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Duffydc 1983 Ecology.Pdf ktill.rg\ *A(ll. IYSI. pp I Ill-((9 0 1981 by the Et.ologic,it Society of America THE ECOLOGY OF TICK PARASITISM ON DENSELY NESTING PERUVIAN SEABIRDS' DAVID CAMERONDUFFY? Depanmen! of Biolosy, Princeton University, fitncmn, Nfw Jersey 08544 USA Absiracf. Densely nesting Guanay Cormorants (Phuluct-ocot-axhouguinviliti) , Peruvian Booby (Sdu variegafu), and Peruvian Brown Pehcan (Pelecunus occiciett~ulis thugus) deserted eggs and young in response to high densities of argasid ticks (Ornirhoilorus umbius). Alternative causes of desertion such as human disturbance, predation, disease, heat, and food shortage did not fit the evidence. Tick predators were less effective at controlling ticks than were frequent human harvestings of guano. A review of the avian and medical entomological literature indicates that ticks and other ectoparasites may be strong determinants of nesting success and of nest structure and duration of use. Key words colonid nesfins; ei.'fopurasnes:Ornithodoros amblus; Pelecanus occidentalis thagus; Phalacrocorax bougainvillii, Population replurioii, seabirds; Sula variegata, rieks. tween 15 and 55 min. Related species (Anastos (957) Biologists have generally ignored the effects of par- can survive for many years as unfed adults, and this asitism on social animals (but see Smith 1968, Alex- is probably true for 0.amblus. ander 1974, Hoogland and Sherman 1976, Hoogland This species of tick is a vector for at least two ar- 1979). Among colonial seabirds, there is growing evi- thropod-borne viruses (arboviruses) designated " Hu- dence that ticks (Acari: Ixodoidea) cause desertions acho" and "Salinas" (Clifford et al. 1980), which were by nesting adults with resultant mortality of eggs and recovered from the Punta Salinas colony just east of Isla Mazorca, my primary study site. The effects of young (Feare 1976, King et al. 19770, h). This paper investigates the role of the argasid tick these arboviruses on the birds are unknown, but hu- Orni~hodorosumhlus in causing desertions of nests of mans suffer severe swelling, itching, and occasional the three main Peruvian guano birds: the Guanay Cor- gangrene and death following multiple tick bites (D. morant Phulucrucorax hu~iguinviilii,the Peruvian C. Duffy, personal com.municufion with various per- Booby or Piquero Suh vuriegata, and the Peruvian sonnel on the islands; Clifford et al. 1980). It is not Brown Pelican Peiecunus occidentaiis fhws. De- clear if all of these are the result of the tick bite itself, spite recent declines in numbers (Tovar 19781, these an arbovirus, or the low standard of medical attention three species still form some of the largest and densest which previously prevailed on the islands. I asked two questions in this study; To what extent colonies of birds (Murphy 1936, Duffy 1980). The col- onies have existed for millenia (Hutchinson 1950). are the ticks responsible for the desertions of nests by Dense, reliable populations of hosts offer excellent the three species of guano birds, and what limits the conditions for large numbers of parasites, because the numbers of ticks? difficult task of finding a host is virtually eliminated (Rothschild and Clay 1952). Ornirhodoros amblus was chosen for study from a variety of ectoparasites of the Study sires birds (Murphy 192 1) because it is large and abundant, I spent 6 mo (September 1977 to March 1978) on and LaValle (19 18) mentions ticks causing desertions Isla Mazorca (It023'S, 77'45'W, 11.6 ha) and between by guano birds, suggesting that at times it does have 14 February and 12 March 1979, I made trips of 1-7 d an effect on the hosts. to eight other islands: Isla Guanape Norte (08O32'S, Clifford et al. ( 1980) and Khalil and Hoogstraal ( 198 ( ) 78'58'W; 35 ha), Isla Macabi (07¡47'S 79*3OfW; 8 ha); have described some aspects of the life history of this Islas Lobos de Tierra (06¡28'S 80°50W; 1426 ha); 1s- tick. It has four to seven nymphal instars before ma- >as Ballestas Norte and Sur ( 13'44'S, 76'24'W; 32 ha); turity, each usually requiring a blood-meal. Females and Islas Chinchas None, Centro, and Sur (13'39'S, have a mean egg production of 457 in captivity, and 76¡25'W Norte: 64 ha; Centro: 66 ha; Sur: 27 ha). under artificial conditions the life cycle is completed in 63 to 401 d. Adults feed for only short periods be- Extent of deserf;'on.'s I used three methods to calculate the extent of de- ' Manuscript received 20 Apnl 198 1 ; revised 1 January 1982, sertion of nests by adult birds: (1) on Mazorca, I fol- accepted 9 March 1982. lowed the nest-by-nest pattern of desertion at 1 3 boo- Present address: Percy RtzPatrick Lnstitute of African Ornithology, University of Cape Town, Rondebosch 7700 by nesting sites; (2) on other islands, I estimated the South Africa extent of desertions from conversations with care- February 1983 TICKS AND SEABtRDS ill takers and by examination of empty nests; (3) at all Alternative agents of de'.ertiwns islands, the bimonthly census maps made by care- A number of other agents could have caused deser- takers (Nelson 1968) allowed an independent measure tions at the colonies. These include: predation, heat, of desertion over time. human disturbance, disease, and food shortage. Most Census records were also useful as an index of de- of these could be assessed through qualitative obser- sertions in past years. Ianalyzed all available records vations. Were predators present? Did desertions begin from three of the most important guano islands: Ma- on the warmer side of the island? What relation was zorca, Macabi, and Isla Don Martin (occasionally re- there between desertions and disturbed areas? Were ferred to as San Martin, I I¡01'S 77'40'W; 16.5 ha) there any symptoms of disease? For each breeding season, I compared the greatest To examine the possibility that shortage of food had area occupied by the three species during incubation led to the desertions, Imeasured the durations of 306 with the area occupied 2 mo later. Young take at least foraging trips by boobies over 10 d and 157 trips by 2 mo after hatching to leave the nest (10 wk for the cormorants over 5 d at Isla Mazorca. Idid not measure cormorant; 12 wk for the booby; and 12 wk for the the duration of pelican trips because this species is at pelican (Vogt 1942, D. C. Duffy, personal ohservu- least partially nocturnal (Vogt 1942, D. C. Duffy, per- nun), so any decrease in active nesting area was likely sow! ubser~~atiot~).I assumed that the longer the trip, to have been caused by desertions rather than by the the more difficult it was to obtain food. Vogt ( 1942) successful fledging of young. believed that trips of >6 h indicated food shortages and were predictive of desertions by nesting cormo- Behavior of she bid-i during desertion rants. Iassumed that the behavior of the birds would be indicative of the cause of the desertions. [ made qual- Limndons on the' numbers of sicks itative observations of the behavior of deserting and 1 concentrated on the two most likely controls on "normal" adults, noting nest attendance, ambulatory the increase of ticks on the islands: predation and guano behavior, and condition of plumage. harvesting by humans. On the assumption that preening serves at least par- On Lobos de Tierra, my field assistant and I col- tially to limi t ectoparasites (Rothschild and Clay 1952), lected five Tropidur~t.~sp. lizards from the edges of a. I recorded on Isla Mazorca the number of seconds per nesting colony of Peruvian Boobies. The collection minute that adults spent preening at nests in "healthy" was made between I000 and 1500 h. The lizards were areas and in areas where they were deserting. I made preserved in 95% alcohol within I h of collection. 88 I-min observations of boobies in both areas, 108 Stomachs were later dissected and the contents ana- such observations of cormorants in both areas, and lyzed by number of prey items per stomach, frequency 100 of pelicans, only at active nests. Idid not measure of occurrence, and percent of stomach volume. These preening at nests where pelicans were deserting be- data were compared with those compiled by Vogt ( 1939) cause most such desertions took place before I had from 12 Trupidurus taken on Isla Chincha Norte. opportunity to study them. Peruvian authorities harvest the guano at 1-3 yr in- tervals, removing all nests, trash, and guano accu- Ticks us she afens.~oj (if.'ifrlu)n.'i mulated. Dates of extraction are on file in the archives 1 did not actually count ticks feeding on birds be- of the Compania Administradora del Guano and PES- cause of the disturbance this would have entailed, the CA PERU in Lima. short feeding periods of adult and ny mphal 0.umhlus I compared the number of ticks on each island with (Clifford et al. 1980), and the possible bias which would the number of breeding seasons since the extraction have been introduced by any die! variations in feeding of guano. I used all samples taken from the nests of behavior of the ticks. recently deserted nests of Peruvian Boobies (Zone B) On every island, 1 took 500-cm3 scoop samples of in Tables I and 2. guano and debris from nests. 1 distinguished three cat- If ticks are limited by predators, then tick popula- egorles of nests based on proximity to areas with de- tions would be expected to go down with successive serted nests: (a) deserted nests >5 m from occupied seasons since extraction, as populations of the pred- nests : (b) deserted nests within I m of occupied nests; ators recover from the destruction caused by the last and (c) occupied nests at least 5 m from the edge of harvest.
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