Leo Zwarts

Leo Zwarts
Independent Researcher · ecology

About

90
Publications
15,255
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
4,556
Citations
Introduction
Leo Zwarts currently works at Altenburg & Wymenga Ecological Consultants. Leo does research in Ecology. Their most recent publication is ' Large decline of birds in Sahelian rangelands due to loss of woody cover and soil seed bank. Journal of Arid Environments 155: 1-15 '.

Publications

Publications (90)
Article
Full-text available
The Glossy Ibis is among the most widespread bird species in the world. However, the Glossy Ibis erratic occurrence and distribution makes it a difficult species to study, and we know little about its dispersal and metapopulation dynamics. This study summarises previously-scattered and unpublished information by collating, in a single database, the...
Article
Full-text available
Arboreal birds tend to remain in woody vegetation and avoid crossing open areas. Therefore, few tree-dwelling birds are to be expected in scattered trees. We tested this expectation with field data collected in the deserts, savannas and open agricultural parklands of West Africa where woody cover in 1,327 stratified random study sites varied betwee...
Article
Full-text available
The large and widespread decline of European migratory birds spending the northern winter in the Sahel suggests – on top of adverse changes in the breeding quarters ‒ pivotal changes in African overwintering areas. This paper attempts to answer three questions related to the sub-Saharan region: (1) can a change in the woody vegetation explain the d...
Article
Full-text available
When sampling the intertidal macro-invertebrate food supply of wading birds Charadrii, cores of sediment have been taken to depths of 15–30 cm or even more. But for a variety of reasons, samples are often taken to 15 cm. Using data from the Dutch Wadden Sea on the depth-distribution of one widely important prey species—the ragworm, Hediste diversic...
Article
Full-text available
For a study of long-distance migrants in sub-Saharan Africa, a census method was developed that combined precision and accuracy regarding bird numbers and tree choice. The number of birds present in trees and shrubs can be counted accurately, although it is time-consuming. We describe how much time is needed to detect all birds present in trees, us...
Article
Full-text available
In West Africa, tree preferences of wintering migratory birds (and African residents) were quantified in order to assess the importance of wintering conditions on distribution, abundance and trends of insectivorous woodland birds. This study encompassed 2000 plots between 10-18°N and 0-17°W, visited in October-March 2007-2015, and covered 183 woody...
Article
Full-text available
On page 516, in the second paragraph of the subsection (b) Half-asymptote constant, and for a reason that it has proved impossible to discover, there is a significant error. The equation: log e half-asymptote constant = −24 77 + 6.307log e body mass + 5.030log e prey mass + 10.594 dummy variable oystercatcher = 1 is completely wrong and, instead, w...
Article
Full-text available
Mangrove forests attract many insectivorous birds. Bird density in West African mangroves in January-March 2014 is higher in Avicennia (21 birds/ha canopy) than in Rhizophora (11 birds/ha). The Palearctic species are dominant in the most northern mangroves (14-16 degrees N), but further south resident birds become as numerous as migrants (11-12 deg...
Article
Long-distance migrant waders breeding in the Arctic often have globally structured populations, largely because they were isolated in glacial or interstadial refugia or were restricted to fragmented coastal wetlands in winter. Conversely, inland species using continentally distributed wetlands appear to be less structured (more often panmictic), pr...
Article
Mali’s Poverty Reduction Strategy Paper (PRSP) constitutes the sole framework for Mali’s development policies and poverty reduction strategies (GoM 2002). This influential document highlights the need to exploit the country’s hydro-electric and hydro-agricultural potential in the order of 5000 GWh/annum and 2 million hectares, respectively. A revie...
Article
The persistence of the Dutch Black-tailed Godwit population depends largely on high adult survival. Adult survival may be influenced by hunting pressure and land use change in the wintering area, the West African coastal zone. Here we examine hunting pressure on and habitat use of Black-tailed Godwits in West African rice-growing areas. The Black-t...
Book
Full-text available
Of the more th an soo bird species breeding in Europe, some 13 00 - 26oo mi ll ion pairs altogether, about a quarter mi grate south to sub-Saharan Africa . These migrants spend t he greater part of their life in Africa and return north to breed. The Palearctic-African bird migration system draws bi rds f rom Europe, but also from Asia , even from e...
Article
Full-text available
As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes...
Article
Full-text available
One million people in the Inner Niger Delta make a living from arable farming, fisheries and livestock. Upstream dams (one built for electricity generation and one for irrigation) affect this downstream multifunctional use of water. Additionally, the Inner Niger Delta, which is one of the largest Ramsar sites in the world, is a hotspot of biodivers...
Article
Full-text available
As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes...
Article
Full-text available
"Water shortage is a severe problem for millions of people living along the southern fringe of the Sahara desert. Consequently, communities residing in the semi-arid, western Sahel zone fully depend on rivers such as the Niger for their livelihoods. Mali is a classic case of a 'river-dependent economy' that is subject to enormous seasonal variation...
Article
Full-text available
Red Knots (CALIDRIS CANUTUS) are birds of many worlds. These sandpipers breed only on High Arctic tundra but move south from their disjunct, circumpolar breeding areas to nonbreeding sites on the coasts of all continents (apart from Antarctica), between latitudes 58° N and 53° S. Because of their specialized sensory capabilities, Red Knots generall...
Book
Full-text available
The concept of water management of the Netherlands has recently shifted. Water managers increasingly focus on new ways of water-level management due to climatic change, land subsidence, changed demands from the society, and increased awareness concerning nature and environment. The management and control of water levels in our lakes and wetlands ha...
Article
Full-text available
The timing of prey exploitation by oystercatcher 'Haematopus ostralegus' L. was predicted from detailed knowledge of the characteristics of its prey, the clam 'Scrobicularia plana' da Costa. Growth, mortality and depth distribution of a single cohort of 'Scrobicularia' were monitored on a tidal flat in the Dutch Wadden Sea during a period of 7 year...
Article
Full-text available
In 1999 is na een tussentijdse evaluatie het beleidsbesluit Schelpdiervisserij Kustwateren 1999-2003 vastgesteld. Dit hield o.a. in dat het voedselreserveringsbeleid dat in de periode 1993-1998 van kracht was voor de Oosterschelde, werd gewijzigd: De hoeveelheid voedsel die in de periode voor 1999 werd gereserveerd voor vogels in de Oosterschelde,...
Article
Full-text available
In: Chifamba PC (ed) Proceedings of the ULKRS Seminar Series, University of Zimbabwe. ULKRS Bulletin 1/96: 11-18. There has been an extensive debate on the question whether shorebirds feeding in the European Wadden Sea are limited by their food supply. According to traditional stock assessment of the benthic invertebrates, there should be enough fo...
Article
Full-text available
A count of waders and other waterbirds was undertaken in the Baie d'Arguin (an isolated bay in the northern part of the National Park du Banc d'Arguin, Mauritania; Figure 1) in February 1997. In total 85,000 birds were recorded, of which 79,000 were waders. These were similar results to the only other count of the area made in December 1978 (Table...
Article
Full-text available
Prey species have different morphological and behavioural adaptations to escape their predators. In this paper we review how these prey defenses affect prey profitability and intake rate for one predator, the Oystercatcher. Four rules govern profitability. First, within each species large prey are more profitable than small prey, because flesh cont...
Article
Full-text available
Captive Oystercatchers consume daily 25-40 g dry flesh or 550-850 kJ, of which they metabolize 450-700 kJ. Free-living Oystercatchers eat more than captive birds but, contrary to expectation, this is not due to greater activity costs but to a higher body weight. When body weights are equal, free-living and captive Oystercatchers consume the same am...
Article
Full-text available
We predict the intake rate and prey choice of Oystercatchers feeding along the Frisian coast, Dutch Wadden Sea, combining the optimal prey choice model (Chamov 1976) with detailed measurements of the widely fluctuating food supply. Assuming that the birds maximize their intake rate, the birds should never eat Mussels Mytilus edulis during 10 years...
Article
Full-text available
Oystercatchers feeding on grassland in spring mainly take leatherjackets, the larvae of the crane fly Tipula paludosa. The birds roost at night and feed for 40 to 60% of the daylight period. Their intake rate during feeding is 1 to 1.5 mg AFDW or 20 to 30 J s-1, which is rather low compared to the intake rates achieved on tidal flats. The digestibi...
Article
Full-text available
The available feeding time of coastal Oystercatchers varies from day to day due to the effect of wind direction and wind force on the water level. If the birds are not able to feed at all during a day, they lose 30 g, or 6% of their body weight. The body weight increases with the duration of the available feeding time, irrespective whether it is da...
Article
Full-text available
Three individually-tested adult Oystercatchers took different proportions of shallow-buried bivalves Mya arenaria and Scrobicularia plana from a mixture on offer in an experimental situation. Two birds, taking mainly or exclusively Scrobicularia, selected the species as predicted by a random search model based on encounter rate. The third bird acti...
Article
Full-text available
This paper presents methods to determine the sex of Oystercatchers of different age, using measurements of bill length, bill depth, shape of the bill tip, wing length and body weight. The bill length is the most important variable to differentiate between ♂ ♂ and ♀ ♀. The shape of the bill tip, the bill depth, the wing length and the body weight co...
Article
Full-text available
The average body weight of Oystercatchers capture in the Dutch Wadden Sea and adjoining breeding areas varies between 520 g in mid summer and 620 g in mid winter, but the individual differences remain large: between 450 and 650 g in summer and 500 and 725 g in winter. A part of this variation can be explained by body size. Yearlings weigh, on avera...
Article
Full-text available
This paper analyses the relationships between body weight in the Oystercatcher and two measures of its body size, bill length and wing length. The weight variation between individuals due to differences in body size is nearly as large as the seasonal variation in body weight within individuals. Wing length is a better predictor of body weight than...
Article
Full-text available
This paper analyses the weight loss in Oystercatchers on the roost and after capture and attempts to investigate to what degree this weight loss is due to defecation of digested food, to dehydration and to utilisation of nutrient stores. The study emphasizes the need to record weight changes relative to the time of arrival at the roost, rather than...
Article
Full-text available
This paper investigates whether siphon weight limits the elongation of the siphon of deposit-feeding benthic bivalves under natural conditions. Were this to be so, it would imply that foraging and predator avoidance place conflicting demands on these animals, since an increase in the feeding radius on the surface would be associated with a decrease...
Article
The paper describes effects of intertidal height and sediment type on growth rate of the bivalves Cerastoderma edule, Macoma balthica, Mya arenaria, Mytilus edulis and Scrobicularia plana, and of the worms Arenicola marina, Nephtys hombergii and Nereis diversicolor in the eastern part of the Dutch Wadden Sea. In most species, exposure time was nega...
Article
Full-text available
For a full understanding of prey availability, it is necessary to study risk-taking behaviour of the prey. Fiddler crabs are ideally suited for such a study, as they have to leave their safe burrow to feed on the surface of the intertidal flats during low tide, thereby exposing themselves to avian predators. A study in an intertidal area along the...
Article
The Wadden Sea is important as a stop-over and wintering area for several species of waders, foraging for shorter or longer times on its tidal flats. The size of the food stocks these birds encounter varies from place to place and from year to year. We studied characteristics of the variability in time of such prey stocks, using long-term data seri...
Article
Full-text available
For several reasons, waders in the Wadden Sea face a large seasonal and annual variation in their food supply. Observations on a tidal flat in the Dutch Wadden Sea have shown that: 1.- (1) The average energy density of ten invertebrate prey species varies between 21 and 23 kJ·g−1 AFDW. In Scrobicularia plana and Mya arenaria, but not in Macoma balt...
Article
Full-text available
The biomass of the macrobenthic animals living in intertidal flats of the Wadden Sea varies annually and seasonally. However, the variation in prey biomass harvestable by wading birds such as knot Calidris canutus, which feed mainly on the middle range of their prey size classes, is even larger. The majority of first-year Cerastoderma edule, Mya ar...
Article
Full-text available
We quantified the prey selection and intake rate of a wading bird, knot Calidris canutus, when 6 different, intertidal prey species, the mud snail Peringia ulvae and the bivalves Macoma balthica, Cerastoderma edule, Mya arenaria, Scrobicularia plana and Mytilus edulis, were abundant. Knot usually search for food by randomly probing into the mud. Pr...
Article
Full-text available
The paper deals with the seasonal and annual variations in the weight of the soft parts of four bivalve species, Macoma balthica, Scrobicularia plana, Mya arenaria and Cerastoderma edule from tidal flats of the Dutch Wadden Sea. The paper reviews methodology and points to error sources. The variation in ash-free dry weight between individuals of th...
Article
Full-text available
The relationship between relative body condition (deviation from expected mean body weight) and burying depth was investigated in five macro-zoo benthic species living in a marine intertidal habitat. Body weight increased with depth when animals of the same size were compared. The increase amounted to 50% in the clamScrobicularia plana, ca. 40% in...
Article
Curlews feeding on intertidal flats attacked mainly conspecifics for prey, whereas they themselves also fell victim to kleptoparasitic attacks from gulls. There is no convincing evidence that robbers obtained other benefits, such as access to a good feeding site, besides the stolen prey itself. The data are consistent with the view that kleptoparas...
Article
Full-text available
Most flocks consisted of 10-60 individuals, with the larger species occurring in larger flocks. When conspecifics were best "available' (during peak migration and at high tides) starting flocks were largest. During departure flocks sometimes joined others or split up, "stabilizing' between 30-60 birds. Starting flocks usually assembled from cluster...
Article
Full-text available
In order to increase body mass during the 4-6 weeks before their departure from the Banc d' Arguin in spring, some waders, such as Dun­ lin, Knot, Bar-tailed Godwit, increased the total time they spent feeding. They fed more at night and fed throughout neap tides and at high tempera­ tures, circumstances in which feeding activity was depressed in w...
Article
Full-text available
Wader species wintering on the Banc d'Arguin, Mauritania, increased their body mass by c40% during the 4-6 wk before their departure in spring. The total migratory reserve adds 20-80% to winter mass, the rate of mass gain is 0.1-4% per day and the period of mass increase lasts 4 wk on average, but longer if waders prepare for spring migration on th...
Article
Full-text available
Waders wintering on the Banc d'Arguin feed on the intertidal flats at the extremely high average density of 40 birds per ha. Most species eat remarkably small prey at a very high rate. Small wader species select size classes of prey that are ignored by conspecifics when feeding in European estuaries. Without production the food supply might be depl...
Article
Full-text available
Whimbrels Numenius phaeopus are able to consume 2-3 mg ash-free dry mass per second feeding over the short term, but when the intake rate is that high, they have to pause to allow for digestion. If these digestive pauses are taken into account, the maximum crude intake rate is reduced to 1 mg s-l. This maximum intake rate does not increase during t...