Ground Counts Research Articles

Demography of feral horses (Equus caballus): a long-term study in Tornquist Park, Argentina

Context. All over the world, invasive alien mammals present conservation problems. Feral horses are invasive in many countries and their management is challenging. Aims. To determine the demography of feral horses in Tornquist Park, Argentina, so as to develop a quantitative basis for their management. Methods. Ground counts of individually identified female horses were conducted monthly from 1995 to 1997 and from 2000 to 2002, and seasonally in 1998 and 1999. We calculated the age structure, foaling and survival rates of mares and analysed the effects of adult density and rainfall on demographic parameters. The population trend was calculated by fitting a logistic equation to annual adult mare counts. Key results. The foaling rate averaged 0.55 annually and the survival rate in females averaged 0.89 for adults, 0.91 for juveniles, 0.87 for yearlings and 0.94 for foals. We found adult density, rainfall and the previous year’s rainfall to have a significant effect on fecundity but no effect on adult, juvenile and foal female survival rates. Adult density had a significant negative effect on female yearling survival; however, the effect is probably trivial. The logistic equation parameters gave r = 0.327 and carrying capacity (K) = 240 adult mares. Conclusions. A density-dependent response is suggested. The population growth is mainly limited by low fecundity, with the reduced adult female survival probably also playing a minor role. Implications. It is expected that the present long-term demographic study will contribute to the understanding of feral ungulate ecology and improve the management of feral populations.

Rainfall extremes explain interannual shifts in timing and synchrony of calving in topi and warthog

AbstractWe tested the hypothesis that ungulates time and synchronize births to match gestation and lactation with peak food availability and quality in seasonal environments, using ground counts of topi and warthog conducted over 174 months (July 1989–December 2003) in the Mara–Serengeti ecosystem. During this 15‐year period, 2,725 newborn and 45,574 adult female topi and 933 newborn and 7,831 adult warthogs were recorded. Births were distinctly synchronized in both species but far less so than in ungulates in temperate regions. Extreme droughts delayed onset and reduced synchrony of calving and natality rates but high rainfall advanced onset and increased synchrony of calving and natality rates in both species, supporting the seasonality hypothesis. Annual shifts in birth peaks were significantly negatively correlated with the preceding wet season rainfall. Varying the timing and synchrony of births and natality rates are widespread but little understood adaptations of ungulates to climatic extremes. Climate change heightens the need for advancing this understanding because increasing frequency and severity of droughts is likely to decouple phenology of breeding in seasonally breeding ungulates from that in their food plants. Similar studies of African ungulates are either extremely rare or non‐existent. New approaches to estimating the time of peak births and its confidence limits and the degree of synchrony of breeding are also presented.

Estimating Eastern Pacific Coast Populations of Whimbrels and Hudsonian Godwits, with an Emphasis on Chiloé Island, Chile

Abstract. A large proportion of the Hudsonian Godwits (Limosa haemastica) spending the boreal winter along the eastern Pacific Ocean coast are known to occur in the vicinity of Chiloe Island, Chile, but the importance of the region to Whimbrels (Numenius phaeopus) is less known. Ground counts conducted in 2007 and 2008 increased published estimates, at a minimum, of Pacific coast populations by 27% for Whimbrels (33,150 individuals) and 51% for Hudsonian Godwits (21,161 individuals). Bays and shorelines in the Chiloe Island region supported 99% of Hudsonian Godwits and, perhaps, 61% of Whimbrels estimated to occur along the Pacific coast during the boreal winter. Whereas Hudsonian Godwits aggregated in shallow bays on the eastern and northern coast of Chiloe Island, Whimbrels were more dispersed along the island's coastline and reached a density of 7.5 birds/km along sheltered gravel shorelines. Bays in the vicinity of Chiloe's capital, Castro, provided important foraging and roosting habitat for non-bree...

Dynamics of Mara–Serengeti ungulates in relation to land use changes

AbstractAnimal population dynamics can be driven by changing climatic forcing, shifting habitat conditions, trophic interactions and anthropogenic influences. To understand these influences, we analyzed trends in populations of seven ungulate species counted during 15 years (1989–2003) of monthly monitoring using vehicle ground counts in the Maasai Mara National Reserve, Kenya. Abundance of six species declined markedly and persistently throughout the reserve during this period. The declines were contemporaneous with progressive habitat deterioration due to changing land use in pastoral ranches bordering the reserve, habitat desiccation due to rising temperatures, recurrent severe droughts and an exceptional ENSO flood in 1997–1998. The effect of progressive habitat deterioration was accentuated by illicit harvest, competition with livestock and elevated predation. After factoring out the influence of rainfall, ungulate populations declined more markedly in sections of the reserve experiencing greater livestock incursions and poaching. The declines were significantly correlated with increasing number of settlements and people in the pastoral ranches for five species. Heightened predation following a crash in the buffalo Syncerus caffer population during a severe drought in 1993 had little support as the primary cause of the declines.

Can ground counts reliably monitor ibex Capra ibex populations

AbstractAlthough ground counts are often used to monitor ungulate populations, several studies show that counts of ungulates have low precision and often underestimate population size. We assessed the reliability of ibex Capra ibex counts as performed in French national parks, by analysing up to 23 years of annual censuses of six ibex populations for which a subset of animals were individually marked. We compared the population growth rate obtained from census data (estimated by use of four different methods) with the growth rate calculated from a demographic model including parameters estimated from capture‐mark‐recapture methods. The correlations between count‐based estimates and growth rate obtained from demographic models were adequate to suggest that ground counts can monitor trends in population size of ibex, provided that the occasional undercounts are identified. Substantial undercounts in some years led to biologically impossible values of yearly population growth (λ>1.35) and, in the longest time series available, to marked autocorrelations in counts. Managers should replicate counts within the same year to check for underestimated counts. To reduce errors, population biologists analysing time series of ungulate counts should check the plausibility of annual growth rates estimated from two consecutive counts.

Roosting waterbirds at the Osterems, German wadden Sea: seasonal and spatial trends studied by aerial and ground surveys

Waterbirds migrating through and wintering in the Wadden Sea depend on undisturbed roost sites close to their feeding areas. Birds at these remote places are, however, often not counted within standard monitoring programmes. As in the Osterems a new remote sandbank island, the Kachelotplate, is emerging, we initiated a pilot study based on aerial counts to evaluate the importance of this island as roost site for waterbirds in the context of the surrounding islands. To check for the representativeness of the aerial counts, we compared numbers (i) on one occasion with a synchronous ground survey, and (ii) all aerial counts with the long-term average for those areas regularly covered by ground counts. Numbers of birds in flocks counted during synchronous aerial and ground counts were in good agreement. Nevertheless, birds in structured areas and small waders were difficult to count from the air. In general, a calibration of aerial counts and long-term experience is necessary to obtain realistic results by counting waterbirds from an aeroplane. Waterbirds in the Osterems showed the typical seasonal phenology for the East-Frisian Wadden Sea. Given the size of the study area, bird numbers were relatively high. Besides the Kachelotplate, which was important for the Common Eider and gulls for roosting, another remote island, Lutje Horn, was of paramount importance as roost site, in particular for the Eurasian Oystercatcher, Eurasian Curlew, and in spring for terns. A clear distinction could be made between outer, open islands like the Kachelotplate, which were used by birds feeding on pelagic food sources, and sheltered areas, used by benthic foragers, supporting the idea that birds try to reduce travelling costs between feeding and roosting areas. Furthermore, the study underlined the importance of remote, undisturbed roosting areas for waterbirds in the Wadden Sea.

Estimating Numbers of Black Brant Using Sequential Spring‐Staging Sites

Abstract: Estimating total number of birds using a particular staging site during migration is challenging because counts alone do not account for turnover in the local population. Robust statistical methods are needed to more fully assess the conservation value of such sites. We used the multi‐strata model in Program MARK to estimate time‐dependent transition probabilities between sequential staging sites to estimate lengths of stay for spring‐staging Pacific black brant (Branta bernicla nigricans) on the Fraser River delta and Parksville‐Qualicum areas in the Strait of Georgia, British Columbia, Canada. Using resightings of marked individuals in combination with ground counts, we estimated the total number of brant, with associated uncertainties, staging at these sites during the spring migrations of 1999 and 2000. We estimated between 28,927 and 33,181 individual brant transited the Fraser River delta and Parksville‐Qualicum sites in 1999 and between 21,621 and 25,405 individuals in 2000. These totals correspond to approximately 18–26% of the entire Pacific Flyway brant population, suggesting the need for continued conservation and management efforts at these sites. Given the importance of staging sites to migratory populations, we believe this method could be applied to other species and locations.

Accuracy of ‘total counts’ of waterbirds from aircraft in coastal waters

Estimating ‘total counts’ of waterbirds from aircraft is a widely used survey method, and we assessed the effectiveness of this method for geese, ducks, waders and gulls by comparing the results of counts from aircraft with ground counts in the Danish Wadden Sea during 1984–1998. In total, 47 counts were carried out in 12 counting sites and the results were compared for 18 waterbird species, which varied in abundance, flock size and degree of aggregation. Significantly greater numbers of waterbird species were identified from the ground than from aircraft (mean number: 16.1 vs 10.6 species). Depending on the accuracy of aerial counts compared to ground counts, the species were divided into three categories: a) brent goose Branta bernicla, shelduck Tadorna tadorna, mallard Anas platyrhynuchos, eider Somateria mollissima and oystercatcher Haematopus ostralegus had a high level of correspondence between densities obtained from ground and aerial counts (detection rate of >80% with no statistical difference between slopes and intercepts of the observed regression lines and the ideal lines (x=y), differences between mean densities of ground and aerial counts being <15%); b) wigeon Anas penelope, teal A. crecca, grey plover Pluvialis squatarola, dunlin Calidris alpine, bar-tailed godwit Limosa lapponica, black-headed gull Larus ridibundus, common gull L. canus, herring gull L. argentatus, great black-backed gull L. marinus and common/arctic tern Sterna hirundo and S. paradisaea had a medium correspondence between densities obtained from the two platforms (detection rate of >55% with differences between the mean densities of ground and aerial counts of <30%; and c) redshank Tringa totanus, greenshank T. nebularia arquata and curlew Numenius arquata had a low correspondence between the densities obtained from the two platforms (detection rate of <55% and differences between the mean densities of ground and aerial counts of >30%). Species with a high and medium level of correspondence between the two platforms are mostly species that are numerous, of widespread occurrence, and found in large flocks. Species with a low correspondence are minority species, occurring scattered or in small flocks. We recommend to supplement aerial counts with ground counts at sites with mixed flocks of more dabbling duck species present to increase the accuracy of the count results.

Within night correlations between radar and ground counts of migrating songbirds

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Longevity and Size of Wood Stork (Mycteria americana) Colonies in Florida as Guides for an Effective Monitoring Strategy in the Southeastern United States

Abstract Wood Storks (Mycteria americana) breed in colonies widely dispersed across approximately 3,350 km2 within the United States, and effective monitoring of this population presents immediate tradeoffs between coverage, accuracy, and cost. Here, we summarize surveys in Florida 1991-2005 as a first step towards improving existing survey strategies. In order to determine whether counts from aircraft are a suitable technique for quantifying nests, we compared aerial and ground counts at the same eleven colonies in 2004. Across all colonies, aerial counts averaged 8.1% more nests, probably as result of either better visibility or mistakenly including Great Egret (Ardea alba) nests in the count. During the period 1991-2005, statewide totals in Florida ranged from 2,211-6,449 nests, with an apparently increasing trend through time. Annual modal colony size fluctuated from 65-144 nests, with significantly smaller modal size in 2001-2004, suggesting that colony size has decreased over time. Current survey pr...

Rainfall influences on ungulate population abundance in the Mara‐Serengeti ecosystem

1. Rainfall is the prime climatic factor underpinning the dynamics of African savanna ungulates, but no study has analysed its influence on the abundance of these ungulates at monthly to multiannual time scales. 2. We report relationships between rainfall and changes in age- and sex-structured abundances of seven ungulate species monitored monthly for 15 years using vehicle ground counts in the Maasai Mara National Reserve, Kenya. 3. Abundance showed strong and curvilinear relationships with current and cumulative rainfall, with older topi, Damaliscus korrigum (Ogilby); warthog, Phacochoerus aethiopicus (Pallas); waterbuck, Kobus ellipsyprimnus (Ogilby); and impala, Aepyceros melampus (Lichtenstein) responding to longer lags than younger animals, portraying carryover effects of prior habitat conditions. 4. The abundances of newborn calves were best correlated with monthly rainfall averaged over the preceding 5-6 months for topi, waterbuck, warthog, and 2 months for the migratory zebra Equus burchelli (Gray), but with seasonal rainfall averaged over 2-5 years for giraffe, Giraffa camelopardalis (L.); impala; and kongoni, Alcelaphus busephalus (Pallas). The cumulative late wet-season rainfall was the best predictor of abundance for quarter- to full-grown animals for most species. Monthly rainfall exerted both negative and positive effects on the abundances of zebra, impala and waterbuck. Ignoring age, both sexes responded similarly to rainfall. 5. Births were strongly seasonal only for warthog and topi, but peaked between August and December for most species. Hence abundance was strongly seasonal for young topi and warthog and the migratory zebra. Pronounced seasonality in births for warthog and topi obliterated otherwise strong relationships between abundance and rainfall when both month and rainfall were included in the same model. Aggregated density produced relationships with rainfall similar to those for fully grown animals, emphasizing the necessity of demographic monitoring to reliably reveal rainfall influences on ungulate abundance in the Mara. 6. Strong relationships between abundance and rainfall suggest that rainfall underpins the dynamics of African savanna ungulates, and that changes in rainfall due to global warming may markedly alter the abundance and diversity of these mammals. Ungulates respond to rainfall fluctuations through movements, reproduction or survival, and the responses appear independent of breeding phenology and synchrony, dietary guild, or degree of water dependence. Newborns and adults have contrasting responses to rainfall. Males and females respond similarly to rainfall when age is ignored.

Comparison and Assessment of Aerial and Ground Estimates of Waterbird Colonies

Abstract: Aerial surveys are often used to quantify sizes of waterbird colonies; however, these surveys would benefit from a better understanding of associated biases. We compared estimates of breeding pairs of waterbirds, in colonies across southern Louisiana, USA, made from the ground, fixed‐wing aircraft, and a helicopter. We used a marked‐subsample method for ground‐counting colonies to obtain estimates of error and visibility bias. We made comparisons over 2 sampling periods: 1) surveys conducted on the same colonies using all 3 methods during 3–11 May 2005 and 2) an expanded fixed‐wing and ground‐survey comparison conducted over 4 periods (May and Jun, 2004–2005). Estimates from fixed‐wing aircraft were approximately 65% higher than those from ground counts for overall estimated number of breeding pairs and for both dark and white‐plumaged species. The coefficient of determination between estimates based on ground and fixed‐wing aircraft was ≤0.40 for most species, and based on the assumption that estimates from the ground were closer to the true count, fixed‐wing aerial surveys appeared to overestimate numbers of nesting birds of some species; this bias often increased with the size of the colony. Unlike estimates from fixed‐wing aircraft, numbers of nesting pairs made from ground and helicopter surveys were very similar for all species we observed. Ground counts by one observer resulted in underestimated number of breeding pairs by 20% on average. The marked‐subsample method provided an estimate of the number of missed nests as well as an estimate of precision. These estimates represent a major advantage of marked‐subsample ground counts over aerial methods; however, ground counts are difficult in large or remote colonies. Helicopter surveys and ground counts provide less biased, more precise estimates of breeding pairs than do surveys made from fixed‐wing aircraft. We recommend managers employ ground counts using double observers for surveying waterbird colonies when feasible. Fixed‐wing aerial surveys may be suitable to determine colony activity and composition of common waterbird species. The most appropriate combination of survey approaches will be based on the need for precise and unbiased estimates, balanced with financial and logistical constraints. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):697–706; 2008)

Response of waterbird colonies in southern Louisiana to recent drought and hurricanes

AbstractAlthough hurricanes have been implicated in causing shifts in waterbird use of individual colonies, little is known about whether or not these effects are consistent across broader areas affected by a storm. We examined the effects of Hurricane Rita, and to a lesser extent Katrina, and a subsequent drought, on the nesting activity of waterbirds across colonies located in southern Louisiana. Using ground counts, we compared changes in numbers of nesting pairs between 2005 and 2006, the years encompassing the hurricanes and drought, with changes between 2004 and 2005. Following the hurricanes, colonies were more likely to become inactive or experience large shifts in numbers of nesting pairs, compared with the period before the hurricanes. Although one third of the surveyed colonies became inactive following the hurricanes, total numbers of nesting birds of most species increased. We hypothesize that these increases were the result of birds shifting from damaged to active colonies. Colony use was negatively associated with the maximum wind speeds experienced at each site, apparently as a result of damage to nesting habitat. There were no associations of colony use with either storm‐related flooding or localized rainfall during the drought; however, this may be due to manipulation of water levels by management agencies. Our results suggest that monitoring colonies over a broad area is necessary to understand the influence of hurricanes on the nesting activity of waterbirds.

An estimate of the population sizes of Black-browed (Thalassarche melanophrys) and Grey-headed (T. chrysostoma) Albatrosses breeding in the Diego Ramírez Archipelago, Chile

The Diego Ramírez Archipelago, Chile, is the southernmost albatross breeding ground in the world and holds globally important numbers of Black-browed (Thalassarche melanophrys) and Grey-headed (T. chrysostoma) Albatrosses. A census in the Diego Ramírez Archipelago has been attempted only once, in 1980–81, with methods that were unlikely to determine population sizes accurately. The number of breeding pairs of both species was estimated in the incubation period of 2002 using a combination of aerial photography, ground-based photography, yacht-based photography and ground counts. All islands in the archipelago were surveyed. There were an estimated 55000 pairs of Black-browed and 17000 pairs of Grey-headed Albatrosses breeding at the archipelago. Based on the results of this census, and those for the other four known breeding locations, the populations of both species of albatrosses in Chile are considerably larger than previously reported, comprising ~20% of the Black-browed Albatrosses and 23% of Grey-headed Albatrosses in the world, with the largest populations of both species occurring in the Diego Ramírez Archipelago.

Comparison of census methods for black-browed albatrosses breeding at the Ildefonso Archipelago, Chile

A breeding population of black-browed albatrosses has been known to exist at the Ildefonso Archipelago, Chile, for >90 years but the population has never been censused using scientifically defendable methods. To estimate population size, and examine the accuracy and practicality of various census methods, the population was censused in the 2002/2003 breeding season using (a) ground-truthed aerial photography, (b) yacht-based photography, (c) ground counts, (d) quadrat sampling and (e) point-distance sampling. Compared to ground-truthed aerial photography (judged the most accurate) yacht-based photography underestimated population size by 55%, ground counts by 13%, quadrat sampling by 11% and point-distance sampling by 9%. Ground-truthed air photography revealed that in the 2002/2003 breeding season 47,000 pairs of black-browed albatrosses bred at the Ildefonso Archipelago. A repeat aerial census in 2006 suggested the size of the breeding population had not changed in the 4 years between the two censuses. After the Falkland Islands/Islas Malvinas, South Georgia and Diego Ramirez, the Ildefonso Archipelago holds the fourth largest population of black-browed albatrosses in the world.

Estimating the relative abundance of emperor penguins at inaccessible colonies using satellite imagery

Emperor penguin (Aptenodytes forsteri) populations are useful environmental indicators due to the bird’s extreme reliance on sea ice. We used remote sensing technology to estimate relative adult bird abundance at two inaccessible emperor penguin colonies in the Ross Sea, Antarctica. We performed supervised classification of 12 panchromatic satellite images of the seven known Ross Sea colonies. We used regression to predict adult bird counts at the inaccessible colonies by relating the number of pixels classified as “penguin” in the satellite images of the accessible colonies to corresponding known adult bird counts from aerial photographs or ground counts. While our analysis was hampered by excessive guano and shadows, we used satellite imagery to differentiate between relatively small ( 5,000 adult birds). Remote sensing technology is logistically less intense and less costly than aerial or ground censuses when the objective is to document penguin presence and/or large emperor penguin population changes (e.g., catastrophic changes). Improvements expected soon in the resolution of the satellite images should allow for more accurate abundance estimates.

Tests of Dynamic Extensions to a Family of Trip Table Refinement Methods

This paper addresses the problem of using traffic counts to ascertain dynamic origin-destination (O-D) tables when dynamic traffic assignments are performed within a traffic window. A family of dynamic O-D trip table estimation methods containing two previously unexplored members (single factor and biproportional) is proposed to solve this problem. The family is tested on a traffic network for the US-45 corridor in Milwaukee, Wisconsin. The tests show that dynamic traffic assignments can be made to match ground counts by a dynamic extension to a technique conceptually similar to Fratar factoring of both origins and destinations from a static O-D table. The two methods that directly solve for origin or destination factors have computational and statistical advantages over whole-table dynamic O-D trip table estimation procedures. Both methods have results that can be readily interpreted.

Status and distribution of wandering, black-browed and grey-headed albatrosses breeding at South Georgia

Long-term studies at Bird Island, South Georgia, show that numbers of wandering, black-browed and grey-headed albatrosses have been decreasing since the late 1970s. To determine the status of the total South Georgia population, all known colonies were censused in 2003/2004 using a combination of yacht-based digital photography and ground counts. The breeding population estimates from this census are 1,553 pairs of wandering albatross, 74,296 pairs of black-browed albatross and 47,674 pairs of grey-headed albatross. A 30% decline since 1984 was recorded for wandering albatross, and comparison of a sample of black-browed and grey-headed albatross colonies on the mainland of South Georgia photo-censused in both 1985/1986 and 2003/2004 indicates similarly substantial population declines. Unless these decreases can be halted or reversed, doubt will exist as to the long-term viability of these species of albatross at South Georgia.

Borrowing Residential Trip Generation Rates

Residential trip generation rates are a fundamental component of transportation planning. To investigate discrepancies in these rates, residential trip generation rates for nine suburban neighborhoods were computed using four different methods: (1) ground counts conducted at the neighborhoods, (2) household surveys distributed to the neighborhoods, (3) application of national trip generation rates published by the Institute of Transportation Engineers (ITE), and (4) rates derived from the trip generation component of regional urban travel demand models for the neighborhoods. Agencies generally use one of these rates, and by determining all four for the same set of neighborhoods in a controlled study, one can ascertain the extent to which these rates are likely transferable. Rates based on the first three methods were not significantly different. For developments composed solely of single-family detached homes, the average residential trip generation rate was 10.8 based on the site-specific ground counts, 9.2 based on site-specific household surveys, and 9.6 based on ITE's trip generation rates. However, rates based on the fourth method were significantly different, with a mean rate of 6.4. The greatest differences occurred when the long-range regional model used person trips that were converted to vehicle trips rather than estimating vehicle trips directly. Although the summary statistics presented in this paper will not surprise transportation planners, they illustrate two caveats for balancing data collection costs and need for site-specific information. First, a subtle change in how some rates are calculated limits their utility elsewhere. Second, even when equivalent methods for determining rates are used for similar neighborhoods, differences will occur because of the large and random variation inherent in trip generation. Borrowing rates may indeed be tolerable, but only if one gives the full range of rates possible from this probabilistic process rather than just the expected mean rate. Both caveats are treatable provided they are explicitly addressed through uncertainty analysis or adjustments for expected bias.

Estimating elephant densities from wells and droppings in dried out riverbeds

AbstractIn this paper, we present a new method for estimating elephant densities by counting elephant wells and dung boli within dry seasonal flooding rivers. A combination of aerial and ground counts of elephant wells and dung boli in the Ewaso Ngiro River were related to elephant numbers, obtained from an on‐going monitoring program of individually identified elephants in Samburu and Buffalo Spring National Reserves, Kenya. The total number of elephant observations was highly correlated with both densities of wells and dung boli at a spatial resolution of 4‐km river‐section. This indicates that both wells and droppings can be used for estimating relative densities at such spatial resolution. The method can be used as a quick and reliable way of estimating relative elephant densities in semiarid regions but is sensitive to differences in the time when different parts of the river dry out and will be unreliable in areas with secondary un‐censused water sources. A short 4‐week period between the river dry out and the count is recommended, because of an error induced by a level of well reuse and the difficulties in counting areas of high well densities from the air.