BTO: The daily struggle to find food

Robin - Erithacus rubecula. Picture by John Harding/BTO
Robin - Erithacus rubecula. Picture by John Harding/BTO
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During these dark winter months it becomes apparent that our smaller birds may struggle to find food.

The long nights, low temperatures and reduced numbers of invertebrates available can make it difficult for birds to maintain body condition. It has been shown that the provision of supplementary food, in the form of seeds, nuts and suet products, at garden bird tables can make a difference to small birds, enabling them to replenish fat reserves used overnight to maintain body temperature at a safe level.

You might imagine that those birds living in our built up areas might fare better than their country cousins. A higher density of houses should mean more bird tables and more feeding opportunities for the birds. In addition, many of our larger urban areas experience something called the ‘urban heat island effect’. In essence, the heat that escapes from our homes, offices and shops lifts urban temperatures to several degrees above those experienced in the wider countryside. While these differences are small they may be important, as research carried out by the British Trust for Ornithology (BTO) has shown. On average, birds using urban areas first appear at garden bird feeders a few minutes later each morning than their rural counterparts. It is thought that the rural birds, faced with colder temperatures overnight, will have had to find food with more urgency come morning.

Interestingly, you might predict urban individuals to be the early risers because street lighting should enable them to start foraging a bit earlier than is possible in the unlit countryside. New research, again carried out by the BTO, shows that street lighting actually has a negative effect on arrival times (even after accounting for the degree of urbanisation – i.e. rural birds in less well-lit areas arrive earlier than those in better lit rural areas). There are several possible explanations for this: birds in well-lit areas might forage elsewhere before they visit gardens, or they might forage longer into the evening. Alternatively, they might avoid garden feeders in well-lit areas if there is a higher risk of predation there.

Whatever the underlying reasons for these complex patterns, it is clear that our activities (light and heat pollution, garden bird feeding) impact on our garden bird populations. Some of these impacts might be positive (the food provided at garden feeders 
improving overwinter survival), while others might be 
negative. It is important that we understand them, however, and that we can then inform best practice, whether this be for urban planners or for individuals wishing to 
feed the birds visiting their gardens in the best way possible.

When you see garden birds visiting your bird table or hanging feeders this month, think about the decisions they make about where and when to feed. To find out more about which birds visit gardens and when, view www.bto.org/gbw