Optimal foraging: A selective review of theory and tests

Beginning with Emlen (1966) and MacArthur and Pianka (1966) and extending through the last ten years, several authors have sought to predict the foraging behavior of animals by menas of mathematical models. These models are very similar, in that they all assume that the fitness of a foraging animal is a function of the efficiency of foraging measured in terms of some currency (Schoener, 1971) - usually energy - and that natural selection has resulted in animals that forage so as to maximize this fitness. As a result of these similarities, the models have become known as optimal foraging models ; and the theory that embodies them, optimal foraging theory. The situations to which optimal foraging theory has been applied, with the exception of a few recent studies, can be divided into the following four categories: (1) choice by an animal of which food types to eat (i.e. optimal diet); (2) choice of which patch type to feed in (i.e. optimal patch choice); (3) optimal allocation of time to different patches; and (4) optimal patterns and speed of movements. In this review we discuss each of these categories separately, dealing with both the theoretical developments and the data that permit tests of the predictions. The review is selective in the sense that we emphasize studies that either develop testable predictions or that attempt to test such predictions in a precise quantitative manner. We also discuss what we see to be some of the future developments in the area of optimal foraging theory and how this theory can be related to other areas of biology. Our general conclusion is that the simple models so far formulated are supported reasonably well by available data and that we are optimistic about the value both now and in the future of optimal foraging theory. We argue, however, that these simple models will require much modification, especially to deal with situations that either cannot easily be put into one or another of the above four categories or entail currencies more complicated than just energy

A Comparison of Bats and Birds as Pollinators of Banksia integrifolia in Northern New South Wales, Australia.

We captured Queensland Blossom Bats (Syconycteris australis) feeding at the flowers of Banksia integrifolia during the night and several honeyeater species feeding at the same flowers during the day. Nearby were flowering Melaleuca quinquenervia and various forested areas including littoral rainforest. Honeyeaters appear to be more frequent visitors to the Banksia flowers than Blossom Bats but less effective at transporting pollen. When they are feeding at Banksia flowers both birds and bats carry pollen on the parts of their bodies that contact successive inflorescences. Hence, both honeyeaters and bats are likely to be pollinators of B. integrifolia in our study area. However, the flowers produce nectar and dehisce pollen primarily at night, suggesting that Blossom Bats are more important than honeyeaters as pollinators of this plant. Banksia pollen was the most common item in the diet of the Blossom Bats during our study and the bats were able to digest the contents of this pollen. Interestingly, the diet of these animals also included relatively small amounts of Melaleuca pollen, fruit and arthropods. The spatial and temporal patterns of capture of the Blossom Bats suggested that Blossom Bats prefer to forage at Banksia flowers that are near to the forested areas and that adult bats may influence where and when younger bats feed. Banksia integrifolia appears to produce nectar mostly during the night and/or early morning in two different locations, one coastal and one on the tablelands, but shows different daily patterns of pollen anthesis in these locations. </jats:p

Estimated forest bird densities by variable distance point counts

During variable distance point counts a stationary observer records the species and distance for all birds detected during a fixed time interval. Such counts, repeated throughout the area of interest, can be used to estimate bird density so long as the following assumptions are made: 1) the distribution of birds is unaffected by the observer, 2) observers are certain of detecting near birds; 3) there is no error in measurement or estimation of distances; and 4) birds are stationary. Data indicated that at least 2 of these assumptions are not satisfied: estimates of distance based on sound are inaccurate and observers do not always detect near birds. No effect of observer presence on bird distribution was detected nor did there appear to be any significant movement of birds during counts. Bird counts should be based only on sight detections. When bird movement is significant, instantaneous counts be used

Seasonal patterns of capture rate and resource abundance for honeyeaters and silvereyes in Heathland near Sydney

Capture rate per unit effort employing mist nets is used as an index of abundance for honeyeaters and silvereyes. The non-resident and presumed migratory species (Yellow-faced Honeyeater Lichenostomus chrysops, White-naped Honeyeater Melithreptus lunatus, Silvereye Zosterops lateralis, Eastern Spinebill Acanthorhynchus tenuirostris) move through our study area during June or July when nectar production is relatively high. Resident and breeding species (New Holland Honeyeater Phylidonyris novaehollandiae, White-cheeked Honeyeater P. nigra, White-eared Honeyeater Lichenostomus leucotis and Little Wattlebird Anthochaera chrysoptera) are rare or absent only during summer and there is no correspondence between their capture rates and the availability of nectar or insects, their main food sources

Relationship between nectar production and seasonal patterns of density and nesting of resident honeyeaters in heathland near Sydney

Some individual honeyeaters were repeatedly seen near the same location over at least a 2 day period and were considered resident in the present study. Such resident honeyeaters (mostly New Holland and White-cheeked Honeyeaters) were present on the heathland study areas from about February until about October. Production of nectar energy is negligible prior to April and low after October. It is hypothesized that the density of residents is determined by their ability to obtain energy in nearby habitats while establishing nesting sites in the heathland in February and March. Nesting tended to occur between April and July when there was sufficient production of nectar-energy

Patterns of residency and movement among honeyeaters in heathland near Sydney

We present patterns of residency and movement derived from the results of a five-year program of banding birds and a three- year program of observing the location and behaviour of individually colour-banded birds in heathland in Brisbane Water National Park, near Sydney. New Holland and White-cheeked Honeyeaters account for most birds captured or observed in our study area. We define a resident bird to be one that is observed nesting andlor is repeatedly sighted in about the same area over a period of at least two days and then categorise species on the basis of whether or not resident individuals are present throughout most of the year. Those species that satisfy the latter criterion also exhibit high recapture rates whereas other species have relatively low recapture rates. We argue that our definitions are more useful than others in the literature. Based on the above definition and on recapture data, most individual New Holland and White-cheeked Honeyeaters are either transients that are captured just once and are never mapped as resident; or occasional residents that are captured more than once, visit our study area throughout their lives but satisfy our definition of resident only some of the time. Future residents, for example, spend some of their time on our grids for an average of eight months before being mapped as residents. There are also gaps of about three to five months between periods of residency. During summer, the residents appear to spend less time in our study areas than during the rest of the year. However, there appears to be no regular, seasonal movement of New Holland and White-cheeked Honeyeaters into and out of our study area. Residents show little variation over time in the locations of their centres of activity in the heathland and do not move far from these areas. Consequently, emigration of resident birds is negligible and departures of residents must be due to mortality. This mortality of residents, which averages about 54% p.a., is associated with low availability of nectar, the principal source of energy to these birds. Individuals often change partners but not unless their partner vanishes, presumed dead

Relationship between nectar production and yearly and spatial variation in density and nesting of resident honeyeaters in heathland near Sydney

The maximum density of resident honeyeaters in heathland near Sydney was very similar from one area and year to another, despite large variation in production of nectar-energy. The most likely explanation for this is that density is determined by the spacing behaviour of birds rather than by nectar production or other factors. Within years, nesting by these residents was positively correlated with temporal variation in production of nectar-energy and 90% of nesting occurred when the estimated average amount of energy available in the heathland per pair exceeded that required to support parents and young. Most nesting and the highest nectar production consistently occurred between April and July