Cynanthus Latirostris Research Articles

Northward breeding range expansion and diet of the Broad-billed Hummingbird (Cynanthus latirostris) in central Arizona

Northward breeding range expansion and diet of the Broad-billed Hummingbird (Cynanthus latirostris) in central Arizona

David F. DeSante’s Birds of Cabo San Lucas, Fall 1968: A Historic Account

In preparation for his doctoral studies at Stanford University, the late David F. DeSante spent the fall of 1968 studying migratory landbirds at Cabo San Lucas. Although his hopes to capture numerous vagrant warblers were not met, he amassed an impressive list of distributional records for the Baja California Peninsula at a time when few ornithologists were working there. Among these were peninsular firsts for the Ring-necked Duck (Aythya collaris), Broad-billed Hummingbird (Cynanthus latirostris), Broad-winged Hawk (Buteo platypterus), Bobolink (Dolichonyx oryzivorus), and Painted Bunting (Passerina ciris). Another six species, and one subspecies, were found for the first time in Baja California Sur. Venturing away from the immediate confines of Cabo San Lucas, DeSante established the first peninsular record of the Gray-headed Junco (Junco hyemalis caniceps) and first state records for another five species and one subspecies. Although many of these records have been reported previously, the junior authors believe it is appropriate to synthesize them more completely with current context, in Dave’s honor.

Exotic and native plants play equally important roles in supporting and structuring plant-hummingbird networks within urban green spaces.

Urban gardens, despite their transformed nature, serve as invaluable microcosms for a quantitative examination of floral resource provision to urban pollinators, considering the plant's origin. Thus, knowledge has increased, emphasizing the importance of these green areas for hosting and conserving pollinator communities. However, there is a significant knowledge gap concerning the changing availability of these native and exotic floral resources over time and their impact on structuring interaction networks with specific pollinators. Over a year-long period, monthly surveys were conducted to record both native and exotic plant species visited by hummingbirds in an urban garden at Tlaxcala, Mexico. Flower visits were recorded, and the total flowers on each plant visited were tallied. Additionally, all observed hummingbirds were recorded during the transect walks, regardless of plant visits, to determine hummingbird abundance. The interactions were summarized using matrices, and network descriptors like connectance, specializacion, nestedness, and modularity were computed. Plant and hummingbird species in the core and periphery of the network were also identified. Lastly, simulations were performed to assess the network's resilience to the extinction of highly connected native and exotic plant species, including those previously situated in the network's core. We recorded 4,674 interactions between 28 plant species, and eight hummingbird species. The majority of plants showed an ornithophilic syndrome, with 20 species considered exotic. Despite asynchronous flowering, there was overlap observed across different plant species throughout the year. Exotic plants like Jacaranda mimosifolia and Nicotiana glauca produced more flowers annually than native species. The abundance of hummingbirds varied throughout the study, with Saucerottia berillyna being the most abundant species. The plant-hummingbird network displayed high connectance, indicating generalization in their interaction. Significant nestedness was observed, mainly influenced by exotic plant species. The core of the network was enriched with exotic plants, while Basilinna leucotis and Cynanthus latirostris played central roles among hummingbirds. Network resilience to species extinction remained generally high. Our findings provide valuable insights into the dynamics and structure of plant-hummingbird interactions in urban gardens, emphasizing the influence of exotic plant species and the network's resilience to perturbations. Understanding and managing the impact of exotic plants on such networks is crucial for the conservation and sustainable functioning of urban ecosystems.

Assessment of increase in the abundance of two hummingbird species in the last decade in Mexico City

Abstract Hummingbirds are nectarivorous birds with a very fast metabolism, which requires constant access to resources. In cities, resources have distinct abundances and availability from natural environments, which leads to the exclusion of some species. However, other species may increase in abundance within cities, causing a process of biotic homogenization. In this work we evaluated whether the abundance of five resident hummingbird species has increased in Mexico City over the last decade. Additionally, we evaluated whether these hummingbirds used feeders throughout the year. Finally, we determined the reproductive season of hummingbirds and evaluated whether it is longer in Mexico City than in non-urban locations. Our results indicate that during the last decade, the abundance of Berylline Hummingbird and Broad-billed Hummingbird has increased in Mexico City. Both species take advantage of the feeders throughout the year. This constant availability of resources has allowed both species to extend their breeding season, leading to constantly increasing abundance. One way to avoid biotic homogenization in cities is to implement conservation strategies that include variations in the availability and abundance of native floral resources such as pollinator gardens, rather than constantly available artificial resources like feeders.

Nest description, nest survival, and habitat use of three resident hummingbird species in western Mexico

Nesting is a particularly sensitive period during which birds are highly vulnerable to predation, parasitism, and other natural events resulting in high mortality rates. Some factors can increase nesting success, such as habitat selection. In general, only female hummingbirds build nests, incubate eggs, and provide nestling care until chicks can feed independently. The purpose of our study was to describe nests (shape, size, and construction materials), nest survival, and habitat use of 3 resident hummingbirds in western Mexico: Cinnamon Hummingbird (Amazilia rutila), Golden-crowned Emerald (Cynanthus auriceps; a Mexican endemic), and Broad-billed Hummingbird (C. latirostris). In all 3 species, the nest is an open cup built with plant material and bound together with spider web. All 3 species used the same materials (silk cotton, wood, and grass) for nest construction. However, Cinnamon Hummingbird was the only one that used lichens at its outer surface, whereas Broad-billed Hummingbird used only seeds. We did not find interspecific differences among the materials used and their quantities. A low nest survival (26%) can be explained by the high rate of predation but also by the occurrence of natural events such as hurricanes. All 3 species showed preference for nesting in semi-deciduous forests but chose different nest placement sites that differed in (1) nest height, (2) the height of trees or shrubs where nests were built, and (3) the diameter of the supporting branch within a shrub or tree. Other factors, such as distance to the nearest stream, and distance to nesting-branch tip or trunk, did not differ among species.

Hummingbirds budget energy flexibly in response to changing resources

Abstract A key component of individual fitness is the ability to manage energy stores in response to variable resource availability, but because directly measuring energy budgets is difficult, daily energy management is rarely measured. Hummingbirds' energy management is relatively simple to model compared to other endotherms because they have high mass‐specific metabolic rates and store little fat. We determined which aspects of the hummingbird daily energy budget (i.e. thermoregulation, daytime activity costs, night‐time costs) change at the individual level in response to environmental variation. We found that daily energy expenditure varied threefold in two populations of broad‐billed hummingbirds (Cynanthus latirostris). Our model indicated the energy budget was distributed in the following proportions: daytime activity, 59% (range 22%–84%); thermoregulation, 23% (11%–32%); basal metabolism, 7% (3%–16%); and night‐time energy, 17% (6%–37%). Activity costs were higher at the hotter, homogeneous site and during the early‐wet season at both sites. Increased daily energy expenditure was related to decreased nectar availability and not significantly related to temperature or bird mass. With climate change, the indirect energetic costs of shifting resources could have greater impacts on endotherm energy budgets than direct costs such as thermoregulation. Increased foraging and activity costs could decrease the energy available to birds for somatic repair and reproduction, potentially causing differential fitness across seasons and sites. A free Plain Language Summary can be found within the Supporting Information of this article.

Colisión de aves en ventanas del Centro Universitario Victoria, Tamaulipas, México

Las colisiones de las aves con diversas construcciones humanas (e.g., ventanales de cristal) son el segundo factor de mortalidad más importante en paisajes urbanos después de la depredación por gatos. Se ha estimado que alrededor de 988 millones de aves mueren anualmente solamente en los Estados Unidos y Canadá por estos factores. Entre agosto de 2015 y septiembre de 2016 evaluamos la frecuencia de colisiones aves-ventanas en el Centro Universitario Victoria de la Universidad Autónoma de Tamaulipas en el noreste de México. Para detectar aves muertas por colisiones realizamos recorridos en busca de cadáveres dentro de una franja perimetral (tres metros de ancho) alrededor de cuatro edificios durante dos sesiones diarias de muestreo (09:00-10:00 h y 16:00- 17:00 h). Identificamos 21 aves en el suelo de 16 especies, de éstas el 50% de las especies fueron residentes y del total de colisiones 16 fueron fatales. Las especies con mayor incidencia fueron la paloma ala blanca (Zenaida asiatica) con cuatro individuos y el colibrí pico ancho (Cynanthus latirostris) con tres. Durante el muestreo también detectamos cinco especies de aves residentes con conducta agonística contra su reflejo en las ventanas con vidrio reflejante de los edificios del campus universitario. Proponemos algunas medidas para mitigar el número de colisiones de aves en el CUV (e.g., colocación de bandas de colores en las ventanas). Las especies que colisionaron representaron el 5.2% de las especies descritas para el municipio de Victoria. Determinamos mayor frecuencia de colisión en los edificios con vidrios reflejantes. Es necesario seguir desarrollando más estudios de colisiones de aves usando metodologías sistemáticas, que incluyan puntos de conteo simultáneos para considerar la prevalencia de las especies en el sitio de estudio con el fin de documentar el impacto de la urbanización en las poblaciones y comunidades de aves en diferentes centros y paisajes urbanos de México. La determinación de los factores que más influyen en las tasas de colisión de aves-ventanas ayudará en el desarrollo de acciones efectivas de manejo para reducir las colisiones en diferentes escalas espaciales y temporales.

Hovering in the heat: effects of environmental temperature on heat regulation in foraging hummingbirds.

At high temperature (greater than 40°C) endotherms experience reduced passive heat dissipation (radiation, conduction and convection) and increased reliance on evaporative heat loss. High temperatures challenge flying birds due to heat produced by wing muscles. Hummingbirds depend on flight for foraging, yet inhabit hot regions. We used infrared thermography to explore how lower passive heat dissipation during flight impacts body-heat management in broad-billed (Cynanthus latirostris, 3.0 g), black-chinned (Archilochus alexandri, 3.0 g), Rivoli's (Eugenes fulgens, 7.5 g) and blue-throated (Lampornis clemenciae, 8.0 g) hummingbirds in southeastern Arizona and calliope hummingbirds (Selasphorus calliope, 2.6 g) in Montana. Thermal gradients driving passive heat dissipation through eye, shoulder and feet dissipation areas are eliminated between 36 and 40°C. Thermal gradients persisted at higher temperatures in smaller species, possibly allowing them to inhabit warmer sites. All species experienced extended daytime periods lacking thermal gradients. Broad-billed hummingbirds lacking thermal gradients regulated the mean total-body surface temperature at approximately 38°C, suggesting behavioural thermoregulation. Blue-throated hummingbirds were inactive when lacking passive heat dissipation and hence might have the lowest temperature tolerance of the four species. Use of thermal refugia permitted hummingbirds to tolerate higher temperatures, but climate change could eliminate refugia, forcing distributional shifts in hummingbird populations.

Genetic Diversity of the Endemic Xantus’ Hummingbird Using 16 Novel Polymorphic Microsatellite Loci, and Their Cross Amplification between Six Related Species

A set of 16 microsatellite loci was developed and characterized for the Xantus’ hummingbird (Hylocharis xantusii) by using 454 next-generation sequencing. Twenty-five H. xantusii samples from one population were genotyped; all loci were polymorphic, with the number of alleles ranging from three to ten. The mean observed heterozygosity was 0.681 for all loci. No significant linkage disequilibrium was detected, but five loci (Hxan05, 06, 09, 13 and 14) showed deviation from Hardy-Weinberg equilibrium. These microsatellite loci are the first to be characterized for H. xantusii. A moderate to high level of cross-species amplification was observed across the six hummingbird species (31% - 87.5%), with the best cross amplification results observed in the closest related species (H. leucotis, Cynanthus latirostris, Calypte costae). The availability of these molecular tools allows assessing questions integrating population genetics, ecology, conservation, and evolutionary history for H. xantusii and for other phyogenetically related species.

Flight mechanics and control of escape manoeuvres in hummingbirds. I. Flight kinematics.

Hummingbirds are nature's masters of aerobatic manoeuvres. Previous research shows that hummingbirds and insects converged evolutionarily upon similar aerodynamic mechanisms and kinematics in hovering. Herein, we use three-dimensional kinematic data to begin to test for similar convergence of kinematics used for escape flight and to explore the effects of body size upon manoeuvring. We studied four hummingbird species in North America including two large species (magnificent hummingbird, Eugenes fulgens, 7.8 g, and blue-throated hummingbird, Lampornis clemenciae, 8.0 g) and two smaller species (broad-billed hummingbird, Cynanthus latirostris, 3.4 g, and black-chinned hummingbirds Archilochus alexandri, 3.1 g). Starting from a steady hover, hummingbirds consistently manoeuvred away from perceived threats using a drastic escape response that featured body pitch and roll rotations coupled with a large linear acceleration. Hummingbirds changed their flapping frequency and wing trajectory in all three degrees of freedom on a stroke-by-stroke basis, likely causing rapid and significant alteration of the magnitude and direction of aerodynamic forces. Thus it appears that the flight control of hummingbirds does not obey the 'helicopter model' that is valid for similar escape manoeuvres in fruit flies. Except for broad-billed hummingbirds, the hummingbirds had faster reaction times than those reported for visual feedback control in insects. The two larger hummingbird species performed pitch rotations and global-yaw turns with considerably larger magnitude than the smaller species, but roll rates and cumulative roll angles were similar among the four species.

Reproductive biology and nectar production of the Mexican endemic Psittacanthus auriculatus (Loranthaceae), a hummingbird-pollinated mistletoe.

Many mistletoe species produce 'bird'-pollinated flowers; however, the reproductive biology of the majority of these species has not been studied. Psittacanthus auriculatus is a Mexican endemic mistletoe, most common in open, dry mesquite grassland. Knowledge of the reproductive biology of P. auriculatus is essential for understanding species formation and diversification of Psittacanthus mistletoes, but it is currently poorly understood. Thus, we studied floral biology and phenology, nectar production and breeding system and pollination of this species. The hermaphroditic red-pink flowers open from the middle to the tip and petals are curly, but remain partially fused forming a floral tube of ca. 20-mm long. Flowers are partially protandrous, produce large amounts of nectar, last 2 days, and stigma receptivity is highest during the second day. We recorded hummingbirds (Cynanthus latirostris, Hylocharis leucotis, Amazilia beryllina, A. violiceps, Calothorax lucifer, Archilochus colubris) and less commonly butterflies (Agraulis vanillae, Anteos clorinde, Papilio multicaudatus, Phocides urania, Phoebis sennae) as floral visitors. P. auriculatus flowers are self-compatible. However, this mistletoe is an obligate animal-pollinated species, as the sensitive stigma avoids self-pollination. Under natural conditions, reproductive success was higher than in manually selfed or cross-pollinated flowers, likely due to the traplining foraging behaviour of hummingbirds. We suggest that the apparent efficient foraging behaviour of hummingbirds maintains gene flow among P. auriculatus, promoting outcrossing.

Do hummingbirds have a sweet-tooth? Gustatory sugar thresholds and sugar selection in the broad-billed hummingbird Cynanthus latirostris

Nectar is a solution of mainly three sugars: sucrose, glucose and fructose. Studies have demonstrated that pollinators have preferences according to the sugar composition presented in their diet, and these preferences may be caused by sugar assimilation capacities. However, sugar flavor could also play an important role for sugar preferences of nectar-feeding animals. We evaluated the sugar gustatory thresholds of the broad-billed hummingbird Cynanthus latirostris for sucrose, glucose, fructose and a 1:1 mixture of glucose–fructose. We presented eight C. latirostris to paired feeders containing either a sugar solution or pure water. Additionally, we conducted sugar preference tests at three different concentrations (146, 730 and 1022 mmol L − 1 ), to relate sugar preferences with sugar gustatory thresholds. C. latirostris had different gustatory thresholds for the three different sugars tested. At low sugar concentrations (146 mmol L − 1 ), sugar selection followed the gustatory thresholds. Hummingbird sugar preference patterns can be affected by different mechanisms, both pre- and post-ingestive. At low concentrations gustatory thresholds may play an important role to determine sugar selection. However, at intermediate and high concentrations, sugar assimilation rates, and velocity of food processing generated by osmotic constraints, can be the mechanisms that explain the sugar selection of these animals.

Hummingbirds and the plants they visit in the Tehuacán-Cuicatlán Biosphere Reserve, Mexico

We describe the relative abundance, plant species visited, and plant communities used by hummingbird species inhabiting the Tehuacan-Cuicatlan Biosphere Reserve, a semiarid area in South-central Mexico. We recorded 14 hummingbird species and 35 plant species distributed in 4 plant communities during our study. We found 86 different hummingbird-plant interactions. Amazilia violiceps and Cynanthus latirostris were the most common hummingbirds, while C. latirostris, A. violiceps, and Cynanthus sordidus were the hummingbirds that visited more plant species. Hummingbirds were distributed differentially between plant communities inside the reserve, with 12 species being present in the arboreal plant community of the lowlands, 11 both in cactus forest and perennial spine shrub plants, and 6 in perennial unarmed shrub plants. Cercidium praecox (Fabaceae) was the plant species with the highest number of visiting hummingbird species (10 species). Cactus forest and perennial spine shrub plants were the plant communities with largest number of possible interactions (57 and 51, respectively). The mean connectance value of the interaction matrix was similar between plant communities (near to 22%), but lower than those reported previously in other places. In the Tehuacan-Cuicatlan Biosphere Reserve the hummingbird-plant interaction system will be preserved if the hummingbirds C. latirostris, A. violiceps, C. sordidus, and L. clemenciae, and the plants C. praecox, I. arborescens, E. chiotilla, and N. glauca, are protected.

NOTES ON THE POLLINATION BIOLOGY OF JUSTICIA CANDICANS (ACANTHACEAE) IN CENTRAL SONORA, MEXICO

Several aspects of the pollination biology of Justicia candicans were studied in a population located in central Sonora, Mexico. The species flowered from January to April during the studied year (2003). Its flowers secreted up to 4 µL of nectar that had 41% sucrose. It is self-compatible and visited primarily by the resident hummingbird, Cynanthus latirostris, and occasionally by the common bee, Apis mellifera.

Reproductive Biology in Wild and Silvicultural Managed Populations of Escontria chiotilla (Cactaceae) in the Tehuacán Valley, Central Mexico

Reproductive biology of the columnar cactus Escontria chiotilla was analysed in wild and silvicultural managed populations in the Tehuacan Valley, Central Mexico, where this cactus is under domestication because of its edible fruits. Anthesis is diurnal, flowers opening between 8:00 and 17:00 h, the period between 12:00 and 13:00 appearing to be crucial in pollination success since at this time the maximum opening of flowers, turgidity of stigma, and activity of flower visitors, as well as the highest nectar production and pollen release occur. The bees Apis mellifera, Bombus pensylvanicus and Xylocopa mexicanorum, and the hummingbirds Amazilia violiceps, Cynanthus sordidus and Cynanthus latirostris were the most frequent flower visitors during the crucial period, and were observed contacting both stamens and stigma of E. chiotilla. No differences were found in anthesis nor in flower visitors among both population types. Distances separating the populations studied and bridges of individuals of E. chiotilla between them are within the radius that bees and hummingbirds may travel, indicating that spatial barrier to pollen flow are unlikely. Breeding system is self-incompatible. Production of flowers occurs during the whole year in both population types, allowing continual pollen flow among them. This observation confirms reports of high gene flow among the populations studied in previous population genetics studies. The high pollen flow among wild and managed population explains that however the occurrence of artificial selection, the process of domestication is in incipient stages presumably because gene flow counteracts effects of artificial selection.

THE SEASONAL DISTRIBUTION AND ABUNDANCE OF HUMMINGBIRDS IN OAK WOODLAND AND RIPARIAN COMMUNITIES IN SOUTHEASTERN ARIZONA

Abstract We examined the distribution and abundance of hummingbirds at two study sites in southeastern Arizona, where we banded over 8000 individuals and 11 species in a 6-year period. We trapped approximately once a week from April to October at each site, from 1988–1992 at Sonoita, in oak woodlands, and 1991–1993 at Harshaw Creek, in a riparian area. Anna's (Calypte anna), Black-chinned (Archilochus alexandri), and Rufous (Selasphorus rufus) Hummingbirds were the most abundant species. At Harshaw Creek, Broad-billed (Cynanthus latirostris) and Costa's (Calypte costae) Hummingbirds occurred in significant numbers. A massive fall migration occurred at both sites, but few hummingbirds moved northward in spring. The large numbers of migrants were spaced over time within seasons, and the timing of peak migration for a species varied among years. Fall-migrant Black-chinned Hummingbirds peaked earliest with adult males preceding adult females and juveniles, followed by Rufous Hummingbirds (predominantly juveni...

Floral Abundance, Territoriality, and the Importance of Echeveria gibbiflora to Cynanthus latirostris (Apodiformes: Trochilidae) during Winter

Floral Abundance, Territoriality, and the Importance of Echeveria gibbiflora to Cynanthus latirostris (Apodiformes: Trochilidae) during Winter

Reproductive Biology, Pollen and Seed Dispersal, and Neighborhood Size in the Hummingbird-Pollinated Echeveria gibbiflora (Crassulaceae)

Little is known about the reproductive biology of the Crassulaceae. We studied a population of Echeveria gibbiflora in the Pedregal de San Angel ecological preserve in Mexico City, Mexico. Each flower is open and producing nectar 7 to 8 days. On the days of maximum nectar production (flowers 4–6 days old) an average of 14.5 μl accumulates in a flower per day. The maximum rate of nectar production is between 0700 and 0900 hours. The average sugar concentration in the nectar is 43.7%. In a given flower, pollen is exposed and the stigmas are receptive at the same time. The average natural fruit-set and seed-set are 56.6% and 35.5%, respectively. The pollen-ovule ratio is 124, and the plants are fully self-compatible. The flowers are pollinated by only one species of hummingbird (Cynanthus latirostris) and are never visited by insects. Pollen movement is very limited (mean of pollinator flight distances = 0.72 m, mean distance fluorescent dyes = 0.92 m). Seed dispersion is by gravity and wind, and also is very limited (an average of 1.07 m). The total genetic neighborhood area is 15 m2 to 17 m2, producing a neighborhood effective population size (Nb) of 5.01 to 39.7 individuals. This is a very small Nb, indicating that genetic drift may be a dominant force in the evolution of this species.

Nectar extraction by hummingbirds: response to different floral characters.

Handling times of hummingbirds (Amazilia rutila and Cynanthus latirostris) visiting artificial flowers were a positive function of corolla length, nectar volume and nectar concentration. Corolla angle had no consistent effects on handling times. A multiple regression model explained 83% of the variation in handling times for these two species. The model also closely fit independent data from another hummingbird, Archilochus colubris, suggesting that it is general enough to apply to other medium-sized, short-billed hummingbird species. When examined across the range of variation normally encountered by hummingbirds in nature, corolla length and nectar volume had the largest effect on nectar extraction rates. At corolla lengths longer than a hummingbird's bill handling time increases markedly. Hummingbirds maximize their net rate of energy intake by selecting flowers with the shortest corolla, the highest nectar concentrations and the highest nectar volume. Since there is a positive relation between bill length and nectar extraction rate, it is surprising that most hummingbirds have relatively short bills.

Late Breeding on the Tres Marías Islands

There is evidence that some passerine bird species breed later on the Tres Marias Islands than on the mainland of Mexico at the same latitude (Grant, 1964). Observations on breeding activities were made on both mainland and islands in the years 1961-1963, and it was concluded that equivalent phases of the breeding season of eight species are reached from three to eight weeks later on the islands (modal value, 4 weeks) than on the mainland. These findings have been substantiated and enlarged by using other sources of data, namely references in the literature and the condition of the gonads of collected specimens (table 1). The results are as follows. Whereas at least 16 of the 19 passerine species breed later on the islands than on the mainland, according to available data, there is no clear indication that nonpasserines do so. Secondly, the breeding of most nonpasserines starts earlier in the year than does the breeding of passerines. Furthermore, at least four of the nonpasserine species have a longer breeding season than do the passerine species on both islands and mainland. The season of Amazilia rutila (Cinnamon Hummingbird) spans 10 of the 12 months of the year; and writing of Cyanthus latirostris (Broad-billed hummingbird), Schaldach (1963) refers to specimens being in breeding condition at all seasons of the year. The reasons for these differences are not obvious. The passerine species feed more extensively on insects than do the nonpasserines, a difference that might be correlated with the difference in time and duration of the breeding season if the pattern of available insect food is different than that of vegetable food. If this is so, the long breeding season of the predominantly nectar-feeding hummingbirds is perhaps related to a nearly year-round availability of flowers for food. Lack (1954) and several others stress the biological importance of the fact that the time at which most parent birds are feeding nestlings (maximum feeding activity) coincides approximately with the greatest available food supply during the breeding season. On the Tres Marias Islands the maximum feeding activity of the passerines follows the emergence of leaves on the deciduous trees. Leaf emergence is dependent upon rainfall. Perhaps, therefore, the difference in breeding season between mainland and island passerine birds is due to a different pattern of rainfall in the two regions. The rainy season starts in early to mid-June in both regions; by the end of June more than 150 millimeters of rain have fallen on the mainland at Puerto Vallarta in Jalisco and at San Blas and Tepic in Nayarit, whereas on the islands that quantity has not fallen until the beginning of August (see Contreras, 1942). Thus the breeding seasons may be related to the time of maximum food supply, which is governed by leaf emergence and in turn by rainfall. Ennion and Ennion (1962) have adopted a similar explanation to account for the unusual fact that on Tenerife (Canary Islands) some terrestrial bird species regularly breed two months in advance of the normal breeding season on that island. Any future assessment of the significance of late breeding on the Tres Marias Islands should take into account the unusual fat deposition in several island forms, which is not exhibited by their mainland counterparts (Grant, 1965). Fat is accumulated in March and April, two to three months before the advent of the rains and the onset of passerine breeding. It is used just prior to and in the early stages of breeding. There is scant evidence with which to interpret the significance of the fat deposition. It might be related to the lateness of breeding, and the following sugges-