BRANTA — Martin Boschert
The Curlew (Numenius arquata [Linnaeus 1758]) at the Upper Rhine Valley of Baden - Contributions to a detailed, comprehensive science-based conservation programme (transl.)
Institution: University of Tuebingen, Germany
Supervisors: D Ammermann, H-H Bergmann (Univ. Osnabruck)
Details: PhD 2004 (Completed)
Address: c/o Bioplan - Institut fuer angewandte Biologie und Planung GbR, Nelkenstr. 10, D-77815, Buehl/Baden, Germany (Oct 2005) Email
Subject Keywords: Population biology, autecology, synecology, ethology, pollution, conservation
Species Keywords: Eurasian Curlew Numenius arquata
Studies of Curlew biology were carried out during the breeding season, from the 1990s until 2003 and included data and results from studies since mid 1980s, in several breeding areas (plains of the rivers Elz, Schutter, Kammbach, Rench and Acher) in the Upper Rhine Valley (Baden-Württemberg, Germany).
The breeding area of the Curlew in the second part of the 19th century and the first part of the 20th century extended over the whole Upper Rhine Valley. The current distribution in Baden is much reduced with the last breeding sites being limited to a small area stretching for about 90 km in length from south to north. The number of breeding pairs decreased from about 150 pairs during the mid-1970s to less than 50 pairs currently. The most prominent population declines in the 1970s and 1980s correspond with structural changes in agriculture, especially the conversion of meadows into arable farmland. In comparison with other investigations of the breeding biology of Curlews in Europe no fundamental differences could be found.
In the period from 1977-2003 the first Curlews arrived in the Elz plains between February 21st and March 8th. The mean arrival date changed significantly from March 1st in the 1980s to February 25th in the 1990s. At the beginning of the 1990s pairs were observed for the first time in the Upper Rhine Valley to occupy and defend a territory without actually breeding. Since then a variable number of pairs does not breed in any given year. Depending on the climatic conditions in the period under investigation, the first egg was found on March 31st and the last on May 6th. There is no indication for an earlier egg laying date since 1986. During 2000-2002, clutches contained on average 3.67 ┬▒ 0.51 eggs (n = 57) with marked differences between the breeding areas: from 3.89 ┬▒ 0.33 eggs in the Acher plains to 3.41 ┬▒ 0.59 eggs in the Elz plains, which compares to a significantly higher clutch size of 3.80 ┬▒ 0.50 eggs in the 1980s. A significant difference in clutch size between first and replacement clutches was recorded in the 1980s, but could not be found in the recent study.
The hatching rate differed between years and between area, but decreased clearly during the 1990s. Since 1977, annual reproductive success varied from 1.62 to 0.0 fledglings, with an average of 0.16 fledglings/pair each year, which seems to be insufficient to sustain the breeding population in the Upper Rhine Valley. The mean fresh egg mass calculated from 51 clutches with a total of 159 eggs amounted to 77 ┬▒ 5 g. Egg fresh mass did not correspond with laying date, but small differences between egg mass of the first and replacement clutches existed. No differences could be detected between the breeding areas. The mean egg volume of 49 clutches with a total of 151 eggs was 73.3 ┬▒ 6.0 cm3. Egg volume did not change with laying date. No differences in egg volume could be found between the breeding areas. Slight, but not significant differences between eggs of first and replacement clutches existed. Curlew obviously react to habitat loss and lower habitat quality (reduction in food supply) by reducing the clutch size, not by reducing egg volume.
Females deserted the breeding areas during the first days of June, at the latest. More than half of the females left their offspring in the first ten days of June, irrespective of the age of the chicks; the majority had gone by mid-June. Time of desertion in first and replacement clutches was significantly negatively correlated. A significant difference existed between successful and unsuccessful families. Families where females attended for the first ten days after chick hatching were significantly more successful. Curlew broods clearly benefited from females staying longer with their offspring. The main role of the female seems to be covering and warming of young chicks, which are not yet homoeothermic at this age. No relationship was found between offspring desertion and habitat quality.
Curlew used four different feeding-techniques: (1) pecking – bill touches substrate, (2) jabbing – characterized and distinguished from (3) probing by brevity of bill inserting into soil, (3) probing – bill partly or fully inserted in soil, and (4) taking prey items from the air. Beyond these, several more foraging techniques and strategies used in the breeding areas could be described. The different foraging techniques are used under different, mostly specific circumstances. But together they show the flexibility of the Curlew to search prey in different situations. Four methods of analysis (direct observations as well as stomach, faeces and pellet analysis) were used to determine dietary composition. By using direct observations a wide dietary range could be shown; this result was verified by examining faeces and pellet samples. Stomach analyses (of dead birds) were used to complete the overview about the dietary range.
Curlew are omnivorous in the breeding area. The most important groups of prey were earthworms, several beetle families, crickets, grasshoppers and locusts, but their diet also encompassed a very wide range of other prey items, especially insects such as flies, particularly crane-flies, ants or other invertebrate groups such as snails, millipedes and spiders. Vertebrates such as small rodents were rarely found. Several factors affected prey choice of Curlew in the breeding area. During the breeding season the diet changed from prey items living within the soil to prey living on the soil or in vegetation. The most preferred size classes ranged from 5 to 15 cm. Larger prey items (> 15 cm) were clearly preferred. The Curlew could be described as a feeding opportunist, who chooses its food according to availability, but on the other hand showing clear preferences for some groups.
Chicks and juveniles employ the same feeding techniques and foraging strategies; like adults they are omnivorous and take a wide range of different prey items from several invertebrate groups like earthworms, beetles, locusts, flies, ants and caterpillars or spiders. The diet changes during the growth period. Initially mainly prey items living in the vegetation or on the soil surface are searched for; later more and more prey items living in soil are exploited. As with adults, chicks and juveniles can also be described as feeding opportunists, by choosing easily available food, but preferring some groups.
The home ranges of Curlew families (n = 33) can be divided into two categories: (1) spatially separated sectors (n = 18) and (2) homogeneous home ranges (n = 15). Habitat choice and habitat use is determined by the age of the chicks, by hatching time, food supply and food availability, vegetation structure, supply of water, and by the presence of not too intensively used meadows. Optimal habitats were found to be extensively used fresh meadows.
During the period 1992-2003, 55 Curlew chicks were colour ringed, with 15 recoveries obtained, including two long distance recoveries. The majority of the recoveries of ringed Curlews, however, occurred within 40 km of their natal site. Nine of these birds were recorded as breeding birds at the earliest in the third calendar year.
In the different Curlew breeding areas in the Upper Rhine Valley a combination of different threat factors were found. The major problem found in all areas was the continuous loss of habitat with direct and indirect effects on the quality of the breeding habitat. These losses are mainly attributable to industrial and building areas, gravel pits and road building, areas for leisure time activities and loss of suitable habitat by the continuous change of agriculture. The main factors causing the decline of the breeding population (down from 33 pairs in 1970 to 5 pairs recently) were losses of landscape and structural changes in agriculture. The enormous increase in land use also led to habitat fragmentation and an additional loss of suitable breeding habitat through side effects of leisure time activities. In all parts of this breeding area a combination of several threat factors acting together were responsible for the population decrease.
The reasons for nest losses in 2000-2002 were identical in all breeding areas and attributable to three categories: natural, anthropogenic and unknown factors. The main factor (56 %) was predation. Out of 79 nests only 14 % produced hatched chicks. The reasons for nest losses have changed dramatically over time. The predation rate has multiplied in all regions. The red fox was identified as the main predator, whereas crows and other bird species did not appear as egg predators. Former clutch losses by crows had been influenced by disturbance of the breeding Curlews by humans. Other factors formerly responsible for nest losses have been greatly reduced, often due to conservation measures. Exceptionally, climatic influences could exert an influence in single years.
In the Elz plains space utilization by Curlews is shown to be directly influenced by the use of an adjoining model aircraft strip. If undisturbed, a virtually even utilization of available space was noticeable. In the presence of model aircraft, however, the Curlew were restricted to the use of peripheral areas of their territories further from the air strip or even forced to utilize areas outside their actual territorial boundaries. In addition to the devaluation and shift of territories, the interruption of brooding and the direct threat to the birds, two cases are reported in which direct effects of model aircraft disturbance on clutches of the Curlew could be established. This suggests additional far-reaching consequences for the well-being to the total population under study. The flying of model aircraft is found to be among the leisure activities with the severest consequences for birds in the Rhine Valley in general. In addition to this disturbance factor the investigations show that Curlew also keep minimum distances corresponding to different classes of roads, both in the choice of foraging areas and of nest sites.
Between 1986-1989 organochloro-residues were investigated in 37 eggs from 20 clutches out of the breeding area of the Elz plains. DDT, PCB and HCB were detected in all samples, sometimes in relatively high concentrations. Aldrin, dieldrin and heptachlor were also found but less frequently. The degree of PCB and DDE contamination varied from low to extremely high. The intra-clutch variation was smaller than that among the different clutches. In 1989 p,p’-DDT in its unmetabolised state was found in all eggs. Contamination with HCB, heptachlor, heptachlorepoxide, lindane, dieldrin and aldrin was at low levels. The differences in content of these residues could have arisen in four possible reasons. Additionally, some eggs from other parts of the Upper Rhine Valley, Lake Constanze, Bavaria and Nordrhine-Westfalia were investigated. The results were comparable to these of the Elz plains.