We followed this until one species or the other was displaced, or until neither species was 1900 1910 1920 1930 1940 1950 1960 able to displace the other (a "standoff") and both YEAR resumed other activities. Birds on nests were con­ FIGURE2 sidered to have "won" if they displaced the other species or if a "standoff" occurred. When both an­ Total Namibian guano extractions from 1898 to tagonists were either nesting or non-nesting, 1959, and Namibian platform guano extractions "standoffs" were recorded separately from dis­ from 1937 to 1959. placements. Only jabbing or fighting were con­ sidered interspecific aggression because other behaviour might not constitute communication forms in Namibia began in 1937 with 332 t. Most of between species at the or ordinal levels in­ the guano was produced by Cape Cormorants, the volved here. primary nesting species on the platforms (Rand Observations were made at Mercury (February 1952, 1963b ). Harvests increased rapidly to be­ 1982), Ichaboe (February 1982), Bird (Lambert's tween 1000 and 2 000 t/year (i = 1468,7 t; SD = Bay) (September 1981, October 1982), Malagas 73,9; n = 22; Fig. 2; data from SFRI: Rand 1952). (October 1981), Vondeling (April 1983), Dassen Mean production for all guano-producing sites off (November 1982), and Dyer (November 1981; Oc­ Namibia from 1898 to 1936 was 4085 t (SD = tober 1982) Islands. 1306,8; n = 38; 1904 data missing). After the ad­ dition of the platforms, yields rose to a mean of RESULTS 4 747,2 t (SD = 920,4; n = 23; 1958 data missing). The difference is not significant (t-test, p > 0,05), Space limitation suggesting that guano production and the Cape As in Peru (Jordan & Fuentes 1966; Duffy Cormorant population did not increase with the 1983b), changes in management practice for guano provision of additional nesting space. production allow insight into the birds' population Although over 400 ha of islands exist off South dynamics. Artificial nesting platforms were cre­ Africa (excluding Robben Island, 507 ha, which ated at Cape Cross (21 45S; 13 56E; 3,1 ha), Swa­ has been extensively modified by human activity kopmund (22 35S; 14 31E; 5,2ha) and (De Villiers 1971; Brooke 1983)) and approxi­ (22 57S; 14 29E; 1, 7 ha) along the Namibian Coast mately 200 ha on islands off Namibia, only 30% (Rand 1952, Berry 1975). If space had been lim­ of potentially available space on South African ited, this additional nesting area should have in­ islands (Duffy & Siegfried 1985) and 10% on creased guano harvests as breeding populations in­ islands off Namibia (data from Rand 1963a, b) creased. If birds merely moved from other sites, were being used by nesting birds in 1956. For ex­ while the total breeding population remained the ample, much of Dassen Island (222 ha or 53% of same, the platforms should have shown increased the total area of South African islands) is covered guano yields with overall production in Namibia in low shrubs which would be unsuitable habitat remaining the same. for nesting gannets or cormorants. A wall was Guano harvesting from artificial nesting plat- erected on Dassen Island sometime after 1930 to 190 DuFFY & LA CocK: SEABIRD NEST PARTITIONING OSTRICH 56 prevent penguins from nesting in the interior and 110 -- to facilitate egg collection (Frost et al. 1976). The interiors of several other islands appear to be suit­ I STANDARD ERROR able only for low-density nesting by penguins, because the islands are too rocky, too hot, or are en covered with vegetation. - 90 -,..- 1-en w Species tolerances of different nesting habitats z Nesting season 0 z 70 Bank Cormorants and Jackass Penguins can be - found nesting throughout the year, although both w show seasonal peaks which vary from island to N island. Most breeding occurs between January and en September (Rand 1960; Randall & Randall1981; >- 50 Duffy eta/. 1984). Cape Gannets have a fixed an­ z nual breeding cycle with nesting commencing in 0 ....1 the spring. Gannets may disperse from nesting 0 -;- islands for about three months during the non­ 0 breeding season in the winter (Rand 1959; Broek­ Ill 30 huysen eta/. 1961; Jarvis 1971; Randall et al. 1981; ::::> -- Randall 1983). Cape Cormorants tend to nest in en the spring and summer but exhibit great variability -- from year to year in South Africa (Siegfried et al. 1975; pers. obs. ). -- 10 Colony formation __._-r- I

The two cormorants and the penguin often have PENGUIN GANNET CAPE C. BANK C. relatively little synchrony of nesting state between adjacent nests or within a subcolony. For example, FIGURE3 on Mercury Island in February 1982 adjacent nests of both Bank and Cape Cormorants had eggs, Size of sub-colonies on Mercury Island; Cape C = young or courting pairs (Duffy pers. obs.). In con­ Cape Cormorant, Bank C = Bank Cormorant. trast, Cape Gannet nesting appeared to have be­ gun with a small nucleus which expanded outward with nonbreeders and the most recent nests at the photographs in Rand 1963a). periphery (pers. obs.). This pattern may be less clear for gannets at other colonies where strong at­ Site tolerance tachment to particular nest sites by pairs may lead to lack of synchrony between adjacent nests (A. Site tolerances appeared broadly similar for all Berruti pers. comm.). four species, but subtle differences occurred. On On Mercury Island, Cape Gannets and Jackass Mercury Island, Cape Gannets and Bank Cormo­ Penguins had the largest subcolonies (Fig. 3). rants placed their nests on flat or nearly flat sur­ Bank Cormorants formed the smallest subgroups. faces whereas Jackass Penguins and Cape Cormo­ On flatter islands, such as Ichaboe, Bird (Lam­ rants chose sites with up to 15° slopes (Table 1). bert's Bay), Malagas, Dassen and Dyer, subcol­ Considering the overall slopes of nesting areas, onies of all the species except Bank Cormorant are Cape Gannets and Bank Cormorants also tended much larger, numbering into the hundreds (cf to nest on nearly flat areas (<5° slopes); Cape Cor- 1985 DuFFY & LA CocK: SEABIRD NEsT PARTITIONING 191

TABLE 1 PERCENTAGE OF THE FOUR SPECIES NESTING ON DIFFERING DEGREES OF SLOPE (MEASUREMENT INTERVAL; 0,15 M) UNDER NEST SITES ON MERCURY ISLAND

Angle of Cape Gannet Jackass Penguin Cape Cormorant Bank Cormorant slope (n = 108) (n = 108) (n = 109) (n = 109) oo 20 55 39 75 1-5° 74 19 28 25 6-15° 6 26 33 0

TABLE2 PERCENTAGE OF THE FOUR SPECIES NESTING ON DIFFERING DEGREES OF OVERALL SLOPE (MEASUREMENT INTERVAL: 2M) ON MERCURY ISLAND

Angle of Cape Gannet Jackass Penguin Cape Cormorant Bank Cormorant slope (n = 108) (n 108) (n 109) (n = 109) oo 7 22 18 38 1-SO 64 30 0 28 6-15° 26 30 48 18 16-ZSO 3 19 34 17 morants concentrated on slopes; and Jackass Pen­ TABLE3 guin nesting appeared independent of slope (Table PERCENTAGE OF NESTS OF THE FOUR SPECIES IN RELATION 2). Gannets and penguins nested away from drop­ TO DISTANCE TO NEAREST ONE-METRE DROPOFF offs, whereas Cape and Bank Cormorants oc­ Distance from edge curred more frequently in these areas (Table 3). ~pecies <1m >1m On islands such as Dassen and Dyer, where only Jackass Penguin (n = 108) 29 71 penguins and Cape Cormorants nested in large Cape Gannet (n 108) 19 81 Cape Cormorant (n 109) 58 42 numbers, penguins tended to occupy flats, either Bank Cormorant (n 109) 66 34 with burrows or surface nests, and cormorants rocky outcrops or flats. On Possession, Bird (Lam­ bert's Bay) and Malagas, where Cape Gannets oc­ TABLE4 curred, Cape Cormorants also nested on rocky NUMBER OF BREEDING PAIRS ON MALAGAS, lCHABOE, AND MERCURY ISLANDS: 1978-1980 (ROUNDED TO NEAREST outcrops. Penguins did not nest in surface colonies HUNDRED; DATA FROM COOPER 1981; CRAWFORD 1981; on Malagas. Bank Cormorants on South African CRAWFORD & SHELTON 1981; COOPER et a/.1982; CRAW- islands (e.g. Bird (Lambert's Bay), Marcus, Mala­ FORD eta/. 1983; SHELTON eta/. 1984). gas, Jutten, Dassen, Dyer) and Possession Island off southern Namibia nested on giant boulders or Island rocky outcrops adjacent to the sea (Rand 1963a, b; Species Malagas Ichaboe Mercury Williams 1978). Island populations never ex­ Jackass Penguin 600 3 600 3 200 Cape Gannet 21 700 28 400 1 900 ceeded 211 nests and averaged 53 nests (Cooper Cape Cormorant 8 700 45 800 8 800 1981). In contrast, on the central Namibian islands Bank Cormorant 150 4 100 2 300 of Ichaboe and Mercury, Bank Cormorant popu­ lations were 4 102 and 2 305 pairs respectively in present a decreasing gradient of steepness. Mer­ 1978 (Cooper 1981), and nesting extended on to cury has few level areas while Malagas is generally nest sites not immediately adjacent to the sea flat. All four species were present on the three (pers. obs.). islands, but their relative abundances differed (Table 4). A maximum of 42% of the area of Ma­ Long-term patterns of use lagas has been used for nesting over the years (Fig. 4a-e). Cape Cormorants have occupied a maxi­ The islands of Mercury, Ichaboe, and Malagas mum of 27 % over the years, Cape Gannets 26 %, 192 DuFFY & LA CocK: SEABIRD NEST PARTITIONING OSTRICH 56

and Bank Cormorants 1 %. Jackass Penguins A Malagas Is. Nov. 1967 nested under rocks, in burrows, or in groups too small to detect on photographs. Bank Cormorants nested on large boulders along the intertidal zone. Overlap of use between Cape Gannets and Cape Cormorants was 9 % over the years and less than 1% for other species-pairs (Table 5). On Ichaboe (Fig. 5a-e), 94% of the island has been used for nesting, 87 % used by Cape Gannets in one year or another, 43% by Cape Cormorants, 7% by Jack­ ass Penguins and 14% by Bank Cormorants. The island has been almost completely covered with nesting and roosting birds throughout the period covered by the photographs. Overlaps of nesting were slightly higher than on Malagas (Table 5), the most extensive being between Cape Cormorants and Cape Gannets (21 %) . On Mercury (Fig. 6a-d) 82% has been used by nesting birds. Unfor­ tunately, we could not distinguish Cape and Bank Cormorants so values are presented only for com­ bined cormorants. Gannets used 14% of the Key island's area over the years, cormorant species 1/ll Gannet used 69 % and penguins 35 %. Cape Cormorant ~ Bank Cormorant Interspecific Interactions Cape Cormorants on nest sites won 99% (n = 108) of interactions with non-nesting Cape Gan­ B Malagas Is. Nov. 1969 nets. A non-nesting gannet displaced a cormorant from its nest once, then displayed briefly before leaving. Gannets on nests displaced non-nesting Cape Cormorants in all three encounters. When neither was on the nest, gannets won four and nei­ ther species won the other four. When both species were on the nest (n = 20), neither won in 95% of the incidents but again a gannet displaced a cormo­ rant in one encounter. In encounters between Jackass Penguins and Cape Gannets, we observed interactions only be-

TABLES PERCENTAGE OVERLAP OF NESTING AREAS BETWEEN YEARS, BASED ON AERIAL PHOTOGRAPHS ( t = LESS THAN 1 %; * = BANK AND CAPE CORMORANTS NOT SEPARATED ON MER- CURY ISLAND) Species overlapping Malagas lchaboe Mercury Gannet-penguin t 37% 38% Gannet-Cape Cormorant 9% 21% 13% Gannet-Bank Cormorant 0 9% . Penguin-Cape Cormorant t 10% 9 %* Penguin--Bank Cormorant 0 2% • Key Cape Cormorant-Bank '§@'Gannet Cormorant 0 5% . Cepe Cormorant Bank Cormorant 1985 DuFFY & LA CocK: SEABIRD NEST PARTmONING 193

C Malagas Is. Dec. 1978 E Malagaa Is. Dec. 1980

Key Key lil!Gannet 11!! Gannet C.~ Cormorant Cape cormorant II Sank Cormorant Bank Cormorant

FIGURE4A-E (Pages 192-193)

D Malagas Ia. Nov. 1980 Areas occupied by study species in different years on Malagas Island.

tween nesting penguins and non-nesting gannets (n = 9). Penguins won eight of nine encounters with gannets but a nesting penguin was displaced once. In interactions between Cape Cormorants and penguins, nesting penguins won all seven in­ teractions with non-nesting cormorants. Nesting cormorants won 12 of 13 interactions with non­ nesting penguins, but a nesting Cape Cormorant was displaced once. Standoffs occurred in the two cases where both species were on nests, but pen­ guins displaced cormorants in three of the four cases where neither was on a nest. Bank Cormorants on nests displaced non-nest­ ing penguins twice but when both were on nests, the only encounter observed resulted in a standoff. Penguins won both encounters when neither was l0L_ on a nest. Nesting gannets won twice over non­ nesting Bank Cormorants. Nesting Cape (n = 5) 'lil GannetKey ~ and Bank (n = 1) Cormorants displaced non­ Bank Cormorant breeding members of the other species. Standoffs 194 DuFFY & LA CocK: SEABIRD NEST PARTITIONING OSTRICH 56

A lchaboe Is. 191 0 B lchaboe Is. Nov. 1967 '

Key Key ~ Gannet ill* Gannet Penguin

Cape Cormorant 11111 Bank Cormorant IIi"'! Bank Cormorant

FIGURE 5A-E (Pages 194-196)

Areas occupied by study species in different years on Ichaboe Island. occurred when both were on the nest (n = 5) or nesting space on wooden platforms. were not defending sites (n = 1). Cooper et al. (1982) have suggested an alterna­ tive explanation that the artificial nesting plat­ DISCUSSION forms merely replaced natural sites which were be­ coming unsuitable because of topographical Is space limited? changes in Cape Cross lagoon (2143S; 13 SSE) and Although nesting space may be locally limited, at Sandwich Harbour (23 22S; 1413E). There is no as perhaps on Mercury and Ichaboe Islands, or un­ evidence of breeding by Cape Cormorants on es­ available, as in the areas between Lambert's Bay tuarine islands in Sandwich Harbour south of Wal­ and the islands off southern Namibia or between vis Bay (22 57S; 14 29E), before the creation of the Mercury Island and Walvis Bay, space does not ap­ platforms (Anon. 1885; Rand 1952). Cooper et al. pear to have limited the overall population of Cape (1982) give a single recent breeding record for this Cormorants, the main species using the platforms, site of 15 individuals in 1971. Guano harvests from along the Namibian coast, based on the failure of Sandwich Harbour actually increased after the the guano harvest, and presumably the popu­ construction of the platforms around Walvis Bay lation, to increase with the provision of additional although the difference in amount harvested is not 1985 DuFFY & LA CocK: SEABIRD NEsT PARTITIONING 195

C lchaboe Is. Nov. 1969 o lchaboe Is. Nov. 1978

Key

)® Gannet Key Penguin Gannet illll Cape Cormorant ,.;. Penguin Bank Cormorant Cape Cormorant r:iM Bank Cormorant

statistically significant ( p > 0,05; Fig. 7; data from Berruti pers. comm. ). Cape Gannets are known to Rand (1952): before platforms i = 868 t, SD = have colonized Bird Island (Lambert's Bay) in or 271; after platforms established i = 1089 t, SD = just before 1912 and Possession and Halifax 638). If birds had left breeding colonies in the area, Islands sometime in the late 19th century, and to harvests should have decreased. The Sandwich have recolonized Ichaboe and Malagas Islands fol­ Harbour site and others along the coast may have lowing the disruptions of the initial period of been winter roosts for nonbreeding Cape Cormo­ guano extraction (Crawford et al. 1983). The col­ rants which migrated north from the guano islands onization of Halifax and Possession may have been off southern Namibia ( cf. Anon. 1885; Hutchinson linked to the events on Ichaboe but there seems to 1950; Brooke 1981 b). be no explanation of the colonization of Bird Cape Gannet populations do not appear to have Island (Lambert's Bay). been limited by a shortage of topographically suit­ Bank Cormorants do not appear to be limited by able nesting islands, although space on the islands space on South African islands. Very large bould­ may have been occupied by other species. Hollams ers at the edges of islands appear to be favoured Bird (24 38S; 14 32E), Halifax (26 37S; 15 04E), but boulders or bedrock outcroppings on the Seal () (34 08S; 18 35E) and perhaps islands themselves are also used (Cooper 1981; Dyer Island are former gannetries (Crawford eta/. pers. obs.). The construction of a causeway to 1983; Shaughnessy 1984). Only Dyer has been re­ Marcus Island rendered one nesting rock unsuit­ colonized by gannets and then only in late 1984 (A. able, but the cormorants moved to other rocks on 196 DuFFY & LA CocK: SEABIRD NEST PARTITIONING OSTRICH 56

E lchaboe Is. Feb. 1981 3

WALVIS BAY PLATFORMS CONSTRUCTED

2

0 ...z :::J CJ

1930 1935 1940 1945 YEAR

FIGURE7

Guano harvests from Sandwich Harbour for 1927 to 1947.

form very large dense colonies on flatter islands such as Ichaboe, Bird Island (Lambert's Bay), and

Key Malagas, and much smaller groups on steeper Gannet Mercury. Cape Cormorants nest mainly on rocky Penguin outcrops or very steep slopes not used by gannets. Cape Cormorant They also nest on mainland cliffs (Cooper et al. Bank Cormorant 1982). Penguins form large surface or burrow col­ onies on flat areas on islands where gannets do not occur (Dassen) but, in the presence of gannets, oc­ cupy smaller colonies on both slopes and flats the island (Cooper 1981), suggesting that space (Mercury, Ichaboe), as well as rock crevices and had previously not been limiting. On the islands burrows (Possession, Bird Island in Lambert's off Namibia, nesting space for Bank Cormorants Bay, Mala gas). Mainland nesting is minimal (Shel­ may be limited by Cape Cormorants (Crawford & ton et al. 1984). Bank Cormorants nest only in Shelton 1981 ), or more probably by gannets, since small colonies on the peripheries of islands off gannets and Bank Cormorants appear more simi­ South Africa and of Possession Island off Namibia. lar in their nest site preferences. Larger populations of Bank Cormorants occur on Ichaboe and Mercury Islands, presumably because Nesting habitat of large stocks of Pelagic Go by Sufflogobius bibar­ Species of seabirds appear to have generally batus (Crawford & Shelton 1981; Cooper et al. similar nesting requirements on guano islands in 1982), and perhaps because these islands are the Benguela upwelling region, so that there is cooler than those in South Africa (Siegfried et al. considerable potential for competition for space 1975). On these islands, Bank Cormorants also where it is locally in short supply. The interspecific nest in habitats more characteristic of gannets and differences, nevertheless, appear sufficient to re­ Cape Cormorants. sult in relatively little overlap in areas used by the There appears to be a dominance hierarchy of different species over the years, based on the aerial gannet> penguin> cormorants but it must be rela­ photographs. tively weak since interspecific displacements from Cape Gannets nest mainly on flat areas. They nests were rarely observed, even when colonies of 1985 DuFFY & LA CocK: SEABIRD NEsT PARTITIONING 197 two species were contiguous. Such a weak domi­ birds. Several of the islands had guano caps which nance hierarchy would appear relatively unimpor­ would have considerably altered their primeval tant in partitioning potential nesting space on the shapes, providing substrata for burrowing pen­ islands. guins. These caps were removed in the mid-19th Possession of a nest site seems to be a more im­ century (Hutchinson 1950). There is no evidence portant determinant of success in defending nest for prehistoric man exploiting birds or seals on the sites than species, as also occurs in Peru and Gala­ islands since prehistoric cultures did not possess pagos (Duffy 1983b, 1984). The presence of indi­ boats in (Avery & Siegfried 1981), viduals of one species already defending nest sites but heavy hunting by early European visitors may limit nesting of a second species, if the latter (Raven-Hart 1967) reduced seal populations arrived later. For example, penguins, although not which might previously have restricted nesting dominant in interspecific interactions may, by oc­ space for seabirds (Shaughnessy 1984). The Euro­ cupying nesting colonies throughout the year, pre­ peans also slaughtered penguins and gannets vent nesting by gannets over large areas of other­ (Rand 1949), reducing bird populations which wise suitable islands. On Ichaboe in the 19th might have expanded to fill the space vacated by century, most of the avian guano was produced by the seals. More recently, commercial egg-harvest­ penguins (Hutchinson 1950 and sources therein). ing reduced penguin populations (Siegfried & The removal of the guano cap and disturbance Crawford 1978). The construction of walls, sup­ caused by its exploitation apparently affected the posedly to concentrate penguin nests for easier egg penguins more than the gannets (Avon. 1885; Hut­ collection or to exclude penguins from nesting chinson 1950), perhaps allowing gannets more areas of other species (Anon. 1885; Sclater 1904; space for nesting. Green (1950) reported that early Rand 1963a), may also have reduced populations 20th-century photographs show Bird Island (Lam­ by drastically decreasing penguin nesting space on bert's Bay) covered with nesting penguins and islands such as Dassen and Ichaboe. without gannets. Die-offs of penguins were re­ Collapses of fish stocks also appear to have been ported in 1899, 1902, and 1910 (Gilchrist 1914; particularly damaging to penguin populations Hutchinson 1950) which might have opened up since these flightless birds need predictable, ac­ more nesting s~ace for cormorants and gannets. cessible food supplies (Frost et al. 1976) and can­ Indeed, the summer breeding habits of the gannet not exploit new, more distant food sources as can may have evolved to allow use of space on islands gannets (Crawford & Shelton 1981). All these when penguin nesting is least frequent. Popu­ complications make a reconstruction of species' lations of cormorants in the 20th century may have abundances on the primeval islands virtually im­ been limited on islands such as Mercury and Icha­ possible (Shaughnessy 1984), but conclusions in­ boe that were formerly occupied by large popu­ ferred from observations of crowded islands today lations of gannets. On these two islands, gannets should be indicative of past mechanisms of re­ have decreased between 1956 and 1978 (cf. Rand source partitioning. 1963b; Crawford & Shelton 1981) and there has been a corresponding increase of Cape and, more CONCLUSION recently, Bank Cormorants. Although the in­ creases may have been responses to more abun­ The creation of additional nesting space did not dant prey populations, such increases would not result in increased guano harvests and presumably have been possible if gannets had continued to oc­ greater bird numbers in the Namibian part of the cupy as much nesting space as previously. Benguela upwelling region, as it did in Peru in the 1950s (Duffy 1983b ), suggesting that even before Historical complications the creation of artificial sites, space did not limit The partitioning of nest space described above bird numbers in that part of the Benguela may be a relatively recent one, the partial result of ling region. The evidence for present-day hunting pressure by man, removal of guano and specific competition for nesting space between Cape Fur Seals by man, and the collapse of pil­ Benguela species is not strong, despite chard and populations exploited by the in nesting-habitat preferences. 198 DuFFY & LA CocK: SEABIRD NEsT PARTITIONING OSTRICH 56

A Mercury Is. Nov. 1956 B Mercury Is. Nov. 1969

0

Key Key

§§! Gamet 'fJJ1 Gaooet .. Penguin :> Penguin Cormorant• Cormorants 1985 DuFFY & LA CocK: SEABIRD NEST PARTmoNING 199

c Mercury Is. Nov. 1978 D Mercury Is. Aug.1979

Key w. Gannet Key Penguin f/IJ, Gannet .. Penguin Cormorants

F!GURE6A-D (Pages 198-199)

Areas occupied by study species in different years on Mercury Island. 200 DuFFY & LA CocK: SEABIRD NEsT PARTITIONING OSTRICH 56

Other factors such as disease and ectoparasites 1961. The South African Gannet Morus capensis. 1. may have affected populations in the Benguela up­ Distribution and movements. Ostrich 32: 1-19. BROOKE, R. K. 1981a. The place of South Africa in the welling region, as they have in Peru (Vogt 1942; world of seabirds and other marine . In Duffy 1983c). Nesting failures and adult die-offs CooPER, J. (Ed.). Proceedings of the symposium on during El Nino-like events in southern Africa may birds of the sea and shore, 1979. Cape Town: African have been important (Hutchinson 1950; Duffy et Seabird Group: 135-147. al. 1984; La Cock MS) but certainly not to the ex­ BROOKE, R. K. 1981b. The seabirds of the Mocamedes province, Angola. Gerfaut71: 209-225. tent encountered in Peru (Vogt 1942; Jordan & BROOKE, R. K. 1983. On the 17th century avifauna of Fuentes 1966; Duffy 1983a). Human exploitation Robben Island, South Africa. Cormorant 11: 15-20. of birds and seals (Siegfried & Crawford 1978; BROOKE, R. K. & LouTIT, R. 1984. Marine cormorants Shaughnessy 1984) may have been the most im­ using moored boats as nest sites in southern African west coast harbours. Cormorant 12: 55-59. portant factor affecting Benguela seabird popu­ BuRGER, A. E. & CooPER, J. 1984. The effects of fisher­ lations. More recently, the close relationship be­ ies on seabirds in South Africa and Namibia. In NET­ tween changes in fish stocks and guano harvests TLESHIP, D. N., SANGER, G. A. & SPRINGER, P. F. during commercial fishing (Crawford & Shelton (Eds). Marine birds: their feeding ecology and com­ 1978) suggests that food presently limits seabird mercial fisheries relationships. Special Publications. Ottawa: Canadian Wildlife Service: 150-160. populations in the Benguela as it has in Peru dur­ BURMAN, J. & LEVIN, S. 1974. The Saldanha Bay Story. ing overfishing (Duffy 1983a). While Benguela Cape Town: Human & Rousseau. seabird numbers may ultimately be limited by CooPER, J. 1980. Breeding biology of the Jackass Pen­ food, a changing array of other factors may have guin with special reference to its conservation. In JoHNSON, D. N. (Ed.). Proceedings of the Fourth more immediate effects at particular times or Pan-African Ornithological Congress, Mahe 1976. Jo­ places. Inferences on food supplies drawn from hannesburg: South African Ornithological Society: historical records of bird numbers may be mislead­ 227-231. ing, if these other factors are not considered. CooPER, J. 1981. Biology of the Bank Cormorant, Part 1: distribution, population size, movements and con­ servation. Ostrich 52: 208-215. ACKNOWLEDGMENTS CooPER, J. 1982. New breeding locality data for south­ We would like to thank the Fisheries Development ern African seabirds. Cormorant 10: 57. Branch and the Sea Fisheries Research Institute for per­ CooPER, J. 1985. Biology of the Bank Cormorant, Part mission to work on the islands and for considerable lo­ 2. Morphometries, , bare parts and moult. gistical support without which this paper would have Ostrich 56:79-85. been impossible. A. Berruti, M. J. de Duffy, B. L. Fur­ COOPER, J., BROOKE, R. K., SHELTON, P. A., & CRAW­ ness, D. Leng, and B. Stead provided assistance in the FORD, R. J. M. 1982. Distribution, population size and field. A. Berruti, F. Kriel, P. A. Shelton and C. Thomas conservation of the Cape Cormorant Phalacrocorax facilitated out examination of aerial photographs. A. capensis. Fish. Bull. S. Afr. 16: 121-143. Berruti, J. Cooper, W. R. Siegfried, and R. P. Wilson CooPER, J., WILLIAMs, A. J., & BRITTON, P. L. 1984. commented on the manuscript. This paper is a report of Distribution, population sizes and conservation of the Benguela Ecology Programme, sponsored by the breeding seabirds in the Afrotropical region. In CRox­ South African National Committee for Oceanographic ALL, J.P., EvANS, G. H. & ScHREIBER, R. W. (Eds). Research. The status and conservation of seabirds. Technical Report 2, Cambridge: International Council for Bird This publication is a contribution to the celebrations Preservation. of the 25th anniversary of the Percy FitzPatrick Institute CRAWFORD, R. J. M. 1981. Distribution, availability and of African Ornithology, University of Cape Town. movements of anchovy Engraulis capensis off South Africa, 1964-1976. Fish. Bull. S. Afr. 14: 51-94. REFERENCES CRAWFORD, R. J. M. & SHELTON, P. A. 1978. Pelagicfish and seabird interrelationships off the coasts of South ANON. 1885. Proceedings of the Angra Pequena and West and South Africa. Biol. Conserv. 14: 85-109. West Coast Joint Commission. March-September CRAWFORD, R. J. M. & SHELTON, P. A. 1981. Population 1885. Cape Town: Saul Solomon. trends for some southern African seabirds related to AvERY, G. & SIEGFRIED, W. 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S. Afr. J. Mar. Sci. 1: 153-174. birds. The distribution, abundance and feeding habits DE VILLIERS, S. A. 1971. Robben Island. Cape Town: of the Cape Gannet Morus capensis off the south­ Struik. western coast of the Cape Province. Invest! Rep. Div. DuFFY, D. C. 1983a. Environmental uncertainty and Fish. UnionS. Afr. 41: 1-28. commercial fishing: effects on Peruvian guano birds. RAND, R. W. 1960. The biology ofthe guano-producing Bioi. Conserv. 26: 227-238. sea-birds. 3. The distribution, abundance and feeding DuFFY, D. C. 1983b. Competition for nesting space habits of the cormorants, Phalacrocoracidae off the among Peruvian guano birds. Auk 100: 680-688. south-western coast of the Cape Province. Invest! DuFFY, D. C. 1983c. The ecology of tick parasitism on Rep. Div. Fish. UnionS. Afr. 42: 1-32. densely nesting Peruvian seabirds. Ecology 64: RAND, R. W. 1963a. The biology of guano-producing 110-119. sea-birds. 4. Composition of colonies of the Cape DuFFY, D. C. 1984. Partitioning of nesting space by Islands. Invest! Rep. Div. Sea Fish. S. Afr. 43: 1-32. Masked and Blue-footed on Isla Espanola, RAND, R. W. 1963b. The biology of guano-producing Galapagos. Condor86: 301-304. sea-birds. 5. Composition of colonies on the South DuFFY, D. C., BERRUTI, A., RANDALL, R. M. & CooPER, West African islands. Invest! Rep. Div. Sea Fish. S. J. 1984. Effects of the 1982-3 warm water event on the Afr. 46: 1-26. breeding of South African seabirds. S. Afr. J. Sci. 80: RANDALL, R. M. 1983. Biology of the Jackass Penguin 65-69. Spheniscus demersus (L.) at St Croix Island, South DuFFY, D. C. & SIEGFRIED, W. R. 1985. Historical vari­ Africa. University of Port Elizabeth: Ph. D. thesis. ations in food consumption by breeding seabirds of RANDALL, R. M. & RANDALL, B. M. 1981. The annual the Humboldt and Benguela upwelling regions. In cycle of the Jackass Penguin Spheniscus demersus at St CROXALL, J.P. (Ed.). The feeding ecology of seabirds Croix Island. In CooPER, J. (Ed.). Proceedings of the and their role in marine ecosystems. Cambridge: symposium on birds of the sea and shore, 1979. Cape Cambridge University Press. Town: African Seabird Group: 427-450. FRosT, P. G. H., SIEGFRIED, W. R. & CooPER, J. 1976. RANDALL, R. M.' RANDALL, B. M., BATCHELOR, A. L. & Conservation of the Jackass Penguin (Spheniscus de­ Ross, G. J. B. 1981. The status of seabirds associated mersus) (L.). Bioi. Conserv. 9: 79-99. with islands in , South Africa, 1973-1981. FURNESS, R. W. & CooPER, J. 1982. Interactions be­ Cormorant 9: 85-104. tween breeding seabird and pelagic fish populations in RAVEN-HART, R. 1967. Before Van Riebeeck. Cape the southern Benguela region. Mar. Ecol. Prog. Ser. Town: Struik. 8:243-250. ScLATER, W. L. 1904. Saldanha Bay and its bird-islands. GILCHRIST, J.D. F. 1914. An enquiry into fluctuations in Ibis 8( 4): 79-88. fish supply on the South African coast. Mar. Biol. SHAUGHNESSY, P. D. 1984. Historical population levels of Rep. (C.P.6-14)2:8-35. seals and seabirds on islands off southern Africa, with GREEN, L. G. 1950. At daybreak to the isles. Cape special reference to Seal Island, False Bay. Invest! Town: Howard Timmins. Rep. Sea Fish. Res. Inst. S. Afr. 127: 1-61. HuTCHINSON, G. E. 1950. Survey of existing knowledge SHELTON, P. A .. CRAWFORD, R. J. M., CooPER, J. & of biogeochemistry, Vol. 3. The biogeochemistry of BROOKE, R. K. 1984. Distribution, population size and vertebrate excretion. Bull. Amer. Mus. Nat. Hist. 96: conservation of the Jackass Penguin (Spheniscus de­ 1-554. mersus). S. Afr. J. Mar. Sci. 2:217-257. JARVIS, M. J. F. 1971. Interactions between man and the SHELTON, P. A., CRAWFORD, R. J. M., KRIEL, F. & South African Gannet Sula capensis. Ostrich Suppl. 8: CooPER, J. 1982. Methods used to census three species 492-513. of southern African seabirds, 1978-1981. Fish. Bull: JORDAN, R. & FuENTES, H. 1966. Las poblaciones de S.Afr. 16:115-120. aves guaneras y su situaci6n actual. Inf. Inst. Mar SIEGFRIED, W. R. & CRAWFORD, R. J. M. 1978. Jackass Peru-Callao 10: 1-31. Penguins, eggs and guano: diminishing resources at LA CocK, G. D. MS. The Southern Oscillation, environ­ Dassen Island. S. Afr. J. Sci. 74: 389-390. mental anomalies, and mortality of two southern Afri­ SIEGFRIED, w. R., WILLIAMS, A. J., FROST, P. G. H. & can seabirds. Climatic Change. (submitted). KINAHAN, J. B. 1975. Plumage and ecology of cormo­ RAND, R. W. 1949. Some early references to the Cape rants. Zoo!. Afr. 10: 183-192. Penguin. Ostrich 20: 2-5. VoGT, W. 1942. Aves guaneras. Bal. Camp. Admora RAND, R. W. 1952. Guano enterprise in South West Guano Peni 18: 1-132. Africa. Ostrich 23: 169-185. WILLIAMS, A. J. 1978. Nests and cormorant biology. Cor­ RAND, R. W. 1959. The biology of guano-producing sea- morant4: 22-27.