Galápagos Mockingbirds Research Articles

Demography of the Cooperatively Breeding Galapagos Mockingbird, Nesomimus parvulus, in a Climatically Variable Environment

Demography of the Cooperatively Breeding Galapagos Mockingbird, Nesomimus parvulus, in a Climatically Variable Environment

Group structure, within-group conflict and reproductive tactics in cooperatively breeding Galápagos mockingbirds, Nesomimus parvulus

Reproductive conflict within groups can be an important feature of cooperative breeding systems, especially when more than one individual of a sex breeds within a social group. Relationships between group structure, dominance, within-group conflict and reproductive tactics of cooperatively breeding Galápagos mockingbirds were examined on Isla Genovesa. Territorial groups of 2–24 adults included up to three breeding females, with 42% of the groups containing more than one (plural groups); females in most plural groups nested separately. Territory size increased with group size, but the area available per pair in plural groups was smaller than in singular groups (groups with only one breeding pair). Most pairings were monogamous, and males usually outnumbered females; high-ranking males obtained mates more frequently than subordinate males. In 3 relatively dry years, but not in a wet El Niño year, subordinate pairs in plural groups fledged fewer young than dominant pairs or pairs breeding in singular groups. Interference by dominant breeders, often leading to abandonment of nests by subordinate pairs, appears to account for these differences: through nest disruption in drier years, dominant individuals may reduce the cost of sharing their territories and increase the chances of recruiting helpers. Dominant males in plural groups may also father young through extra-pair copulations with subordinate females. Despite costs imposed by within-group conflict, subordinate breeders have higher long-term reproductive success than birds that defer breeding. Plural group structure is maintained because unpredictable climatic variation favours opportunistic breeding by subordinates.

Influence of kinship on helping behavior in Gal�pagos mockingbirds

The social organization of the Galápagos mockingbird (Nesomimus parvulus) in unusual in that groups frequently include more than one breeding pair (plural breeding), and helping behavior is flexible: some birds neither breed nor help, while others do both. To investigate the influence of kinship on helping behavior, I categorized each bird as a helper or non-helper with respect to each nest within its group where it had an opportunity to help. The incidence of helping varied with relatedness: more birds helped when nestlings available to be fed were close relatives than when not. This result was independent of a higher incidence of helping among males than among females and of variation with age among males. Proportionally more nonbreeding than breeding males helped, but breeding and nonbreeding females helped equally infrequently; breeders helped most often after their own nests failed. The incidence of helping was highest among birds with opportunities to feed offspring of breeders that had fed the potential helper as a nestling, suggesting a mechanism for kin discrimination based on associative learning. Juveniles with opportunities to choose among alternative recipients preferentially fed closely related nestlings, but insufficient information was available to determine if adults also did so. Kinship did not influence the rate at which nestlings were fed by helpers. Juveniles fed nestlings at lower rates than did adult helpers, but helping effort was otherwise unaffected by age, sex, or relatedness. Limitation of help to former feeders functions as a mechanism for directing aid to relatives in a plural breeding system where degrees of kinship vary among potential recipients within the same group.

Cooperative breeding by the Gal�pagos mockingbird, Nesomimus parvulus

The costs and benefits of helping behavior were analyzed for 36 pairs of the Galapagos mockingbird, Nesomimus parvulus, and their associates. Helping at the nest is usually done by sons or males suspected to be offspring of the breeders. Costs and benefits to breeders were assessed by comparison of pairs with and without helpers, and costs and benefits to helpers were assessed by comparison of birds which help and those which establish themselves as novice breeders.

The Breeding and Feeding Characteristics of Darwin's Finches on Isla Genovesa, Galapagos

The breeding and feeding of four species of finches were studied on Isla Genovesa, Galapagos from January to May 1978. Food supply and finch diets were studied in the nonbreeding season (November) as well. The species in order of decreasing body size were Geospiza magnirostris, G. conirostris, G. difficilis, and Certhidea olivacea. Initiation of breeding was not a simple function of body size; the medium—sized cactus finch, G. conirostris, started breeding first, followed by G. difficilis and G. magnirostris in that order. Eggs of G. conirostris were first laid within °1 wk of the first rains. Certhidea laid a maximum of three clutches, G. magnirostris and G. conirostris laid up to four, and one pair of G. difficilis laid five. Similarities among all four species were found in incubation periods (10—14 d, mode 12 d), nestling periods (11—17 d, modes 13—15 d), and uniform success of clutches of different sizes. Similarities among the three Geospiza species were found in the interval between successive clutches (°30 d); the modal size of clutches (four eggs); seasonal variation in clutch sizes, in which average size was largest among second clutches and smallest among fourth clutches; egg and nestling production; and the average interval between fledging from one nest and egg laying by the parents in the next nest. This average interval was 1—4 d, but some pairs of all three species started laying 5—6 d before the nestlings of the previous brood had fledged. The species differed in the number of fledglings produced per pair; from the first three clutches G. difficilis produced 9.5 ± 0.5 per pair, G. conirostris produced 7.9 ± 0.7, G. magnirostris produced 5.1 ± 0.9, and Certhidea produced 3.2 ± 1.5. The relatively low success of G. magnirostris was attributed to predation of nestlings by owls (Asio flammeus), which occurred most frequently within 1 wk of expected fledging. The low success of Certhidea is attributed to disturbance at the nest by G. difficilis and mockingbirds (Nesomimus parvulus), and possible predation by mockingbirds. Compared with finch species in a climatically similar region of the Ecuadorian mainland, Darwin's Finches have large clutches, a much longer nestling period, higher nesting and breeding success, but similar, and low, incidences of egg infertility. Certhidea fed nestlings on an exclusively arthropod diet, whereas the three Geospiza species fed nestlings on a mixed diet of arthropods, fruits, and seeds. All Geospiza species fed size selectively on spiders in proportion to their average body and beak size. Diets of the three Geospiza species were largely overlapping throughout the breeding season. Differences reflected the differences in beak sizes. Seeds of Bursera graveolens, which are difficult to crack, were absent from nestling diets of (the smallest) G. difficilis, and most frequent in nestling diets of (the largest) G. magnirostris. Only G. conirostris fed nestlings on parts of Opuntia helleri (cactus). Diets of the three species diverged markedly in the nonbreeding season, at which time they clearly reflected the adaptations of beak sizes and shapes. The results are discussed in terms of the strong seasonality in food production and availability, coupled with interannual variation in amount, onset, and pattern of rainfall, and hence food production. It is concluded that adaptations for food gathering in the nonbreeding season, when food is scarce, have indirect influences upon reproductive traits, resulting in small but significant demographic differences between species.

The tortoise and the rail

Giant tortoises ( Geochelone gigantea ) on Aldabra respond to light tactile stimulation of their soft posterior regions by adopting an immobile erect stance, previously thought to be a threat posture. It is argued that this is probably a cooperative posture assisting any potential symbiont to clean ectoparasites from the areas of soft skin thus exposed. Most Aldabran tortoises over 25 cm long respond in this way and the response appears to be unaffected by activity, sex, time of day and ectoparasite abundance. The visual stimulus of a white-throated rail Dryolimnas cuvieri within 1 m may also elicit this response, and rails were observed removing ectoparasites from tortoises which had adopted the cooperative posture. The distributions of the tortoise and the rail overlap only slightly and the mutualism is thought to be ecologically unimportant to both the ectoparasites and the rail. Comparisons are made with the mutualism involving Galapagos giant tortoises, Geochelone elephantopus , and the ground finches Geospiza fuliginosa and G.fortis ,and the mockingbird, Nesomimus parvulus .

Multivariate Study of Morphological Variation in Galápagos and Ecuadorean Mockingbirds

Compared with the remarkable and wellknown diversification of finches within the Galapagos Archipelago, the mockingbirds rate a poor second. They are almost as widely distributed as the finches but only one form is found on any island, in contrast to a maximum of nine finch species on some of the large, high islands (Bowman 1961). Like the finches, the mockingbirds have been well collected. This study is based on measurements made on 1,100 sexed museum specimens. However, unlike the finches, mockingbirds have never been subjected to proper morphometric study; previous woirkers have worked only on limited collectioijs. Darwin made the Galapagos mockingbirds known to science when he collected five specimens. These were described by Gould (1837), who placed them in the genus Orpheus. Ridgway (1890) proposed the new genus Nesomimus (restricted to the Galapagos) and placed all mainland forms in the genus Mimus. However, we prefer to treat the Gal/apagos mockingbirds as congeneric within mainland mockingbirds (Rothschild and Hartert 1899, Bowman and Carter 1971) . It is generally agreed that the Ecuadorean populations belong to the species M. longicaudatus. In contrast, there have been considerable differences in the taxonomic treat-

Excessive drinking (polydipsia) in a galápagos mockingbird ( Nesomimus)

1. 1. A species of Galápagos mockingbird Nesomimus parvulus on a diet of insects, goat meat and bread drank between 14 and 30 per cent of its body weight/day in distilled water. As the NaCl concentration of the drinking water was increased to 300 mM, the amount drunk increased. Lesser amounts of 400 and 500 mM NaCl were drunk. The maximum cloacal fluid to plasma chloride concentration ratio was 3·2–3·6. 2. 2. Another Galápagos mockingbird species, Nesomimus macdonaldi, on the same diet drank 52 to 89 per cent of its body weight/day in distilled water, an excessive consumption of water. As the salinity of the drinking water was increased in 100 mM steps, the amount drunk remained constant until a precipitous decline occurred on 400 mM NaCl. The maximum cloacal fluid to plasma chloride concentration ratio was estimated as 3·4–4.·1. On a dry diet (chicken mash), drinking varied between 54 and 119 per cent body weight/day. This species feeds partly in the intertidal zone on marine invertebrates and probably has no access to fresh water for long periods of time. The cause of the pronounced polydipsia observed and its relevance to adaptation of this species to arid conditions deserve futher study.

Egg-Eating by Galápagos Mockingbirds

The Galapagos mockingbirds (Mimidae) belong to the endemic genus Nesomimus, and it is generally accepted that there are four distinct species: N. trifasciatus (on Champion and Gardner-by-Floreana), N. melanotis (on San Crist6bal [= Chatham]), N. macdonaldi (on Hood [= Espafiola]), and N. parvulus -seven subspecies on different islands not inhabited by any of the other species (Swarth, Occas. Papers Calif. Acad. Sci. 18:1, 1931). It is therefore of some interest that Hatch (Condor 67:354, 1965) reports that only one species, N. macdonaldi, feeds on seabird eggs. N. trifasciatus and N. parvulus he found did not, and he does not mention N. melanotis, and says that is an interesting example of a behavior pattern differing strikingly between closely related species. On Hood Island N. macdonaldi, which has by far the largest bill of any of the species, is frequently seen attacking the unincubated eggs of the Galapagos Albatross (Diomedea irrorata), Blue-footed Booby (Sula nebouxii), Masked Booby (S. dactylatra), and Swallow-tailed Gull (Creagrus furcatus). L6veque (Alauda 32:5, 1964) reports these mockingbirds as eating the eggs of the American Oystercatcher (Haematopus palliatus), and on several instances I have seen them take the eggs of the Galapagos Dove (Zenaida galapagoensis). However, in the normal course of events it is unlikely that many eggs of any of these species are available to the mockingbirds except when extreme adverse conditions, such as food shortage or human interference, drive the birds from the nests. Also it must be only infrequently that the birds manage to crack an intact egg of the larger seabirds, although they would soon devour a hatching or cracked egg. Alan Root tells me that in 1966 the mockingbirds were pecking at bleeding wounds on the feet of the albatrosses. The mockingbird on Tower Island (N. parvulus bauri) was noted by Hatch (op. cit.) to be indifferent to the eggs of the abundant seabirds and even to freshly broken eggs. However, during field work on Tower Island in 1966-67, it was seen that this species would eagerly eat eggs of the Red-footed Booby (S. sula) and frigatebirds (Fregata minor) broken by other frigates or Lava Gulls (Larus fuliginosus) and would even peck furiously at unattended frigratebird eggs and try to eject them from the nests when the adults were scared away by human intruders in the colony (fig. 1). There is on Tower Island an extremely large colony of the Wedge-rumped Storm-Petrel (Oceanodroma tethys) which lays eggs in the large but well-protected open spaces under the lava flows. Large numbers of broken eggs are found near the nest sites, and Nelson (Ibis 108:430, 1966) thought that these had not been eaten by mockingbirds (he infers by the use of the abbreviation Nesomimus spp. that there is more than one species present, but there is no evidence for this) but were more likely a result of interspecific competition for nest sites. Close inspection of the colony, however, has now shown that the majority of these eggs have been ejected by competition (but intraspecific) and then eaten by mockingbirds. Great care had to be taken while examining petrel nests as the ?~

BEHAVIOUR

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