Individual study: The importance of different crops and fallow fields as over-wintering foraging areas for seed-eating birds in Dolnoslaskie, south-western Poland
Orlowski G. (2006) Cropland use by birds wintering in arable landscape in south-western Poland. Agriculture, Ecosystems and Environment, 116, 273-279
A decrease in farmland bird abundance has been observed in many parts of Europe in recent years. In Western Europe, the decline of many seed-eating birds has been attributed to a greatly reduced over-winter stubble field area (important foraging habitat) due to abandonment of spring sown cereals in favour of winter varieties, as well as disappearance of wild growing weeds due to the extensive herbicide use. Central European farmland, with its less intensive agricultural production, constitutes an important refuge for many bird species.
This study investigated use of different arable field types by wintering birds in south-western Poland.
Study area: The study was conducted on 54 km2 of farmland in Dolnoslaskie province (Lower Silesia). Arable land covered about 93% of the area, the main crops in 2000 included wheat (50%), oil-seed rape Brassica napus oleifera (25%), root crops (10%) and maize Zea mays (8%). 1.5% was covered by woodland, and the rest (5.5%) comprised settlements and communication routes.
Study fields: A total of 117 fields, cultivated (n = 59) and abandoned (n = 58), covering 329 ha were selected. Fields were categorised as one of seven types: bare tilled (ploughed); winter cereals (wheat and barley); oil-seed rape and cereal stubbles (maize, barley); weedy root crop stubbles (sugar and fodder beet, potatoes); fruit and vegetable crops (cucumber, onion, celery, beetroot, chokeberries, strawberries); young fallow; and permanent fallow. (See Table 1, attached for field characteristics).
Root crop stubbles, fruit and vegetable crops were dominated by the weed common amaranth Amaranthus retroflexus. The young fallows were floristically richest, with dense cover formed by many annual weed species. Permanent fallows were dominated by wasteland communities consisting mostly of alien perennials.
Bird surveys: Bird abundance was assessed during two (ploughed fields, winter cereals and permanent fallow) and three (other crops) counts carried out between mid-December 2002 to mid-February 2003. Smaller fields were surveyed by walking a single line along their centre; larger ones along parallel lines spaced about 50 m apart. The weather conditions during the study period varied markedly with regard to temperature (−20 to 10 °C), and snow cover (0 to 30 cm).
Wintering birds were recorded in 69 (59.5%) of the 117 fields. No birds were found in bare tilled fields or winter cereals. Excluding one atypical unharvested field in which very large numbers of birds were observed (see below), a total of 27 bird species (2,256 individuals) were recorded (including four raptors, kestrel Falco tinnunculus, common buzzard Buteo buteo, rough-legged buzzard B.lagopus and sparrowhawk Accipiter nisus observed in small numbers). Seed-eaters (n = 2,030 individuals) accounted for 90% of all recorded birds (range 84-98% dependent on field type).
Tree sparrow Passer montanus was the most abundant species overall (531 individuals), whilst the most frequent in terms of presence in fields were reed bunting Emberiza schoeniclus (37% occupancy), followed by pheasant Phasianus colchicus (21%). Six species were observed in one field only.
Between December 2002 and February 2003 multi-species flocks of seed-eating birds exceeding 1,500 individuals, were observed in an unharvested wheat field (1.2 ha). The maximum number on this field was recorded on 1 January 2003 (−15 °C and 20 cm snow depth) when 1,970 birds, mainly seed-eaters, were counted, including yellowhammer Emberiza citrinella (650 individuals), corn bunting Miliaria calandra (550), tree sparrow(450), linnet Carduelis cannabina (220) and greenfinch C.chloris (50). The number of birds observed on a single day in this one field was enormous compared with 2,256 individuals recorded elsewhere during the three month survey period on all 117 fields. The following results exclude birds from this one aberrant field.
The highest average bird density was recorded in fruit and vegetable crops (61.6 ± 32.7 individuals/ha), with 20 times fewer in permanent fallows. The average number of individuals found within a single field was also highest in fruit and vegetable crops. Permanent fallows, however, held the highest share of insectivorous birds (7%) and predators (5%) albeit in very small numbers.
The highest average number of species was noted in cereal stubbles (c. 3.8) and the lowest in permanent fallows (1.2). The highest average number of individuals in a single field was again found in fruit/vegetable crops (71.2 ± 120.8 individuals/field) and lowest in permanent fallows (c. 7).
From 1 to 10 species were noted in a single field. The number of individuals on a particular field ranged from 1 to 370 (average 23.5 ± 49.8). There was a weak trend between field size and the number of individuals recorded as well as between field size and the number of species. No correlation was found between the field size and the overall density of birds.
Conclusions: The results of the study indicate that during winter the dominant birds using the fields (except for bare tilled and winter cereals, where no birds were seen) were seed-eaters. There was an almost complete absence of birds feeding on small soil invertebrates that typically forage in large numbers on fields in western Europe during the winter, easily explained by much harsher climatic conditions in Poland.
The weak correlation between the field size and the total number of birds, and the lack of correlation between the field size and the overall bird density suggests that overall abundance depends more on field type (and accordingly food availability). However, an accurate assessment of habitat preferences of particular species was not made due to the insufficient number of surveyed fields in some habitat classes, insufficient number of surveys, and the lack of quantitative data on weed densities. The results however give a useful overview of field use and distribution. A general increasing trend in seed-eater abundance was clearly visible in relation to the field weediness, mainly in field types with abundant annual weeds (i.e. fruit and vegetable crops, young fallows, cereal and root crop stubbles). Young (1–3 years old) fallows, profusely overgrown with annual weeds, in particular were characterized by high abundance and occupancy of many species. In contrast, seed-eaters were generally less abundant on permanent fallows which were dominated by perennial weeds, where the lowest occupancy and bird densities were observed. The exceptions were goldfinch Carduelis carduelis and linnet C.cannabina which fed on the seeds of tansy Tanacetum vulgare and burdock Arctium sp.
The majority of weed species found on the study fields form an essential component of the known diet of many seed-eaters wintering on northern European farmland. Those most numerous were: green foxtail Setaria viridis, fat hen Chenopodium album and common amaranth, the staple winter food of species such as tree sparrow, reed bunting and linnet. The winter diet of yellowhammer and corn bunting consists mainly of corn and monocotyledon weeds, which was reflected in the very abundant occurrence of these birds on the unharvested wheat field and also on the cereal stubbles. These results suggest that use of fields in winter by granivorous birds is influenced mainly by the lack of autumn cultivation. In terms of conservation management for such species, it therefore seems important to extend the fallowing period after harvest and refrain from ploughing until the spring of the following year.
Note: If using or referring to this published study, please read and quote the original paper: http://dx.doi.org/10.1016/j.agee.2006.03.005.