Birdsong and anthropogenic noise: implications and applications for conservation


  • Box 1 Is singing low important to great tits?

    There is a general relationship in birds between body size and frequency of pitch: larger species typically sing lower. Small birds with small vocal organs may be lacking the power, the essential size of the vibrating source, or the suitable resonance features of the vocal tract, that allow the production of low-pitched sounds. This creates the possibility of using low frequencies as a way to signal condition or size to competitors and potential mates (Ten Cate et al. 2002). However, we do not know whether intraspecific variation in male body measures is related to the minimum frequency in great tits, nor do we know if female great tits prefer males singing low-pitched songs. If it were hard for great tits with a territory to sing low, you may expect that they sing songs relatively high in pitch when the motivation to sing is low. Such a situation occurs at the end of the breeding season, in both urban and forest habitat. Males still sing their stereotypic song types but the resources at stake in their territory have decreased in value until the start of the next breeding season. A decline in motivation to sing is clear from a decline in number of repeated songs (phrases) in a strophe, independent of habitat: in the city of Rotterdam from six to three phrases, and in the forest called the Liesbos from six to four phrases on average (anova: F2,100 = 27.4, P < 0.001). Interestingly, controlling for habitat differences, we found that the minimum frequency increases dramatically when recordings from relatively early in the season (early April: ‘Early’) are compared to recordings from late in the season (late May: ‘Late’, anova: F2,100 = 7.8, P < 0.01). We therefore hypothesize that it may be an important signal of quality to have low-pitched notes in the song type repertoire, which may cause a trade-off especially for the urban birds in noisy territories. Low-frequency noise may constrain the capacity to signal individual qualities: urban birds may need to choose between being heard well or being rated high as a competitor or mate. High noise levels may increase the number of intruding competitors needing physical combat and may affect the number and quality of potential mates that are attracted. Consequently, although great tits are successful in cities, and even though they show a remarkable flexibility in spectral adjustment to local noise conditions, this does not mean that traffic noise interference is not harmful to individu


    al birds.

  • Box 2 Noise barriers for birds

    Raising a barrier between the noise source and bird breeding habitat can lead to a significant reduction in detrimental noise levels. Vegetation is only of limited use, although the effectiveness will increase with stem and leaf density and width of the zone (e.g. Cook & van Haverbeke 1972; Bucur 2006). A solid barrier as close as possible to the noise source will be most effective (Maekawa 1977; Ishizuka & Fujiwara 2004). Adding an overhang on the road side or increasing the height of a barrier which is already in place to benefit pedestrians at ground level (A), could be a very cost-effective measure to the benefit of flying and tree-dwelling birds at a higher level (B). It is usually also easier to filter out the bird-relevant frequency components of traffic or industrial noise than to block the lower frequencies. Construction details and barrier shape especially can have a big impact in the frequency range between 2.0 and 8.0 kHz. For example, sound-pressure levels at the barrier's edge (= secondary point of radiation) can be significantly decreased by installing a soft, absorbent material on top. This will lead to a decrease in the noise level behind the barrier, especially in the field below the barrier height (A). Specific shapes of the barrier top with multiple diffracting edges may add to this effect. For example, T–shaped noise barriers with soft material on top only have to be three meters high to be as effective as a 10-metre high rigid–edged plain barrier (Ishizuka & Fujiwara 2004). This effect steadily gets stronger with frequencies above 250 Hz, also for the sound field above barrier height (B), which concerns the habitat layer typically most important for acoustic signalling in many bird species (Slabbekoorn 2004). Noise barriers at both sides of a road shield areas in both directions, but multiple reflections and scattering effects between the barriers will reduce the shielding capacity. Noise levels become higher especially above the barrier compared to a single barrier situation (Maekawa 1977). Buildings by the side of a busy road typically also increase noise levels within the street or urban canyon (Oldham & Radwan 1994; Heutschi 1995). Both inside and outside urban areas, the problem can be brought down by noise abatement schemes using a more absorbent ground surface and intermittent or continuous absorbers on the walls of buildings or noise barriers (Horoshenkov et al. 1999). Placing noise barriers at an angle will also reduce the impact of reflections by beaming them more upward into the sky or downward back towards the tarmac. Finally, the choice of construction material or absorbent matter may make noise barriers also bird-friendly in nonacoustic ways. Combination barriers incorporating trees or shrub layers can reduce the negative impact of an artificial, visual barrier in the landscape, and potentially provide nesting and foraging



Hans Slabbekoorn, Fax: 31 71 527 4900; E-mail:


The dramatic increase in human activities all over the world has caused, on an evolutionary time scale, a sudden rise in especially low-pitched noise levels. Ambient noise may be detrimental to birds through direct stress, masking of predator arrival or associated alarm calls, and by interference of acoustic signals in general. Two of the most important functions of avian acoustic signals are territory defence and mate attraction. Both of these functions are hampered when signal efficiency is reduced through rising noise levels, resulting in direct negative fitness consequences. Many bird species are less abundant near highways and studies are becoming available on reduced reproductive success in noisy territories. Urbanization typically leads to homogenization of bird communities over large geographical ranges. We review current evidence for whether and how anthropogenic noise plays a role in these patterns of decline in diversity and density. We also provide details of a case study on great tits (Parus major), a successful urban species. Great tits show features that other species may lack and make them unsuitable for city life. We hypothesize that behavioural plasticity in singing behaviour may allow species more time to adapt to human-altered environments and we address the potential for microevolutionary changes and urban speciation in European blackbirds (Turdus merula). We conclude by providing an overview of mitigating measures available to abate noise levels that are degrading bird breeding areas. Bird conservationists probably gain most by realizing that birds and humans often benefit from the same or only slightly modified measures.