Bird census techniques pdf

For larger birds, aerial or even satellite photography gets around this problem, although identification may be difficult. A general consideration when counting flocks is that observers show a natural tendency to overestimate small flocks and underestimate large flocks, although the extent to which different observers do this varies greatly. Furthermore, most observers estimate the size of larger flocks far less accurately than smaller flocks.

It is always helpful for individual counters to repeat their own section counts and compare them with those from another observer. For flocking species that disperse to feed over wide areas, it is often advisable to count the birds as they enter or leave roost sites at dawn or dusk, particularly where the sites are used traditionally and predictably.

At this time, a high proportion of males can be detected at a relatively small number of often traditionally used sites. One or two counts of the leks may be sufficient to give a reasonable and efficient census of the local population. There are downsides to this method however. For example, you need to be sure that all the leks present in an area have been detected, as birds can move between leks, and the smaller they are, the harder they are to find.

Counts restricted to the largest traditional leks may well sample a specific group of birds and we do not know the area from which the birds came. In addition, some males may not choose to visit leks and this is particularly true for younger males.

Finally, lek counts provide a poor means of surveying female birds. In Israel, counters are arranged in a line across the front of migration and use radios to ensure that no more than one observer records each large flock of migrating birds. As migration can take place at great heights, observers often count in teams, continually scanning the sky and working together. Similar coordinated raptor counts occur across North America where their potential for population monitoring has been explored Lewis and Gould Many smaller migrants call as they migrate, allowing at least minimum numbers to be assessed and species to be identified.

Recently developed methods use microphones and complex computer programs to try to estimate total numbers of calling birds passing, as well as their height and speed Evans and Rosenberg , www. Radar has been used to not only detect passing flocks, but also to estimate their numbers, direction of flight, speed, altitude, and even wing beat rate, but not their specific identity.

This method requires access to extremely sophisticated, and usually militarily sensitive equip- ment and is generally beyond the reach of most researchers. Counts of migrants passing in front of the moon, or passing through the beams of bright lights, are of limited use, because only a small proportion of birds can be seen and most cannot be identified. A further indirect method of measuring changes in numbers of migrants, although not the absolute numbers, is ringing banding , and a high proportion of ringing effort is concentrated at migration stopover points Dunn et al.

These methods are described in detail in Chapter 7. Occasionally, however, this may not be the case, as in species that live in dense undergrowth, or in the forest canopy, which may be rarely seen or heard. Under such circumstances, one way to census them is to catch them using mist nets. Capture techniques have been widely used in the tropics where they can be usefully combined with other census methods e. Broadly, two separate approaches can be used; either capture-mark-recapture also known as mark-release-recapture, MRR which allows estimations of population size, or catch per unit effort which can be used to produce population indices.

Capture methods can be time consuming and require substantial training to develop the skills necessary to catch, handle, and mark birds. The safety and welfare of the birds are always of paramount importance. In many countries, these techniques are licensed, and anyone considering using them should apply to the relevant authority well in advance. As we have seen in the previous chapter, mist netting is a relatively poor method for surveying birds.

Further information on methods of capture and marking are given in Chapter 4. Despite these disadvantages, capture techniques yield much information besides popu- lation size and trend estimation. Chapter 5 covers these issues in detail. The principle behind standard effort capture-mark-recapture is that, if birds are caught and individually marked e.

Imagine that on the first day of capture at a site, birds of a particular species were caught in nets, marked, and released. A week later, the nets were put back up. This time 50 of the same species were caught, 25 of which had been marked on the first day.

If we assume that the population is closed and the original birds caught had become fully mixed back in the population over the inter- vening week, then the total population size of the species on the site is Because we know that the number of marked birds is , then the total population is twice that, that is, In practice, there is no need to actually catch birds on the second date, as they could be recorded by walking around the site trying to see as many birds as possible and recording those that were marked.

While the capture-mark-recapture approach may seem simple, it is in practice fraught with problems because it relies on a suite of assumptions, many of which may be untrue. While many of these assumptions may be broken, it is possible to plan fieldwork to minimize their influence on the results. For example, if the first and second capture dates are reasonably close together, the study site is well defined, and the study is undertaken outside of the breeding and migration periods, then the population will more approximate a closed one.

An array of mathematical models has been developed to analyze data from capture-mark-recapture studies. The simplest of these, which is known as the Lincoln index or Petersen method assumes one capture and one recapture or re-sighting event only, and that the population is closed. The calculations for this model are essentially those described above. More complex models allow for multiple capture re-sighting events, and for open populations.

The latter types of model, generally known as Jolly-Seber models, provide information on both population size and survival rates. Further information on these models is given in Chapter 5. The principle behind standard effort capture is that populations of birds can be reliably monitored by capture methods if capture effort is kept constant over time, and done at the same season each year.

Several programs for monitoring birds with this method exist, but perhaps the best known is the Constant Effort Sites scheme of the British Trust for Ornithology Peach et al. For the Constant Effort Sites scheme, the capture method involves placing the same types e.

These methods are held constant from year to year. All birds caught are identified, aged, and sexed, and all un-ringed birds are ringed. While it might be tempting to vary net lengths from visit to visit, particu- larly if the number of fieldworkers varies from visit to visit, this could influence the catches. Simply calculating the number of birds per 10 m of net is insuffi- cient, because doubling net lengths does not necessarily double the number of birds caught.

Similarly, catching for twice as long with half the length of nets on some visits is not advised as capture success may vary with time of day.

Because some dense habitats, such as scrub and reed bed, can be successional, care needs to be taken to ensure that population trends reflect real changes in bird numbers rather than local habitat change around the nets.

Although the Constant Effort Sites scheme uses mist nets, any accepted capture technique Chapter 4 can be used, providing that effort is standardized same number of traps, places, time periods, etc. As a general survey or monitoring tool, catch per unit effort has some limi- tations, such as requiring specialist equipment and training and thus being expensive to maintain.

Examples of such species are those that have skulking behavior, live in dense habitats, are nocturnal or crepuscular or nest down burrows. The probability of detecting these species can sometimes be increased by the use of tape playback, in which the taped call or song of a bird is played, and a response listened for. Recordings of the calls and songs of many species are now commercially available, and can be copied to tape.

Ideally, use a tape loop, so that a short length of call can be repeated continuously for as long as is required. The results from census work involving tape playback need careful interpreta- tion. If the aim is simply to determine whether a given species is present in an area, then tape playback may simply increase the chance of finding it.

If, however, the aim is to estimate population size or to produce a population index, then more care is needed. To generate a reliable population index, the probability of birds responding to the tape needs to be held as constant as possible.

This can be helped, for example, by standardizing the manner in which the tape is played same volume, recording, playback length, time of day, season, etc , and ensur- ing that the tape is not played to any one individual too frequently, causing it to habituate and respond less frequently.

Tape playback has been used widely for monitoring populations of marsh birds, owls and raptors Gibbons et al.

Estimating absolute population size from tape playback is more complex, as the probability of the average bird in the population responding to playback needs to be known. Frequently, detailed additional work will be required to determine response probabilities.

Such work has been undertaken on owls and nocturnal burrow-nesting seabirds. Detailed observations on incubating birds showed that this was because males and females shared incubation equally, but that only males responded to playback. Playing the tape into numerous bur- rows, counting the number of responses, and doubling this number could thus yield an estimate of the overall population.

Unfortunately, response probabilities are not always constant. In their studies of Storm Petrels Hydrobates pelagicus Ratcliffe et al. To estimate population size, it is thus necessary to determine year-specific and colony-specific response probabilities. Acoustically distinct calls of this kind have considerable potential in monitoring and conservation, particularly for birds that occur in dense vegetation or are otherwise difficult to observe, but this potential has not always been realized McGregor et al.

The method involves recording songs or calls with a directional microphone and examining sound spectrograms using freely available software. The spectrograms from an individual bird are often recognizable by eye and discrimination can be formalized using statistical techniques. Work on Bitterns Botaurus stellaris, in Britain has shown that their booming calls are individually quite distinct.

This has allowed their numbers to be monitored more accurately and their year-to-year survival to be estimated Gilbert et al. The churring call of male European Nightjar Caprimulgus europaeus, a mainly nocturnal and mobile species, has been shown to differ between individuals Rebbeck et al. Interestingly, males were shown to move some distance within a breeding season, but return to the same territory year after year.

It is hard to see how these insights could have been gained by other methods. One can also apply capture-mark-recapture methods to re-sightings based on vocalizations to estimate population size. In contrast, although the calls of Black-throated Diver Gavia arctica are distinct, the method proved impractical as a monitoring tool because calls are infrequent and difficult to record McGregor et al.

In each case, quantitative rules were developed to help discriminate one bird from another, but this is not always straightforward and, in some cases, ambiguity remains.

The disadvantages are: that it requires high quality recording of birds that often live at low densities across scattered sites; ideally, one needs an independent means of identification, such as marking or radio tracking, to corroborate the findings; it requires specialist and quite expensive equipment; it often tells us only about breeding males; and it can be time-consuming, unless the analysis is automated see Rebbeck et al.

For example, are we interested in relative or absolute abundance, or a population index instead of a population estimate? As we have seen, it is vital to establish the objectives of the survey at the outset and consider their practicality and relative priority. The survey objectives will interact with, and be influenced by, the sampling strategy choosing where to count and the field method how to count ; these taken together define our survey design.

A number of generic rules help us decide how to select our survey plots; random stratified and regular sample designs are best. Stratification should always be considered. Furthermore, a number of rules allow us to choose between survey methods and apply them in an appropriate fashion. We recommend line and point transects as the two most adaptable and efficient methods for most surveys.

While each survey must be tailored to a particular situation, the common application of field methods will greatly enhance our ability to compare across studies; and we make some practical suggestions. A number of specialized and often more intensive techniques are available for survey and research purposes. References Arendt, W. Status of the volcanically threatened Montserrat Oriole Icterus oberi and other forest birds in Montserrat, West Indies.

Bird Conserv. Double sampling to estimate density and population trends in birds. Auk, , 36— Bennun, L. African Forest biodiversity: a field survey manual for ver- tebrates, eds. Davies, and M. Hoffmann, Earthwatch Europe, Earthwatch, Oxford. Bibby, C. Making the most of birds as environmental indicators. Ostrich, 70, 81— Expedition Field Techniques: Bird Surveys. Royal Geographical Society, London. Bird Census Techniques, 2nd ed. Academic Press, London. BirdLife International Threatened birds of the world.

Brooke, M. Sexual differences in voice and individual recognition in the Manx shearwater Puffinus puffinus. Buckland, S. Oxford University Press, New York. Carter, M. F, Hunter, W. Setting conserva- tion priorities for land birds in the United States: the partners in flight approach. Auk, , — DeSante, D. A field test of the variable circular plot censusing technique in a California coastal scrub breeding bird community.

In Estimating Numbers of Terrestrial Birds, eds. Ralph, and J. Studies in Avian Biology, 6, — Dunn, E. Monitoring songbird population change with autumn mist netting. Wildlife Manage. Evans, W. Acoustic monitoring of night-migrating birds: a progress report. October 1—5, Farnsworth, G. A removal model for estimating detection probabilities from point count surveys. Gibbons, D. In Ecological Census Techniques: A handbook, ed. Sutherland, pp. Gilbert, G. Bird Monitoring Methods—a manual of techniques for key UK species.

RSPB, Sandy. Local annual survival of booming male Great Bittern Botaurus stellaris in Britain, in the period — Ibis, , 51— Greenwood, J. Basic techniques. Gregory, R. Development of breeding bird monitoring in the United Kingdom and adopting its principles elsewhere.

The Ring, 22, 35— Large-scale habitat use of some declining British birds. Procter, D. The population status of birds in the United Kingdom, Channel Islands and Isle of Man: an analysis of conser- vation concern — Birds, 95, — H, Raven, M. Using birds as indicators of biodiversity.

Ornis Hungarica in press. Haselmayer, J. A comparison of point counts and sound recording as bird survey methods in Amazonian southeast Peru. Condor, , — Lewis, S.

Survey effort effects on power to detect trends in raptor migration counts. Wildlife Soc. Call-response surveys to monitor marsh bird population trends. Loh, J. Living Planet Report WWF Gland Switzerland. Marchant, J. H, Hudson, R. Population trends in British breeding birds. British Turst for Ornithology, Thetford. McGregor, P.

Communication behaviour and conservation. In Behaviour and Conservation, eds. Morris Gosling and W. Nichols, J. A double-observer approach for estimating detection probability and abundance from avian point counts. The Auk, , — Niemi, G. A critical analysis on the use of indicator species in management.

Nemeth, E. Distribution, habitat selection and behaviour of the East Coast Akalat Sheppardia gunningi sokokensis in Kenya and Tanzania. Bird Conser. Newton, I. Ecology and Conservation of owls. Peach, W. The use of constant effort mist- netting to measure between-year changes in the abundance and productivity of common passerines. Bird Stud. Expedition Field Techniques Skip to content.

Bibby Publisher: Elsevier ISBN: Category: Nature Page: View: In this book there are entire chapters devoted to the most widely used bird counting techniques, and attempts to amalgamate other counting methodologies into major groups were made. Examples of the use of methods are provided wherever possible and the relative value of various approaches for answering specific questions is also addressed.

A newly revised edition of the immensely successful Bird Census Techniques An entirely new chapter covering the census methods recommended for tropical habitats Provides a concise guide to various census techniques and their opportunities and pitfalls.

Almost all ecological and conservation work involves carrying out a census or survey. This practically focussed book describes how to plan a census, the practical details and shows with worked examples how to analyse the results. The first three chapters describe planning, sampling and the basic theory necessary for carrying out a census. In the subsequent chapters international experts describe the appropriate methods for counting plants, insects, fish, amphibians, reptiles, mammals and birds.

As many censuses also relate the results to environmental variability, there is a chapter explaining the main methods. Serious ecologist or ornithologist either engaged in research, as a consultant or a serious and interested amateur. Purpose and Design in Counting Birds. Census Errors. Territory Mapping Methods. Line Transects. Point Counts and Point Transects. Catching and Marking. Counting Individual Species.

Distribution Studies. Description and Measurement of Bird Habitat. Colin Bibby Colin Bibby is Director of Science and Policy at the International Council for Bird Preservation where he is interested in the role of birds as indicators for global biodiversity conservation. In both capacities, he has been struck by the small number of birds of conservation concern which have been counted adequately.

He has counted birds in Britain and Europe, as a professional, as an amateur participant, and as an organizer of surveys for the British Trust for Ornithology.