Robert Gwiazda

Selective Foraging of Grey Heron (Ardea cinerea) in Relation to Density and Composition of the Littoral Fish Community in a Submontane Dam Reservoir

Abstract Grey Herons foraged in a high number only in one of three preferred foraging areas in the Dobczyce Reservoir, S. Poland: (1) the backwater area at the main tributary inlet (MTI), (2) the shore of the near-dam pool (NDP), and (3) shallow lateral bay (SLB). Median value of the fraction of herons foraging in MTI was 64% of all counted in the reservoir in 2001-2002 despite the smallest fish density there (2 times smaller than in NDP, and by one order of magnitude than in SLB). Roach (Rutilus rutilus) was the most abundant both in the heron diet (13 species) and in littoral fish communities (8 species). Total length of prey ranged within 4.2-26.5 cm. The individual size range of littoral fish was similar (4.6-31.5 cm). However, the size distributions in three foraging areas and in heron diet were different. Fish longer than the median total length of heron prey, i.e. ≥8 cm in total length constituted 51.9% of heron diet, 34.7% of the fish community in MTI, 8.5% in the psammolittoral of NDP, and only 5.2% in the phytolittoral of SLB. The strategy of selective choice of longer fish from those occurring at foraging sites allowed greater reward with roughly unchanged foraging cost. This may explain why Grey Heron foraged mainly in the habitat with the lowest fish abundance and highest water turbidity but with the largest prey size.

Impact of a small cormorant (Phalacrocorax carbo sinensis) roost on nutrients and phytoplankton assemblages in the littoral regions of a submontane reservoir

Cormorants (median numbers of 54.5 ind.) excreted ca. 102.7 kg yr−1 of nitrogen and ca. 80.5 kg yr−1 of phosphorus in the area of the Dobczyce Reservoir (ca. 980 ha). Concentrations of N-tot and P-tot were 3 times higher in soil from the roosting area of the cormorants than outside this area. Nutrient concentrations in the littoral sediment of the reservoir were also higher in the area frequented by these birds. Differences in concentrations of pH, NO3−, NO2−, NH4+, PO4−, and P-tot in water between the area associated with the cormorants and the reference site were not found. Differences in the planktonic algal communities from around the roost and the reference site were not found, with the exception of the chlorophytes which were more abundant in the area occupied by the cormorants. Water movement and mixing in the reservoir can influence the effect of the nutrient load on the water chemistry and planktonic algae.

Foraging ecology of the Great Crested Grebe (Podiceps cristatus L.) at a mesotrophic-eutrophic reservoir

The foraging ecology of the Great Crested Grebe wasstudied at Dobczyce Reservoir in southernPoland. The mean diving frequency of grebes withoutyoung was 19.3 h−1, while grebes with young dived onaverage 32.5 h−1. The mean number of apparent prey caughtby grebes without young was 0.5 fish h−1. Birds with young caught a greater number, on average4.5 fish h−1. Probably, birds fishing forthemselves ate much prey underwater. The total lengthof the fish consumed by adult grebes was on average10.0 cm. Adults ate larger fish in the breeding seasonthan in the autumn migration period. Young birds werefed fish about 8.3 cm long. More than 50% of preywere bleak. The mean success of dives of birds withoutyoung was 3.3% and for individuals with young 14.8%.A comparison of the weight of the daily food demandwith the weight of fish brought to the surface showed that it was 70% lower than the dailyfood demand. The poor relation between prey size anddiving success shows that grebes can eat both small andlarge fish below the water surface.

Impact of waterbirds on chemical and biological features of water and sediments of a large, shallow dam reservoir

Large numbers of Mallard Anas platyrhynchos (max. 10,490 ind.), Black-headed Gull Chroicocephalus ridibundus (max. 3,430 ind.) and Great Cormorant Phalacrocorax carbo (max. 1,449 ind.) were recorded on the Goczałkowice Reservoir, Poland (2,754 ha). Most of the waterbirds occurred in the backwater of this reservoir. The amount of phosphorus and nitrogen loaded by the most numerous waterbirds into Goczałkowice Reservoir was estimated at 958 kg and 2,621 kg, respectively in 2011 and 1,043 kg and 2,793 kg, respectively in 2012. In 2011 and 2012, the waterbirds introduced a considerable amount of phosphorus, nitrogen and a large number of coliforms into the backwater of the reservoir. The concentration of different forms of phosphorus and nitrogen, chlorophyll-a and bacteria coli in the water was not greater at the site of birds’ concentration (except dissolved organic nitrogen). The concentration of nitrates in the water at the site near the breeding colony of gulls in comparison with the reference site was not different. The amounts of P-tot and N-tot in the sediment were similar at the site affected by waterbirds and at the reference site. The dynamics of water masses was not the reason for the lack of differences between the studied sites.

Reproductive Parameters of Caspian Gull Larus cachinnans Pallas, 1811 in Different Habitats Nearby and Away Fish Ponds

ABSTRACT Reproductive performance of gulls depends on a variety of factors, but food abundance and its availability are among the most important. Clutch and egg sizes in gulls are found to be strongly influenced by food availability, thus better reproductive performance in the colony with greater fish availability (near fish ponds) was expected in that study. We compared the reproductive traits (clutch size, volume of eggs in the full clutch, relative volume of the C-egg (the third egg in gulls clutches) and hatching success) of Caspian Gull Larus cachinnans in five inland colonies in Poland located at a gravel pit, a lake, a river and two dam reservoirs. Differences in the clutch size between sites were found, with the lowest at a lake. We found similar clutch volume in all studied colonies. C-eggs were slightly smaller than A- and B- eggs, in all colonies and all study years, but the relative volume of C-egg in colonies located near fish ponds (<10 km) was significantly greater compared to colonies located far away. This may be explained by high fish availability in fishponds in comparison to other habitats. However hatching success (the ratio of the number of hatched chick to the number of eggs laid) was highest in the colony at the lake. This indicates that both inland habitats a gravel pit and a lake offered good food conditions for large gulls when fish ponds are nearby.

How equally sized piscivorous birds and fish sharing common food resources may reduce possible feeding interactions between them

The diets of sympatric predators may overlap, especially when their body sizes are similar and foraging area is relatively small. It may be also supposed that some differences in their foraging strategies may counteract competitive interactions among them, and therefore be of advantage to these species. To reveal such phenomena the composition of food of cormorant and adult pikeperch was studied in the Dobczyce Reservoir (S Poland) from June to November 2002. The main prey species were the same and the range of prey size was similar for both piscivores. Despite these similarities, the potential for dietary overlap was strongly reduced due to two differences in their foraging patterns: (1) different preferred prey species (cormorants foraged mainly roach, whereas pikeperch selected juvenile percids); (2) different size of simultaneously selected prey (in summer, cormorants selected larger prey, while in autumn larger prey was selected by pikeperch). These differences may be explained by some general features of birds and fishes, which determine the costs to the individual of capturing prey. The observed selection of different prey species and sizes may be also important for the co-occurrence of other piscivorous birds and fishes sharing common food resources.

Diet and feeding of nestling Little Bitterns Ixobrychus minutus at fishponds: testing a new method for studying a difficult-to-monitor species

ABSTRACT Capsule: The diet and feeding of nestling Little Bitterns Ixobrychus minutus was studied using trail cameras. Little Bitterns nestlings were mostly provisioned with amphibians and small fish. They were fed in the morning and evening, but not during the night, and both parents provided similar amounts of food.

Do flocks of great cormorants and goosanders avoid spatial overlap in foraging habitat during the non-breeding season?

Species distribution, ecology, and behaviour are shaped, amongst other things, by interspecific, antagonistic interactions, and this phenomenon is particularly noticeable among predators. We studied the spatial co-distribution of two top piscivorous bird species foraging on inland waters outside breeding season. We considered the hypothesis that goosanders, Mergus merganser, and great cormorants, Phalacrocorax carbo, avoid foraging in close proximity to each other. Data collected on five river-reservoir systems in the Western Carpathians (Poland and Slovakia) during two periods (2014–2015 and 2015–2016) showed that goosander numbers reduced significantly and their foraging areas changed when large flocks of cormorants arrived and began foraging. We also found that inter-flock distances were greatest between flocks of goosanders and cormorants, suggesting that the former species avoided the waters occupied by the latter. Distribution of flocks of both species was additionally determined by the location of foraging place in river-reservoir system, weather, and presence of other piscivorous birds (e.g. grebes) and raptors (e.g. eagles). Together with the results of research in adjacent Bohemia, this study suggests that competition between cormorants and goosanders may arise when bodies of water suitable for piscivorous foraging are scattered and limited in number, as in the mountainous areas of Central Europe.

Can poor foraging habitat (an inundated opencast sulphur mine) be attractive to the great crested grebe (Podiceps cristatus)

Can poor foraging habitat (an inundated opencast sulphur mine) be attractive to the great crested grebe (Podiceps cristatus)? During investigations of an inundated opencast sulphur mine (an open, deep water habitat with a relatively small littoral area) only five species of breeding water birds were recorded. The density of breeding great crested grebes Podiceps cristatus was relatively high (c. 1.0-1.1 pairs 10 ha-1 of water, and 1.6-1.8 pairs ha-1 of the macrophyte area) although fish density was very low. High water transparency probably compensated for low prey density, because potential prey could be readily detected. Most of the fish were small and occurred in the upper layers of the water column (they could not live below 10 m where the water was anoxic), so were readily available and easily caught by the grebes.