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In 1978 and 1979 we studied brood reduction in Black-legged Kittiwakes (Rissa tridactyla) on St. Paul Island, Alaska. In two-egg clutches, first-laid eggs were largest; they were incubated before second eggs were laid and consequently hatched first. Chicks from first-laid eggs begged more frequently, fed more frequently, grew faster, and were more aggressive than their sibs. These sibling differences frequently facilitated the ejection of younger siblings from the nest by first-hatched young. Lower feeding and growth rates and higher aggression levels occurred in broods in which the second chicks subsequently died than in broods in which both chicks fledged. Chick loss was most frequent during inclement weather. These data are consistent with the hypothesis that brood reduction is related to the amount of food that chicks receive.

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Agriculture is a sector of the economy that influences the natural environment both directly and indirectly. Traditional and industrial farming not only alters the soil and water conditions, but also affects the animal and plant life of agrosystems (Horrigan, Lawrence & Walker 2002). Sustainable farming, and organic farming in particular, are activities that are much more beneficial to the natural environment (Stolze et al. 2000). The fundamental principle of sustainable agriculture is to introduce the least possible amount of foreign matter so as to preserve the best possible environment quality for future generations (Horrigan, Lawrence & Walker 2002). Sustainable agriculture aims to reconcile various aspects of the uncontaminated environment with socioeconomic profitability (Horrigan, Lawrence & Walker 2002; Rodriguez et al. 2008). In order to more effectively protect the environment and plants against pests and diseases, sustainable farming recommends appropriate crop rotation (Corselius, Wisniewski & Ritchie 2001; Robak 2007) i.e. that there should be an interval of at least four years between the cultivation of crops of the same species in any particular field (Szwejda 2007). Practices such as crop rotation and the use of intercrops and aftercrops are measures that slow down the spreading of weeds, diseases and pests, help preserve favourable soil structure and maintain the desirable chemical and physical features of the soil and its biological activity. They also help prevent soil erosion in areas with pronounced relief (Corselius, Wisniewski & Ritchie 2001; Smagacz 2007; Szwejda 2007). Tree planting as well as the cultivation of semi-wild varieties of plants (Horrigan, Lawrence & Walker 2002) also play an important role in the development of biodiversity and the conservation of agrosystems (Bałazy, Weyssenhoff & Ziomek 2007). Sustainable agriculture refers to two basic types of farming: organic and integrated. On organic farms, man-made chemicals (i.e. pesticides, growth regulators, artificial additives to fodder and mineral fertilisers) are not used and only the use of calcium fertilisers is permitted (Stolze et al. 2000). However, farming in this way requires substantial outlays in terms of time and labour and for this reason, organic farming is recommended mainly to small farms. These farms are usually self-sustaining, a feature which also makes it possible to contain the circulation of matter within the farm's system of soil, plants and animals (Ciećko et al. 2007; Horrigan, Lawrence & Walker 2002; Stolze et al. 2000). The main goal of integrated farming is the proper selection of plants that are suited to the specific environmental conditions of the farm in order to ensure satisfactory crop yields and thereby reduce mineral fertilisation and use of pesticides. Important components of both integrated and organic farming approaches are associated with the development of the agricultural landscape by ensuring desirable proportions for the layout of fields, meadows and pastures, and the enrichment of the...

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