The Effect of Climate Change on Birds

by David Leech

4.2 Food availability

a. Food accessibility during British winters

The accessibility of food resources during the winter may be influenced by the extent of snow cover or frost during the winter period, particularly for ground-feeding birds and waterbirds. Reduced access to food may result in increased mortality rates.

• The population size of a variety of bird species, tends to be smaller following winters with a greater number of snowy days (Fig 4.2) (Greenwood and Baillie, 1991).

Fig 4.2. Graph showing the relationship between the rate at which wren population sizes changed between winter (D) and winter snow cover. Positive values of D indicate that populations increased in size the following spring, and negative values of D indicate thhat populations decreased in size the following spring. (From Greenwood & Baillie, 1991).

• Song thrush abundance during the breeding season was found to decline following winters with a high incidence of frosts (Baillie, 1990).

The size of song thrush breeding populations is reduced following frosty winters.

• Peach et al. (1995) reported that wren (Troglodytes troglodytes) survival rates were lower in winters with longer periods of snow cover.

Prolonged snow cover reduces the survival rate of the wren.

• Grey heron (Ardea cinerea) survival rates decrease during colder winters as areas of open water on which they rely for much of their food freeze over (Besbeas et al., in press).

The predicted trend towards increasing temperatures throughout the winter period in the UK suggests that the duration of snow cover and number of frosty days may decrease over the present century, which may in turn lead to increased rates of over-winter survival for some British species.

b. Winter food availability for Palaearctic-African migrants

• Marchant et al. (1992) identified a positive relationship between African rainfall levels and population size changes in a range of migrant passerines with population sizes increasing after wet winters.

• Over-winter survival rates of both swallows (Hirundo rustica) and whitethroats (Sylvia communis) increase with increasing rainfall on their African wintering grounds (Baillie and Peach, 1992).

Swallow survival rates are reduced during droughts on their African wintering grounds.

• Peach et al. (1991) observed that the both adult sedge warbler (Acrocephalus schoenobaenus) survival rates and the size of the UK breeding population increased with increasing rainfall in sub-Saharan Africa during the preceding wet season (Fig 4.3).

The size of the UK sedge warbler breeding population is reduced following periods of drought in Africa.

Fig 4.3. Graph showing the relationship between sedge warbler survival rates and the amount of rainfall in their West African winter quarters (higher index values signify increased rainfall). The numbers on the graph represent the season (years) to which each point corresponds. (From Peach et al., 1991).

c. Food availability for breeding birds

• Kittiwake (Rissa tridactyla) fledging success is related to phytoplankton and zooplankton abundance, which in turn is influenced by the degree to which westerly weather systems predominate during the breeding season (Aebischer et al., 1990).

Kittiwake breeding success is influenced by the predominance of westerly weather systems.

• It has been suggested that seasonal shifts in the peak abundance of sand eels in the North Sea in response to increasing water temperatures has led to a decrease in the breeding success of a variety of seabird species, including common guillemots (Uria aalge), shags (Phalacrocorax aristotelisi) and kittiwakes (Rissa tridactyla) (Rindorf et al., 2000).

• Montevecchi and Myers (1997) observed that slight variations in oceanic currents due to climatic warming may have major consequences for fish population sizes, and therefore for the abundance and distribution of seabirds such as the gannet (Sula bassana).

• The abundance of macro-zooplankton in the north-western Pacific increases as sea surface temperatures decrease, leading to an increase in fledging success for planktivorous auklets. However, high sea surface temperatures increase fish abundance and therefore also increase the fledging success of piscivorous puffins (Kitaysky and Golubova, 2000). Climatic warming may therefore be advantageous for some species and detrimental for others.

Changes in ocean currents may affect the abundance of gannets by influencing prey availability.

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