Field methods

2.2.1.1. Study sites and study period

Fieldwork was carried out on alkaline lakes of the Kiskunság National Park (KNP) in central Hungary, from 1998 to 2000. Alkaline lakes with bare, flat shorelines and islands are natural nesting and feeding habitats of avocets at this location. The lakes vary in surface area from 2-3 ha to 500 ha and in depth from a few cm to a maximum of 1 m. These shallow lakes change

considerably among years depending on winter and spring precipitation and the growth of

vegetation. Thus, avocets did not use every site every year and fieldwork was concentrated in areas of high nesting density and good accessibility for researchers. In addition to the natural breeding sites, I also included artificial habitats, such as a drained fishpond and a reconstructed wetland because of the importance of these areas to avocet nesting. Fieldwork started in early May in 1998 and in mid-March in 1999 and 2000 and ended in mid-July in each year with occasional visits to the study sites until October. Fieldwork was conducted on a total of 265 days.

2.2.1.2. Nest searching, measurements, and monitoring

Nests were located by searching areas used by avocets for nesting. Every nest was numbered, marked and its location was recorded on a map. I recorded the type of substrate and vegetation and the distance of the nest to the nearest water. I measured the maximum length and maximum width

of each egg using a vernier caliper. Finally, I floated the eggs in a cup of water to determine their stage of incubation. Floating the eggs is not harmful for embryo development (Alberico 1998).

Modifying the egg immersion technique of Nol & Blokpoel (1983) I used the following scheme to infer the incubation stage of eggs:

 Stage 1: axis of maximum length of the egg is horizontal: 0-1 days,

 Stage 2: axis is tilted in ca. 45: 2-6 days,

 Stage 3: axis tilted in 90 (egg is standing on sharp end on bottom of cup): 7-12 days,

 Stage 4: egg is floating but does not reach the water surface: 12-15 days,

 Stage 5: egg is floating on the water surface but is not tilted: 16-19 days,

 Stage 6: egg is floating tilted on water surface: 20-23 days.

If the incubation stage of eggs within a clutch differed, I used an average stage for that clutch. I estimated the date of the start of incubation and calculated the expected date of hatching by adding 23 days (average incubation period, Cramp & Simmons 1983) to the start of incubation.

To decrease potential disturbance on avocets, I visited nests only three or four days prior to the expected date of hatching. Colony visits were limited to 1 h in cold weather and 0.5 h in warm weather.

2.2.1.3. Definition and determination of nest fates

A nest was considered successful, if at least one egg of the clutch hatched. If eggs disappeared from the nest before the expected hatching date, the nest was considered depredated. In ambiguous cases, I determined nest fates by the presence or absence of tiny eggshell fragments, which are characteristic of nests in which eggs hatched (Mabee 1997). A nest was considered depredated if

all eggs were missing and there were no tiny eggshell fragments in the nest. Mammalian predators were identified by footprints on the ground, toothmarks on the eggshells, eggs crunched in the nest (yolk in the nest-cup), eggs dropped during carrying or by eggs incompletely buried near nests.

Avian predators were identified when the eggshell remnants indicated that eggs were cracked from one side and their content was drawn out. I found such eggshell fragments in nests in which eggs were seen being eaten by Harriers (Circus aeroginosus).

Incubation period was defined as the number of days between the laying of the last egg (exclusive) to the hatching of the first chick (inclusive). Hatching period was given by the number of days between the day of hatching of the first chick (inclusive) and the day of hatching of the last chick (exclusive) within a clutch

2.2.1.4. Banding and measuring chicks

Colonies in which nests were hatching were searched for young chicks at least once per day.

Chicks were considered belonging to the nest in which they were found. I restricted nest checks to early mornings when it was cold and chicks were more likely to be in the nest than later during the day, when they were often at the edge of the water closest to the nest. If chicks were found in a nest, they were covered with a piece of cloth to keep them warm and to prevent them from leaving the nest while I was handling chicks in other nests. Chicks in a brood were banded with two plastic bands of seven different colors (white, yellow, orange, red, blue, green, and black) and a metal band from the Hungarian Ornithological Society on their legs above the tarso-metatarsus. The two color bands and one metal band were placed on chicks in a brood-specific combination. The chicks were distinguished by attaching pieces of tape of four different colors (white, yellow, red, and black) on the middle section of the metal ring.

I took three body size measurements on as many chicks as was possible. First, the length of the culmen was measured to the nearest 0.1 mm with a vernier caliper from the edge of down feathers between the eyes to the tip of the bill. Second, tarso-metatarsus length was measured with a vernier caliper ( 0.5 mm) by bending the chicks’ right leg at the intertarsal joint and the ‘ankle’

and measuring the distance between the extreme bending points. Finally, the mass ( 0.5 g) of chicks was measured to the nearest gram using a Pesola spring scale.

2.2.1.5. Monitoring broods

Brood movements were monitored by regularly locating avocet families near the nesting colony, on brood-leading routes and in the brood-rearing areas. Searches were conducted by using a telescope to find vigilant adults and counting and identifying their chicks. I attempted to cover all areas used by avocet broods at least once in three days. During observations I used mobile and stationary hunting blinds or a car as a mobile observation point. The observation distance was always chosen to be large enough to minimize disturbance on avocets and on birds of other species. I used territory mapping to determine the number of broods at any one site and the location of territories and composition of avocet broods occupying them were recorded on enlarged copies of 1:10000 habitat maps.

2.2.1.6. Definition of the fate of chicks and broods

A chick was defined as fledged if it was seen at or after age 35 days, when avocet young are able to fly. Broods were considered successful if at least one chick fledged from them. Chick carcasses were difficult to find and in most cases the death of the chicks was only inferred by their

disappearance from the broods. A chick was considered dead if it was not seen on at least three consecutive observation occasions.

2.2.1.7. Sampling of food availability

To quantify territory quality, I estimated the abundance of potential food items available to chicks on the territory of as many broods as possible. The food items of avocet chicks in my study sites are mostly aquatic and semiaquatic waterbugs (Heteroptera: Corixidae, Notonectidae and Gerridae), and dragonfly larvae (Odonata: Anisoptera and Zygoptera), water beetle larvae (Coleoptera: Dytiscidae, Gyrinidae), chironomid larvae (Chironomidae) and annelid worms (Oligochaeta) to a smaller extent (E. Boros, B. Kiss & Sz. Lengyel unpubl. data).

Sampling of food availability was conducted once in 1998 and twice in 1999 and in 2000, when I collected samples from territories of early as well as late broods. Territories were selected for sampling only if the brood occupying the territory consisted of young (< 2 wk old) chicks and if the brood was observed on the same territory at least twice in 4-5 days. I used territory maps recorded one day before sampling to identify territories for sampling.

The exact sampling location was standardized either in the center (two-dimensional territories) or the middle point (linear territories), because chicks fed in the center of a territory more often than on its edges. After locating the approximate center of the territory, I pushed down a plastic cylinder (diameter: 0.45 m) open on both ends into the lake floor covered by ca. 3-4 cm of water, the water depth preferred by young chicks for feeding. Next all aquatic invertebrate

organisms were collected from the water enclosed by the plastic cylinder using a sweepnet (mesh size 0.2 mm). Macroscopic (>0.5 mm) aquatic invertebrates were put in jars, selected from the debris on trays in the field station and preserved in 70% ethanol until they were identified in the laboratory several weeks later.

The abundance of invertebrates was calculated by dividing the total number of individuals collected in a sample by the volume (circumference of sampling cylinder multiplied by water depth) of water from which they were collected, and expressed as the number of organisms per liter water.