de Vries, Tjitte

Aquatic Commons

WHY ARE FRIGATE-BIRDS COLONIAL? 
by 
Tjitte de Vries 
Universidad Ca t6lica del Ecuador. Apartado 2184. Quito 
Colonial breeding occurs in many bird species. Some of the selective forces favouring coloniality are: 
(I) reduced nest predation resulting from communal defence ; 
(2) predator swamping; 
(3) shortage of suitable nest sites ; 
(4) increased foraging efficiency; and 
(5) socially induced breeding. 
Only the last of these forces seems to be related to the natural history of the frigate-birds, of which there are 
two species in the Galapagos - Fregata magnificens and Fregata minor. Since 1975, together with a group 
of students from the Universidad Cat61ica of Quito, I have studied frigate-birds on Tower and Seymour 
(Coello et a l. 1977). With my wife Cecilia Hernandez I have visited Tower every year since then in 
July/August (except in 1978 when Ko de Korte went) to observe the colony of F. minor in Darwin Bay. 
With almost one thousand birds ringed, more has now become known on breeding cycles of individually 
marked birds and the first age of breeding. In 1983 Carlos Valle joined the frigate-bird project 
investigating inter-island movements and feeding ranges with dyed birds , concentrating on the colonies of 
F. minor on Tower, Pitt, and Hood Island. The present story refers mainly to Fregala minor on Tower. 
Oceanic islands rarely have land predators and avian predators are insignificant on Tower; bushes and 
trees for nesting are abundant (Cryp tocarpus pyriformis and Bursera gra veolens). Fregala minor feeds far 
out in the ocean, perhaps sometimes in groups following schools of tuna to catch flying fish which are 
trying to escape from the hunting tuna. 
Piratical Frigatebirds Photograph by Fritz Pol king 
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Noticias de Galapagos, vol. 40 1984
Is aggression inevitable? 
In "The Biology of War and Peace", Eibl-Eibesfeldt devotes a chapter to the question: Is aggression 
inevitable? I have often pondered this question while observing frigates. As they seem so destructive to 
their own species, why should they be colonial? They could easily avoid each other. Males will not only 
destroy an unattended nest by taking the twigs, but will also devour the eggs or smal\ chicks. Food robbery 
is a frequent occurrence, particularly at times when a parent is regurgitating food for large young. When 
the young are less than three weeks old, feeding takes places less conspicuously while the parent is still 
brooding the chick; but when the parent comes flying in to feed the older, noisily waiting young, several 
males will be seen soaring above, ready to dive and snatch the fish away with a rapid stab and grab 
movement. In this context it is interesting to note that in a colony of F. minor a number of chicks will start 
calling, in contrast to F. magnijicens colonies, where it is only the proper chick which responds to the 
calling parent (Coello et al. 1977). 
One could also ask why it is that only the males are so piratical? Without-having a definite answer it is of 
interest to note that the male is the smaller of the sexes and may consequently be more rapid and 
manoeuvrable (see de Vries, 1980). Also, it would be of interest to know which males are involved in food 
robbery - the non-breeding, moulting males, which would be at a disadvantage in hunting flying fish as 
they lack two or more primaries and some of their tail feathers, or the non-moulting, breeding males? I 
suspect it will turn out to be the non-breeders. 
Does such aggressive behaviour by the males perform functions in the service of the preservation of the 
species? Wynne-Edwards (1967) treats the eating of unprotected nestlings in a chapter on "socially-
induced mortality". 
Frigate natural history 
Several aspects of frigate biology demonstrate that there is a tendency to keep productivity low. Frigates 
lay a single egg and make such a flimsy nest that, even if the egg does not fall through, it can easily roll off. 
Eggs and small young of their own species are eaten when left unattended; slightly larger young are used in 
a cruel game, being passed like a ball from bill to bill until dropped and left to die. Both parents (in F. 
minor) attend to their young for as long as 16 months and so have a breeding cycle of two years. However, 
it was found that many ringed birds did not start breeding again for three or even four years, so perhaps 
most females only succeed in rearing one juvenile every five years. 
Our ringing of breeding pairs revealed that birds mated just for one breeding season and did not return to 
their previous nest site. So far we have noted two exceptions: two males that lost their egg were breeding on 
the same site the next year, but with another female. Some 15-20% of previously ringed birds were found to 
have returned to the same colony at Darwin Bay (Hernandez, in prep.), where on average 180 pairs breed 
yearly (Hernandez and de Vries, in press). Although other colonies on Tower were checked, no ringed 
birds were found there, but no doubt these are not so easy to encounter in a population of well over 20,000 
birds. 
It was calculated that in August 1982 some 4000 pairs were breeding on Tower. Given the same number 
breeding the year before and therefore occupied with moult and/or taking care of juveniles, and half as 
many moulting birds that had bred successfully two years before, the population would be well over 
20,000 adult birds. No doubt a successful species - as far as numbers are concerned. With no predators at 
sea or on land except themselves (and a very occasional Lava Gull), the popUlation has a regulatory valve 
only when young. 
Few ringed young have so far returned to their place of origin, but preliminary results suggest that on 
Tower F. minor does not breed before the age of six or seven years (Hernandez, in prep.). Such a delay in 
maturity is a further factor in controlling the numbers of the population. 
Colonial versus solitary 
But let us return to our first question; why are frigates colonial? On Tower there are about five major 
colonies of some 200-600 breeding pairs each, whereas the rest (perhaps slightly less than 50%) are 
breeding in smaller groups or in some cases in complete isolation. Future studies will be needed to 
demonstrate whether these isolated pairs are less successful in breeding and whether they continue to be 
isolated breeders or later become colonial. 
There is still another aspect of frigate-bird life to be taken into account. Is it the male who chooses the nest 
site or, if not, and it is in fact the female who chooses, then one would expect that she would remain faithful 
to the nest site, a supposition which was found not to be true. 
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Noticias de Galapagos, vol. 40 1984
Male Frigatebird carrying nesting material Photograph by Fritz Polking 
If females are attracted by a group of males displaying together and ignore isolated male display, then this 
would be a factor which would favour aggregation in frigates. This seems in agreement with observations 
that, within a colony, one finds small groups which are at the same breeding stage. If one male is successful 
in obtaining a female, then it seems that other females quickly decide to pair with the neighbouring males 
- a form of socially induced breeding. Subadult and nearly adult males often fight for an abandoned nest 
(where an egg or chick recently disappeared) even at the end of the breeding season. I could often catch and 
ring four or five males in a quarter of an hour when they came to roost at such a site . 
On Tower eggs are laid over a period offour months (from the end of February till the end of June; in July 
and August an odd male may still be found displaying). Different islands have different breeding seasons, 
a fact not well understood, but most likely related to distinct feeding regions. Differences in timing of 
breeding are not uncommon in sea birds; in Galapagos it was also recently discovered that the Dark-
rumped Petrel (Pterodroma phaeopygia) had distinct breeding seasons on islands as close to one another as 
Roreana, Santa Cruz and Santiago. 
Ringing results have not shown that the same frigate will breed on more than one island. 
An anti-autopredatory strategy 
There exist breeding associations between two or more species where at least one species is a potential 
predator of the other. Wiklund (1982) demonstrated that Fieldfares breeding away from Merlins had a 
lower breeding success than pairs associated with Merlins. However, these results in a lowland deciduous 
forest are in contrast with studies by Hogstad (1983) in a homogeneous birch forest where the Hooded 
Crow was the main predator. In this situation there was apparently no advantage in colonial compared to 
solitary breeding, and the number of Fieldfare pairs breeding solitary was in fact higher than the number 
of colonial breeders. Hogstad concludes that "nest predation is scarcely a decisive selective factor 
favouring colonial breeding in Fieldfares in the heath birch forest. ... The colonial breeding (or clumped 
distribution) of Fieldfare pairs might therefore be a result of a scarcity of suitable nesting areas, and only 
secondarily an anti-predatory strategy. It seems unclear which selective factors favour colonial breeding in 
the Fieldfare, and studies on the effect of food availability and foraging effeciency on the breeding success 
are needed". 
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Such studies in different habitats and with different predation pressures show the importance of 
environmental factors on the formation of colonies. In our context it is interesting to note that the effects 
of food availability and foraging efficiency are not factors related to coloniality in the pelagic feeding 
frigates. 
Red-footed Boobies, Sula sula, breed interspersed with their potential pirate, Fregata minor. Whether 
they profit when under communal attack by frigates, so that at least some boobies get a chance to sweep to 
their nest without being harassed by frigates, has yet to be studied. Nelson (1978) notes that "undoubtedly 
the frigate is the sole beneficiary from the sympatry which must be assumed to stem at least partly from 
deliberate 'policy' on its part". Little is known about the breeding success of solitary Sula sula pairs, but 
frigates are not the sole pirates and predators on Sula; Lava Gulls, Short-eared Owls and Night herons are 
other predators of Sula eggs and small young. Nelson (op. cit.), who estimated its population on Tower at 
around 140,000 pairs, considers the Red-footed Booby to be "probably the frigate'S main victim". It 
would need, I think, further study to assess whether indeed the redfoots are the main victims rather than 
other frigates, as far as food-robbery and predation is concerned. 
Colony forming results in the concentration of prey, an anti-predator mechanism and, in the case of 
frigates, it is conspecific predation. 
Milinsky (1977) asked whether all members of a swarm of waterfleas suffer the same predation by 
sticklebacks. He concluded that stragglers were more attacked than any portion of a swarm itself. By 
analogy, would solitary nesting frigates be more molested by con specific male piracy than colonial 
breeders? 
Regarding aggression within the group, Bibl-Bibesfeldt (1979) mentions under functions of aggressive 
behaviour that "in general, an animal's greatest competitors are its conspecifics. By their aggressive 
behaviour, animals exert a certain pressure on their conspecifics, enforcing their distribution over a wider 
area". This rule does not seem to be true for frigates; on the contrary, intraspecific aggression may well be 
the main force for frigate's coloniality, resulting in reduced nest predation by communal attack, not by 
communal defence as is usually the case in other colonial species. This is what I would call an anti-
autopredatory strategy favouring colony forming. 
The biology of frigates presents several intriguing problems of population dynamics. Why they are 
colonial is just one of them. 
REFERENCES 
Coello, F., C. Hernandez, M.L. Ortega, and Tj. de Vries. 1977. Reproduccion y frecuencia alimenticia de 
Fregata minor en Genovesa y Fregata magnificens en Seymour, Galapagos. Revista Univ. Catolica Quito, 
5 (16): 71-110. 
Bibl-Bibesfeldt, I. 1979. The Biology of Peace and War, - men, animals and aggression. Thames .and 
Hudson. 
Hernandez, C. in prep. EI cicio reproductivo, fidelidad del vinculo de paraja y sitio de anidacion de 
Fregata minor, Genovesa, Galapagos. Tesis de Licenciatura, PUCE, Quito. 
Hernandez, C. and Tj. de Vries. in press. Fluctuaciones de la poblacion de fragatas Fregata minor en 
Bahia Darwin, Genovesa, Galapagos durante 1975-1983. (in del Pino, E. and G. Robinson (eds). The 
f'henomenon EI Nino 1982-1983 in the Galapagos Islands. Charles Darwin Foundation, Quito). 
Hogstad, O. 1983. Is nest predation really selecting for colonial breeding among Fieldfares Turduspi/aris? 
Ibis 125 : 366-369. 
Milinsky, M. 1977. Do all members of a swarm suffer the same predation? Z. Tierpsychol. 45: 373-388. 
Nelson. J.B. 1978. The Sulidae, gannets and boobies. Oxford Univ. Press. 
de Korte, J. and Tj. de Vries. 1978. Moult of primaries and rectrices in the Greater Frigatebird, Fregata 
minor, on Genovesa, Galapagos. Bijdr. Dierk. 48 (I): 81-88. 
de Vries, Tj. 1980. ?Por que hembras de aves rapaces son mas gran des que machos? Publ. Museo Ecuat. 
Cienc. Nat. 2(2): 31-41. 
Wiklund, Ch.G. 1982. Fieldfare (Turdus pi/aris) breeding success in relation to colony size, nest position 
and association with Merlins (Falco columbarius). Behav. Ecol. Sociobiol. II : 165-172. 
Wynne-Edwards. V.c. 1967. Animal dispersion in relation to social behaviour. Oliver & Boyd, 
Edinburgh. 
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Why are frigate-birds colonial?

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Why are frigate-birds colonial?

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