Raymond McNeil

THE OCCURRENCE AND ADAPTIVE SIGNIFICANCE OF NOCTURNAL HABITS IN WATERFOWL

This paper reviews the occurrence of nocturnal activity, particularly foraging, in wildfowl (Anseriformes) and shorebirds (Charadrii), and discusses its significance. Many duck species are mainly active at night while others regularly feed during both the day and night. Some ducks and geese are normally day feeders and occasionally forage during darkness. In a few duck species, courtship also has been observed at night. Most shorebirds forage both by day and night, in temperate and in tropical latitudes. Some are mainly crepuscular and nocturnal feeders and also display at dusk and at night. Some species may use their daytime territory at night.

Nocturnality in Colonial Waterbirds: Occurrence, Special Adaptations, and Suspected Benefits

Nocturnality, the habit of being active during darkness, has traditionally been viewed as characteristic of a minority of bird species, with the great majority being considered entirely diurnal. Primary examples of activity at night are found in the Apterigiformes, Strigiformes, Caprimulgiformes, and Apodiformes (Thomson, 1985; Martin, 1990). In the waterbirds (marine, freshwater, and marsh birds), excluding species that are crepuscular (i.e., active at dusk or dawn only), members of eight orders and 27 families are regularly or strictly active at night (Table I). Some seabirds are nocturnal in nest exchanges and chick feedings (e.g., storm petrels, shearwaters, diving petrels, small alcids) but to a large extent diurnal in their search for food. Many aquatic and wading birds (e.g., herons, ducks, shorebirds) are partly or mainly nocturnal, showing considerable activity by night, even during very dark nights. The Limpkins (Aramus aramus) and most rails and coots (Rallidae) are somewhat crepuscular or nocturnal; many species vocalize extensively at night (Bent, 1926; Van Tyne and Berger, 1976).

Diets of land birds from northeastern Venezuela

We estimate diets of 68 bird species inhabiting dry seasonal habitats and mangroves of northeastern Venezuela, based on 3,419 birds forced to regurgitate using tartar emetic. Most birds were generalist feeders, including a wide variety of both invertebrate and plant taxa in their diet. Coleopterans, ants, and insect larvae were the most frequent invertebrate preys. Hummingbirds had low nectar intakes and differed from other species by feeding extensively on small soft-bodied arthropods. Most species fed on fruits to a variable extent. Actually, several species tended to be more frugivorous at our study sites than in previous reports.

Seasonal Flowering and Fruiting Patterns in Tropical Semi-Arid Vegetation of Northeastern Venezuela

Flowering and fruiting phenology were studied for thorn woodland (annual precipitation of 1179 mm) and thorn scrub (annual precipitation of 578 mm) stands in northeastern Venezuela. The global phenology of each vegetation type was assessed using multivariate techniques (ordination and chronological clustering). All species and their respective phenophases were analyzed simultaneously. The flowering of trees and tall shrubs occurred in both vegetation types at the end of the dry season and lasted throughout the entire rainy season. Short-duration rains late in the dry season may have triggered and synchronized flowering. Fruiting activities of trees and tall shrubs occurred in all months, whereas fruiting maturation was limited to the dry season and the beginning of the rainy season. The fluctuation in the abundance of fruits was related principally to the production of a large amount of dry, lightweight seeds during the dry season, whereas fleshy and dry, heavyweight fruits were more or less steadily produced throughout the year. This suggests an adaptation to favor wind dispersion of seeds during the dry season when the vegetation is leafless, as well as the maintenance of animal populations as dispersal agents throughout the year. Periods of similar phenological characteristics were statistically defined and compared to annual rainfall and soil moisture. Annual reproductive phenology was more synchronized for thorn scrub than for thorn woodland. This may be due to the fact that thorn scrub vegetation is subjected to more severe drought conditions and consequently responds more punctually to the reappearance of rain or soil moisture.

Effect and Efficiency of Tartar Emetic in Determining the Diet of Tropical Land Birds

The effect and effectiveness of tartar emetic was tested on 82 bird species from 25 families and subfamilies inhabiting seasonal habitats of northeastern Venezuela. Of the 3,419 birds forced to regurgitate, 3,033 diet samples were obtained and 2,712 of them had recognizable food. Seventy birds (2%) died after administration of the chemical, but a smaller dosage or concentration reduced mortality in species more sensitive to the emetic. Overall, 24 invertebrate taxa and 59 fruit species were identified, with an average of 6 items per sample. Pollen grains were observed in 55% of the samples from nectarivorous species. Considering the low mortality, as well as the diversity of prey types and sizes found in samples, regurgitation using tartar emetic is probably the best method for determining the diet of land birds from various feeding habits.

Characteristics of feeding guilds and variation in diets of bird species of three adjacent tropical sites

We studied feeding guild characteristics of birds in northeastern Venezuela in three adjacent tropical habitats: thorn scrub, thorn woodland, and deciduous forest. We mist-netted and marked birds twice monthly to obtain data on site tenacity. We determined diet by examining regurgitated food samples. Species were assigned to feeding guilds by a cluster analysis based on the proportion of the different food types found in their stomachs. Characteristics of each feeding guild from each habitat were submitted to a principal components analysis. Five parameters clearly distinguished feeding guilds: number of species, proportion of transient individuals, amount of seasonal variation in abundance, mean body mass, and estimated biomass. These parameters also appeared to be related to diet characteristics (food types taken, diet diversity) at the species level. Differences among habitats in food availability led to strong between-site differences in the diet of several species, and, accordingly, in their temporal and spatial patterns of abundance.

Retinal Morphology and Electrophysiology of Two Caprimulgiformes Birds: The Cave-Living and Nocturnal Oilbird (Steatornis caripensis), and the Crepuscularly and Nocturnally Foraging Common Pauraque (Nyctidromus albicollis)

Oilbirds (Steatornis caripensis) breed in the total darkness of caves and forage at night on fruits. Common pauraques (Nyctidromus albicollis) are crepuscular and nocturnal foragers on flying insects. We examined if their retinal structure and function can be correlated with their types and periods of activity. Electroretinograms (ERGs) were obtained from anesthetized birds in photopic and scotopic conditions to a wide range of light intensities, following which the retinas were processed for histological analysis. Retinal sensitivity is higher in oilbirds than in common pauraques. Under scotopic conditions with maximum flash luminance, the average (± 95% CI) b-wave amplitude of oilbirds is double that of common pauraques (500.4 ± 49.8 and 245.4 ± 40.9 µV, respectively) but, under photopic conditions, the results are the reverse (common pauraque: 69.4 ± 18.1; oilbird: 23.0 ± 4.4 µV). On the other hand, the retina of both species is highly rod-dominated, but rods are highly more numerous in oilbirds than in common pauraques (rods:cones ratio: 123:1 and 5:1, respectively). In oilbirds, rods are largely thinner and their outer segments are 1.0 µm in diameter and 18.6 µm in length. They are distributed over various levels in the photoreceptor layers, an arrangement known for deep-sea fishes, but so far unknown for birds. In common pauraques, rods are patchily distributed and their outer segments are 4.0 µm in diameter and 53 µm in length. The oilbirds rod thinness allows more rods per area unit, and thus to catch more photons per area unit under darkness, while the low cone number suggests that the species has poor daytime vision, which concurs with the species cavernicolous daytime habits. The lower rod number of common pauraques, compared to oilbirds, appears counterbalanced by their patchiness and longer and thicker outer segments to provide high retinal sensitivity. In addition, common pauraques also have a tapetum. These features, combined with a higher proportion of cones, show that common pauraques are well equipped for crepuscular and nocturnal foraging on flying insects in an open environment.

Diurnal and nocturnal visual function in two tactile foraging waterbirds : The American White Ibis and the black skimmer

We compared the diurnal and nocturnal visual function in two tactile foraging waterbird species, the red subspecies of the American White Ibis (Eudocimus ruber ruber, formerly the Scarlet Ibis), which is known to feed exclusively during daytime, and the Black Skimmer (Rynchops niger), which forages primarily during darkness. Live birds were captured in coastal lagoons of northeastern Venezuela. Electroretinograms (ERGs) were obtained at different light intensities from anesthetized birds, and the retinae were subsequently processed for histological observations. The ERGs of the skimmer were of much larger amplitude than those of the ibis in scotopic (rod-mediated) conditions, but, in contrast, under photopic (cone-mediated) conditions, the amplitude of the ERGs always was significantly larger for the ibis than for the skimmer. The scotopic:photopic b-wave ratio, calculated with b-waves obtained at the highest flash luminance, was 6.82 : 1 for the skimmer and 0.89 :1 for the ibis. The retina of the ibis contained, on the average, 18.8 rods/310 μm and 56.6 cones/310 μm, for a rod:cone ratio of 1:3. The retina of the skimmer contained 90.2 rods/310 μm and 16.8 cones/310 μm, for a ratio of 5:1. The higher density of rods in the skimmer is in some way counterbalanced by their thinness. Compared to the nocturnally active skimmer, the ibis has highly inferior rod function and, consequently, potentially inferior nocturnal visual capabilities. The latter would seem to explain the temporal differences observed in feeding behaviors of the two species.

Settlement Period and Function of Long-Term Territory in Tropical Mangrove Passerines

We studied the territorial behavior of four insectivorous passerines residing in a tropical mangrove of northeastern Venezuela. Data were obtained through mist-netting and direct observation of color-banded individuals, regularly scheduled over two complete annual cycles. Seasonal variation in food resources (arthropods) and the birds’ diet also were evaluated. A chronological cluster analysis was used to determine the settlement periods of territories, the former being illustrated by the probabilistic Jennrich-Turner method. Three of the four species hold year-round territories. These long-term territories were settled only once a year, immediately after the breeding season when food was abundant. The territorial settlement was synchronous among all individuals ofa species, favoring strong modifications of the territorial mosaic. Considering the absence of territorial settlement prior to breeding and the high stability of territorial mosaics even during periods of low food abundance, territoriality during the non-breeding season primarily appears to ensure a breeding site for the next reproductive period. These results contrast with those from temperate regions where territoriality over the non-breeding season primarily enhances the survival of individuals during periods of food shortage.

Bird mortality at a power transmission line in Northeastern Venezuela

Abstract A survey of bird casualties resulting from collisions with cables in a section of the Margarita power transmission line at Chacopata Lagoon, northeastern Venezuela, where it runs between two expanses of water, was undertaken on 27 February 1983. A total of 611 birds skulls and freshly killed or crippled birds belonging to 10 species were counted. Brown pelicanus Pelecanus occidentalis, neotropic cormorants Phalacrocorax olivaceus, royal terns Sterna maxima, and black-crowded night-herons Nycticorax nycticorax, listed here in decreasing order of abundance, suffered the heaviest losses. The frequency of collisions with the power line is discussed in function of the species composition in the bird community, the behaviour of the birds, the flight characteristics or flight directions, the familiarity with local features, and the interactions with local fishermen and poachers. Undetermined quantities of casualties resulting from the removal of carcasses by scavengers appear more significant in smaller species, whereas those resulting from the hiding of injured birds seems to be more significant in larger species. Most collisions with the power line apparently occur either at dusk or at dawn. Burying of the cables is considered as the best corrective measure to prevent bird mortality in this case.