Nest-site characteristics of the Red-knobbed Hornbill Aceros cassidix and Sulawesi Dwarf Hornbill Penelopides exarhatus

Nest-site characteristics of two hornbill species, the Red-knobbed Hornbill Aceros cassidix and the Sulawesi Dwarf Hornbill Penelopides exarhatus , are presented and compared at two sites in north Sulawesi, Indonesia. Seventy-four Red-knobbed Hornbill and 12 Sulawesi Dwarf Hornbill nests were located. Sulawesi Dwarf Hornbill nests were at lower densities and more evenly spaced than Red-knobbed Hornbill nests. Although selective, neither species appeared to be nest-site limited, except perhaps for the Red-knobbed Hornbill in hill forest. Principal components analysis indicated that Red-knobbed Hornbills were 'catholic' in their preferences, although nests were generally in tall, mature specimens of large girth, located high and in the upper half of the tree. The Sulawesi Dwarf Hornbill nested exclusively in live trees and nests were on average located half way up in tall canopy trees with large girths. Within certain parameters, both species were flexible with respect to their nest cavity size and shape requirements; Red-knobbed Hornbills choose sites with larger entrances and internal dimensions. Discriminant function analysis produced parsimonious models for each site, with high rates of correct classification, based on structural, floristic and breeding success parameters. Lack of transference of predictive powers of the models between sites demonstrates the apparent variability in hornbill nesting behaviour and indicates the need for caution when extrapolating models to other sites. Possible factors affecting the nesting density and distribution of each hornbill species are discussed.     

Observations on the breeding biology of the Writhed-billed Hornbill (Aceros waldeni) in the Philippines

Summary We report on some aspects of the breeding biology of the critically endangered Writhed-billed Hornbill (Aceros waldeni) on the island of Panay, Philippines. Observations were made at three nests during 1995–1997. Walling-in of the females commenced in the first week of March. One female remained incarcerated for 77 days, two of three broods completed fledging around May 20 (1995, 1997). Details on fledging of the female and her brood and postfledging care by both parents are reported.The food of the males at two nests was ca. 98% fruits and 2% invertebrates. The plants exploited comprised at least 14 species. Over a third of the fruits delivered were figs of a small number of species.Two males had average feeding rates of 0.56 and 0.88 times per hour respectively, and fed 1 to 66 (median 8) items per feeding visit at the nest. The hourly feeding rate increased after hatching, but the composition of the diet did not change noticeably. As a rule, food items were delivered singly and, during one visit, in runs of one, or rarely up to 3, species.In the three weeks following vacation of the nest, the male appeared to be the sole food provider while the female stayed continually with the 3 young (as sentinel?) in the vicinity of the nest.The nest environs were defended by the male against Tarictic Hornbills (Penelopides panini panini). Six vocalisations of the parents are mentioned. One was used in territorial skirmishes with Tarictic Hornbills.With perhaps less than 30 pairs of the Writhed-bill surviving, the future for the species looks bleak. Only drastic conservation measures can prevent the species' demise. Some have been started by the PESCP.This paper is publication No. 11 of the Philippine Endemic Species Conservation Project (PESCP) of the Frankfurt Zoological Society.     

Fitness consequences of helping behavior in the western bluebird

We examined the fitness consequences of helping behavior inthe western bluebird (Sialia mexicana) at Hastings Reservationin Carmel Valley, California, USA, and tested hypotheses forhow helpers benefit from engaging in alloparental behavior.Both juvenile and adult western bluebirds occasionally helpat the nest During a 12 year period, all adult helpers and mostjuvenile helpers were male. Helpers usually fed at nests ofboth their parents and rarely helped when only one parent waspresent. The frequency of pairs with adult helpers was only7%, but nearly one-third of adult males helped among those withboth parents on the study area. At least 28% were breeders whosenests failed. The propensity to help appears to depend uponparental survival, male philopatry, and the breeding successof potential helpers. Feeding rates were not increased at nestswith juvenile helpers, apparendy because breeding males reducedtheir feeding rates. In contrast, adult helpers increased theoverall rates of food delivery to the nest in spite of a reductionin the number of feeding trips made by both male and femaleparents. Helpers did not derive any obvious direct fitness benefitsfrom helping, but they had greater indirect fitness than nonhelpersdue to increases in nestling growth rates and fledging successat their parents' nests. Helpers fledged fewer offspring intheir first nests than did nonhelpers, suggesting that theywere birds with reduced reproductive potential. Although wehave not yet measured the effect of extrapair fertilizationson the fitness benefits of helping, we calculated the differencein fitness between helpers and nonhelpers as a function of thepotential helper's paternity when breeding independently andhis father's paternity in the nest at which he might help. Inconjunction with constraints on breeding and indirect fitnessbenefits, we predict that relatedness of males to the youngin their own as well as their parents' nests will influencehelping behavior in western bluebirds.     

Extrapair paternity, inclusive fitness, and within-group benefits of helping in western bluebirds

In central coastal California, USA, 3–16% of western bluebird ( Sialia mexicana ) pairs have adult male helpers at the nest. Demographic data on a colour-ringed population over a 13-year period indicate that helpers gain a small indirect fitness benefit through increases in the number of young fledged from nests of close kin. A small proportion of adult helpers (16%) that were able to breed and help simultaneously had higher annual inclusive fitness than males that only bred. These males comprised such a minor proportion of helpers that the mean fitness of helpers was still lower than the mean fitness of independent breeders. We used DNA fingerprinting to determine whether extrapair fertilizations alter within-group benefits enough to tip the balance in favour of helping behaviour. Overall, 19% of 207 offspring were sired by males other than their social father and extrapair fertilizations occurred in 45% of 51 nests. Intraspecific brood parasitism was rare so that mean mother-nestling relatedness approximated the expected value of 0.5. Extrapair paternity reduced putative father-offspring relatedness to 0.38. Mean helper-nestling relatedness was 0.41 for helpers assisting one or both parents and 0.28 for helpers aiding their brothers. Helpers rarely sired offspring in the nests at which they helped. Helping was not conditional on paternity and helpers were not significantly more closely related to offspring in their parents' nests than to offspring in their own nests. Although helpers may derive extracurricular benefits if helping increases their own or their father's opportunities for extrapair fertilizations, within-nest inclusive fitness benefits of helping do not compensate males for failing to breed. Breeding failure and constraints on breeding are the most likely explanations for why most helpers help.     

COOPERATIVE BREEDING OF THE NORTHWESTERN CROW CORVUS CAURINUS IN BRITISH COLUMBIA

The behaviour of helpers at nests of Northwestern Crows was studied on Mandarte Island and Mitlenatch Island, British Columbia. Not all nests had a helper and there was only one helper per nest. Helpers participated in varying degrees in the defence of the territory and nest, feeding of the nestlings and fledglings and they cached food on the territory. Adult males fed helpers, and helpers obtained most of their food on the adults' territory. Adults with helpers laid larger clutches and produced more fledglings per nest than adults without helpers. It is suggested that cooperative breeding in the Northwestern Crow is of recent origin.     

Helper contributions in the cooperatively breeding laughing kookaburra: feeding young is no laughing matter

I studied the contributions of individuals to incubation and nestling feeding in a population of cooperatively breeding laughing kookaburras, Dacelo novaeguineae. In most cooperatively breeding birds where nest success is limited by nestling starvation, related helpers increase the overall level of provisioning to the nest, thus boosting the production of nondescendent kin. However, although partial brood loss is the largest cause of lost productivity in kookaburra nests, additional helpers failed to increase overall provisioning. Instead, all group members, but especially helpers, reduced their feeding contributions as group size increased. Breeders and helpers reduced the size of prey delivered, and helpers also reduced the number of feeding visits. An important benefit of helping in kookaburras may be to allow all group members to reduce their effort. Within groups, contributions to care depended on status, sex, group size and the brood size. Breeding males delivered the most food. Breeding females provisioned less than their partner, but their effort was comparable to that of male helpers. Female helpers contributed the least food. Incubation effort followed similar patterns. The relatedness of helpers to the brood had no impact on their provisioning. Across all group sizes, helpers generally brought larger items to the nest than breeders. Copyright 2000 The Association for the Study of Animal Behaviour.     

The Social System of a Communal Breeder,the Bay-winged Cowbird Molothrus badius

Molothrus badius (bay-winged cowbird), an icterine blackbird with cooperative breeding, shares behavioural and ecological characteristics with other communal nesters: it is sedentary, has a high annual survival rate (76.2%) and a strong nest-site tenacity (mean breeding dispersal of 41.9 and 89.4 m for males and females). Behavioural data, including collective agonistic displays, suggest group territoriality. Before egg hatching most breeders occurred as single pairs showing territorial behaviour (82% of nests), and nesting was usually solitary (distances to nearest nests of 25–103 m). Most breeders were apparently monogamous, with a 2.5% incidence of extrapair copulations in the territory during clutch formation. During the nestling stage one to four helpers occurred at 95% of M. badius nests. Most helpers were 1–2 years old, but older breeding adults (mostly males) that failed to rear their own offspring helped at the end of the season. The number of helpers increased (up to 4) with nestling age. Helpers were also recruited during the postfledging period, and group size reached up to 10 adults at this stage. Helpers mobbed predators and brood parasites, and provided 35% of the nestling food. Provisioning rate was positively and significantly correlated with number of helpers, although age of nestlings was the best predictor of overall food delivery rate. The helping system was almost obligate and productivity comparisons between nests with/without helpers are not possible. Data suggest that helpers increased the breeding success per nest. The correlation between the provisioning rates of parents and helpers was negative but non-significant. In 18% of nests 3 to 4 individuals were present before the nestling period, including cases of apparently polyandrous trios and one case of joint nesting by two pairs. Within Brown 's (1987) categories of social organization M. badius is mainly group territorial with plural nesting. Habitat requirements of M. badius are wide and nest sites do not appear to limit breeding. Kinship plays a role in the social system, as 9 of 12 helpers marked as nestlings helped their parents.     

Helper effects in the azure‐winged magpie Cyanopica cyana in relation to highly‐clumped nesting pattern and high frequency of conspecific nest‐raiding

In avian cooperative breeding systems, many benefits obtained by social pairs from the presence of helpers have been uncovered. However, until now, the factors that determine the type of assistance helpers provide and the responses of social pairs have not been well illustrated. We examined the contribution of helpers to cooperative groups and the relevant responses of dominant pairs in the azure‐winged magpie Cyanapica cyana which breeds on the Tibetan Plateau. We used the capture–mark–recapture method to identify helpers. Results showed that helpers were mostly the yearling sons of dominant pairs. They mainly contributed to the cooperative group in three ways, courtship‐feeding the incubation female, provisioning the brood, and defending the nest. For responses of dominant pairs, we unexpectedly found that clutch size was not influenced by the presence of helpers at the nest. However, cooperative groups had higher brood feeding rates than biparental nests and their feeding pattern also differed to that of the latter. Consequently, nestlings in cooperative groups had larger fledging body mass than that in biparental nests. By examining reasons for nest failure, we revealed that conspecific nest‐raiding contributed to more nest failure than any other natural predators. Because of the contribution of helpers in defending against both predators and conspecific nest‐raiders, cooperative groups had higher survival rate than biparental nests. Thus, our findings suggest that in a highly‐clumped nesting pattern, factors concerning the risk of nest predation, rather than that influencing food supply, play an important role in determining helper effects and responses of aided dominant pairs.     

Plural Breeding in the Splendid Fairy-wren,Malurus splendens (Aves: Maluridae), a Cooperative Breeder

Groups of the cooperatively breeding splendid fairy-wren Malurus splendens may include more than one female. Previously this species has been described as singular breeding (only one female breeds). This paper describes the occurrence of plural breeding (PB) groups in 10% of group years, in which two females had separate nests. In all cases, the secondary female (Y) was related to the primary breeding female (X) and was generally a 2-year old female which had helped in the group during the previous breeding season. Plural breeding was correlated with an increase in population density and in the number of female helpers; PB groups were larger than singular-breeding groups. In most cases, the X female was occupied with her own nest or offspring when the Y female began to nest, and there was no aggression between them. Which birds helped the Y female to feed at her nest depended on the time between the hatching of the two nests. If the interval was small, some group members helped at each nest; with longer intervals, the group members began to feed at the earlier nest, and the other female was left to raise her brood alone. Female helpers were very active in feeding at single nestings, and the cost to an X female of a Y female breeding was mainly a loss of this assistance. The success of individual X nests was not affected. Effects on productivity were slight, but fewer X females in PB groups raised second broods than did experienced singular breeding females. Y females were less productive than X females, but no less productive than singular breeding novice females without helpers. It is not known whether Y females copulated with primary or secondary males within their group, or with males from outside the group. Certainly, they did not form an observable pairing with any male in the group. Plural breeding occurred in a minority of group years in response to extrinsic conditions and the current demographic situation, and shows the extreme plasticity of the mating system in M. splendens.     

Social organization and communal breeding in the chestnut-bellied starling (Spreo pulcher)

Chestnut-bellied starlings (Spreo pulcher) live in social groups of 10–30 individuals and during their breeding seasons maintain group ranges which show little overlap with neighbouring groups. A social group may contain two to six breeding pairs, non-breeding adults of both sexes, and immatures. Each breeding female has her own nest and she alone incubates. The parents and up to 12 other starlings feed the nestlings. Individual helpers may successively or simultaneously attend the nests of different breeders. The percentage of nests attended within a group differs for helpers of different sex, age and breeding status: immatures of 1–2 years and non-breeding adult males help most and adult females least. Nests with more helpers have higher fledging success than those with fewer helpers. These results are discussed with reference to the tentative benefits of helping behaviour and kinship relationships within the social group.     

Cooperative breeding, mate guarding, and nest sharing in two parrot species of New Caledonia

We provide evidence for cooperative breeding in two endemic parrot species of New Caledonia, the New Caledonian Parakeet (Cyanoramphus saisseti) and the Horned Parakeet (Eunymphicus cornutus). We intensively monitored 11 breeding attempts in two nests of New Caledonian Parakeets over 5 years, and noticed that two males of different size fed the female and the chicks in each nest. During one breeding attempt, the larger male guarded the female and the smaller male tried to copulate with her inside the nest. Genetic analyses showed that males shared paternity. During 17 breeding attempts in 11 Horned Parakeet nests, we observed two pairs sharing a nest on one occasion but we never saw feeding helpers.     

Breeding ecology of the Fulmar Fulmarus glacialis and the Kittiwake Rissa tridactyla in high-arctic northeastern Greenland, 1993

The breeding ecology of the Fulmar Fulmarus glacialis and the Kittiwake Rissa tridactyla in the high Arctic was studied in relation to the occurrence of the northeast water polynya in northeasternmost Greenland (80̀N). Mean laying dates were 31 May in the Fulmar and 18 June in the Kittiwake; the total nesting season for the Fulmar just matched the time window of the polynya opening period. Fulmar colony attendance fluctuated within a period of 11.6 days because of variation in nonbreeding prospectors but showed no clear diurnal variation. Fulmar incubation shifts, on average, lasted 6.1 days (range 1–13 days), which is significantly longer than elsewhere, and the average chick-guard period of 10.9 days (range 1–17 days) was significantly shorter than in other studies. Egg neglect occurred in 18% of Fulmar nests or 0.7% of nests per day. Overall breeding success (chicks fledged per egg laid) was 0.56 in the Fulmar and 0.67 in the Kittiwake; the latter produced 1.4 young per active nest or 1.2 per completed nest. Mean Kittiwake clutch size was 2.03; larger clutches were laid early. Nest site characteristics (presumably reflecting nest predation risk) and breeding behaviour affected breeding success. in the Fulmar, hatching success was negatively correlated with laying date and the proportion of egg neglect, while overall breeding success was correlated negatively with distance to nearest neighbouring site and positively with the length of the chick-guard period. Kittiwake breeding success was negatively correlated with laying date. Using seabirds as indicators of marine food supply, breeding success in both species suggested moderate to good food supply in the northeast water polynya in 1993, although at least in the Fulmar the high reproductive output appeared partly maintained by behavioural buffering; long incubation shifts, egg neglect and short chick-guard periods were symptoms of foraging constraints.     

Flexible Helping Behaviour in the Azure-Winged Magpie

Helping to rear the offspring of others may be a way for younger birds to gain access to future reproduction especially when turnover of breeding opportunities is low. However, this explanation is not applicable to cases where adults also help, or when roles shift between helpers and breeders. Over a period of 6-yr, we studied a marked population of azure-winged Magpie (Cyanopica cyanus) breeding in a non-territorial, colonial system. Magpies bred in a highly flexible cooperative system, in which individuals helped at different stages of the breeding cycle, including nest building, feeding the incubating female and feeding the young and removing the faecal sacs. On average, 50% of hatched nests were assisted by helpers-at-the-nest, and nest success appeared to be positively related to the presence of helpers. Helpers were predominantly males. Although juveniles were more likely to help, both juvenile and adult birds helped. Individual birds behaved as helpers either as a first-option or after having attempted their own breeding (second-option helpers). An individual helper may assist more than one nest during the same breeding season and in different breeding seasons. Reversals between breeder and helper roles were common in both directions, within a breeding season and between years. Helping behaviour is an option for almost any member of the colony. Therefore, hypotheses related to the enhancement of future breeding opportunities for juveniles can be discarded as general explanation of helping in this species. Although the decision to help appeared to be influenced by proximal environmental conditions hindering successful breeding, the associated benefits of helping as opposed to simply recovering for future reproduction, especially for former breeders, deserve further study.     

Breeding biology of White‐rumped Tanagers in central Brazil

ABSTRACT White‐rumped Tanagers (Cypsnagra hirundinacea) are widely distributed in northern Brazil, Bolivia, and Paraguay, and are classified as vulnerable in the state of Paraná and as endangered in the state of São Paulo, Brazil. Little is currently known about their breeding biology. We studied the breeding behavior of White‐rumped Tanagers in the Cerrado (Neotropical savanna) in central Brazil from 2002 to 2007. The breeding period extended from mid‐August to mid‐December. Nests were cup‐shaped and located mainly in trees of the genus Kielmeyera at a mean height of 3.7 ± 0.3 m (SE). Clutch sizes varied from one to three eggs and the incubation period lasted an average of 16.0 ± 0.3 d. Incubation was by females only and started with the laying of the first egg. Mean nest attentiveness (percent time on nests by females) was 64 ± 0.08%. Nestlings were fed by males, females, and, when present, helpers. The mean rate of food delivery rate to nests was 5.2 ± 0.4 items/h, with rates similar for males (mean = 2.7 ± 0.3 items/h) and females (mean = 2.4 ± 0.3 items/h). The mean duration of the nestling period was 12.1 ± 0.5 d. Compared to many temperate species of tanagers, White‐rumped Tanagers in our study had relatively small clutches, low nest attentiveness, and long incubation periods. As with other tropical species, such characteristics might be due to food limitation or high rates of nest predation.     

Breeding biology and social behaviour of Magellanic Woodpeckers (<Emphasis Type="Italic">Campephilus magellanicus</Emphasis>) in Argentine Patagonia

The Magellanic Woodpecker (Campephilus magellanicus) is a poorly known species endemic of the Austral Temperate Forests of South America, where it is a potential keystone habitat modifier. Here, I summarize data on the social and breeding biology of this woodpecker, based on 22 active nests located from 1998–2002 in forests from northwestern Argentine Patagonia. Woodpeckers normally traveled in pairs or family parties. In late Austral winter, one to three cavities were selected for completion at each territory. Breeding occurred between mid- to late spring and early to mid-summer, and took about 65 days. Monogamous parents shared duties in nest excavation, incubation and young rearing. Egg length (±SD) measured 34.13±0.79 mm and egg breadth 23.91±0.67 mm, and incubation took roughly 20 days. Nestlings were altricial and remained at the nest for about 45 days. Clutch size was one, occasionally two eggs, and one nestling was produced at all successful nests. Young remained with their family group for up to 2 years or more, and were fed by adults, who normally bred every second year. Nest re-use, nest predation and helpers at the nest were not recorded. Holes were placed (±SD) 8.84±3.71 m high and were 32.3±5.32 cm deep. Entrances (±SD) were 8.92±0.46 cm wide and 15.59±2.54 cm high and mostly oval in shape. Peculiarities of the breeding biology and social behaviour of this species are discussed in the light of patterns common to picids, especially Campephilus spp.     

Increased nest predation in a declining and threatened Temminck's Stint Calidris temminckii population

We measured nesting success of the Temminck's Stint Calidris temminckii along the Finnish Bothnian Bay coast during 19 breeding seasons (1983–2001) and conducted a population census (1999–2002). We found 105 pairs, showing a marked decline from the previous survey (170 pairs 1987–95). Of the 424 'known-fate' nests, 47% hatched. Depredation caused 79.9% of the nest losses. Nesting failures increased from 1983–91 to 1992–2001 owing to a rise in nest predation. The proportion of failed nests that failed because of predation rose from 48.9 to 87.7%. When only depredated nests were considered as losses, Mayfield nest survival probability over the incubation period dropped from 69 to 31% (461 nests). This pattern emerged both in man-made and in natural habitats. Survival probability was independent of habitat type (natural vs. man-made). In an experiment involving videotaping of dummy nests, Common Gull Larus canus and Ruddy Turnstone Arenaria interpres were found to be the most important egg predators.     

Effect of Japanese lesser sparrowhawks Accipiter gularis on the nest site selection of azure-winged magpies Cyanopica cyana through their nest defending behavior

The frequency of azure-winged magpies Cyanopica cyana that nest in association with breeding Japanese lesser sparrowhawks Accipiter gularis was compared between the 1990s and 2000s. During 1990–1994 azure-winged magpies nested within 50 m of their nests in all of the hawk nest sites studied. During 2005–2006, however, the magpies nested in only two of nine hawk nest sites. During the 1990s Japanese lesser sparrowhawks chased jungle crows away when they intruded within 50 m of their nest. During the 2000s, on the other hand, the hawks attempted to expel few crows at a distance of more than 10 m from their nest. As a result of it, the predation rate of simulated magpie nests located in the hawk nest sites was significantly higher during the 2000s. This suggests that the magpies have the ability to assess the defending behavior of the hawks and judge whether they can rely on their defense. Most azure-winged magpies may stop nesting in association with breeding Japanese lesser sparrowhawks because of the reduced defending range of their nest sites.     

Helping behaviour in facultatively eusocial hover wasps: an experimental test of the subfertility hypothesis

A candidate explanation for the evolution of eusociality is that helpers are physiologically constrained such that helping is their only realistic option. We tested this subfertility hypothesis in a species of facultatively eusocial hover wasp (Hymenoptera, Stenogastrinae: Liostenogaster flavolineata) by seeing whether helpers that were forced to nest on their own were able to mature their own eggs. One focal helper was left alone on each of 22 nests, from which all other adult wasps (including the dominant) were permanently removed. After 18 days, all but one of the 19 focal helpers that remained on their nests had ovarian development and insemination status characteristic of dominants, and the majority had probably laid eggs. This was in striking contrast to the reproductive status of other helpers removed from the same nests at the start of the experiment. These results provide convincing experimental evidence that females do not become helpers because of some unconditional physiological constraint. There is currently no unequivocal support for the subfertility hypothesis in facultatively eusocial Hymenoptera lacking morphological castes. Copyright 1999 The Association for the Study of Animal Behaviour.     

Prey of White-throated Hawks (Buteo albigula) in the southern temperate forest of Argentina

ABSTRACT.   We examined the diet of White-throated Hawks ( Buteo albigula ) during the incubation and nestling periods in the southern temperate forest of Argentina. Pellets ( N = 74) and prey remains ( N = 59) were collected at 10 nests from 1998 to 2003, and preys delivered to two nests were monitored during the 2001–2002 breeding seasons. White-throated Hawks fed on small mammals, birds, lizards, and insects. The three methods of identifying prey (pellets, prey remains, and direct observation) produced different results. All types of prey except large birds were detected in pellets, and arthropods may have been over-represented in pellets due to secondary consumption. No remains of either arthropods or reptiles were identified among prey remains collected at nest sites and, during nest observations, we were unable to identify many of the prey items delivered by adults. Our results indicate that accurate determination of the diet of White-throated Hawks requires more than one method of identifying prey.     

Reproductive interactions of the Shiny Cowbird Molothrus bonariensis and the Yellow-hooded Blackbird Agelaius icterocephalus in Trinidad

The reproductive interactions of the Shiny Cowbird Molothrus bonariensis , a brood parasite, and the Yellow-hooded Blackbird Agelaius icterocephalus , a host of the cowbird, were studied In Trinidad, West Indies. We gathered information on the breeding biology of the Shiny Cowbird and the Yellow-hooded Blackbird, the frequency of use of the host species, and the effects of brood parasitism on host breeding success. Yellow-hooded Blackbirds are polygynous for the most part; males build nests and attempt to attract females to lay in them by means of song and visual displays directed towards the nests. This behaviour probably makes it easy for cowbirds to locate breeding birds and their nests. Cowbird eggs were found in 153 of 377 (40–3 %) blackbird nests located before the nestling stage. Shiny Cowbird parasitism of the Yellow-hooded Blackbird had little negative impact on host reproductive success, whereas predation accounted for the majority of nest failures. Vigilant nest defense by male blackbirds combined with colonial breeding apparently also minimized the extent of host egg damage and removal by cowbirds, and the parasitized and unparasitized nests were equally successful at producing blackbirds. Cowbirds most frequently parasitized the first or only nesting attempts in blackbird territories, and first or only nests were also successful more frequently than subsequent nests.