For me, the singing of returning birds is the clearest sign that spring is here; the bubbling song of house wrens, the gurgling bars of red-winged blackbirds, the chirping of an American robin, or the melodic, crisp notes of a song sparrow. These sounds are a great pleasure to hear but there is much more behind these sounds than just music. Songbirds rely heavily on acoustic methods to communicate.
Why Do Birds Sing?
Two key reasons that male birds sing is to defend territory from other males and to attract females. When it comes to attracting females there is a lot that goes into it. A 2004 study discussed the three main categories that determine female preference for songs.¹ The first of these three categories is song output; that is how much a male sings; faster songs being preferred in birds such as pied flycatchers and longer songs being preferred in birds such white-throated sparrows. A second category is song complexity which is the number of different song types or the number of syllable types in a song, females being attracted to more complexity in birds such as red-winged blackbirds. The third category discussed is the local song structure or the “dialect” of specific populations of birds. Female white-crowned sparrows, for example, have shown a preference for the songs of males with local dialects over males with a foreign song. Bird songs are critical in the breeding of songbirds but what happens when these songs are obscured? What impacts might that have?
What If Birds Cannot Hear Each Other?
In a 1989 study, male seaside sparrows in Florida were muted and remained songless for two weeks at the beginning of breeding season and then again later in the breeding season. In every case the muted birds did not attract a mate with most of them attracting a mate after their voices returned. As well, when the birds were muted later in the breeding season, every male lost its mate while it was songless with a few regaining a mate after their song returned. Furthermore, the songless sparrows had considerably more trouble establishing territory and, later in the season, holding it when they were unable to sing.² This study really emphasized just how important hearing songs is for the breeding of songbirds. Birds not being able to sing at all obviously has a huge impact, but how sensitive are birds to slight variations in ability to communicate through songs?
An Ontario based study explored this as it explored the reason why birds sing so much at dawn.³ The study discussed the crucial importance that songs have in breeding and the conclusion of this study was that birds sing at dawn to maximize the accuracy of their song since transmission quality is much more consistent at dawn compared to later in the day. This is due to the fact that there is less atmospheric turbulence at dawn. This increased consistency is important because it helps the bird get across an unambiguous message which is of great importance in birds recognizing each other. The study highlighted that very small environmental factors reducing the quality of songs has a negative influence on the birds to the point that birds choose to sing when these factors will have the smallest impact.
The feathered songsters we hear every spring have evolved to use songs as a critical part of breeding and hearing songs clearly is of the utmost importance. What about the noise of our urban development and highways? May these human sources be having unintended consequences on the birds we share the land with?
How Do Humans Affect the Communication of Songbirds?
Humans can negatively impact birds’ ability to communicate through songs in a couple of key ways, auditory changes to habitat and structural changes to habitat. Humans are noisy. Anthropogenic noise or noise caused by humans comes from many different places (construction, roads, etc.) and this noise can mask the songs of songbirds, impeding their ability to effectively hear each other’s songs. Humans also build things. Erecting structures, such as buildings or walls, physically impedes, reflects, or distorts bird songs which negatively impacts songbirds’ ability to communicate.
Both of these are illustrated in a 2011 study.⁴ The study found that birds actually adapted their songs in areas with anthropogenic interference. Because most anthropogenic noise falls at a frequency range below 2000 Hz, bird songs in that range are interfered with the most. This study found that many birds, especially birds with songs naturally in lower frequencies, adjusted their songs to fall above this 2000 Hz range more often to be more clearly communicated. On the other hand, lower frequency songs are better able to pass structural barriers so the study found that in areas with more structural barriers like walls and buildings the birds adjusted their songs to be lower frequency more often, especially birds with higher frequency songs. The biggest problem occurs when both structural and auditory interference is present as birds are impacted on the high and low ends of the spectrum. To me, this study highlights a couple of key things. One is the unbelievable adaptability of nature to deal with challenges that are thrown their way. The other is the lengths birds need to go to try and deal with our effects on them.
The unfortunate thing is that birds can only adjust so much; birds with higher songs can only adjust them to fall in lower frequencies to a degree. As well, birds with lower songs can only use higher frequencies so much. We can push these birds to the point that they are unable to stay near human interferences. A 2010 Australian study⁵ suggested that birds with song frequencies below 1000 Hz might be just unable to adapt to large amounts of anthropogenic noise and would simply not fit in a habitat like that. There is definitely a point that we push species out of the habitat.
Other evidence of negative effects were found in a 2016 study that used an array of speakers to create a “phantom road” at a stopover area for fall migrating songbirds.⁶ This study found that birds avoided the area with the noise — especially hatch-year birds — and the birds that did use this area as a stopover point were found to have negatively affected body condition. A possible reason for the negative effects on the birds that did use this stopover area is that they were less able to use calls to communicate alarms and less able to detect predators. This study showed that there is potential for human caused noise to take a high quality and important place for migrating songbirds and force the birds to avoid it and possibly stop at a lower quality area instead. Since migration is a time when birds are at their most vulnerable, this definitely has the potential for large impacts.
Further studies have shown that noise not only impacts breeding and migrating birds but can even impact the fitness of chicks raised in a noisy environment. One study that focused on a population of house sparrows found that 12-day-old chicks in a noisy environment were of a lower body mass than chicks from a quiet environment. The study suggested that this could be due to the female sparrows perceiving that the chicks were less needy due to the masked begging calls of the chicks. This reduction in communication between the chicks and adults may have resulted in less food being brought to the chicks.⁷
There is a large amount of evidence suggesting that anthropogenic noise impacts birds and it is crucial that we take this into account when conserving bird habitat. Sound cannot be ignored when protecting an area. If we ignore this type of pollution we will either reduce the amount of suitable habitat for species of birds or reduce the success of the birds that use that habitat.
What Are Some Possible Solutions?
A 2007 study from Holland discussed various methods of reducing anthropogenic noise in bird habitat including building effective noise barriers, restricting traffic, or having a larger buffer zone between the source and the habitat.⁸ Another study, from the University of Alberta, suggested that residential areas could be used as refuges for noise sensitive species if proper habitat was presented such as desirable vegetation.⁹
Sound Barriers
Sound barriers next to sources of noise such as roads are one way to reduce our effect on birds. Many studies have been done on the acoustics of effective sound barriers but it seems the most effective type is a T-shaped barrier with softer materials on top.¹⁰ Shrubs or trees can also be incorporated into sound barriers to provide possible foraging or nesting areas for birds and other wildlife.¹¹ The issue with solid barriers, however, is that they would be a barrier to wildlife movement without having animal underpasses or overpasses built in. Another option, although less effective for blocking noise, would be to plant dense vegetation as the sound barrier.
Traffic Restrictions
Another method of reducing our impacts is traffic restrictions. Types of vehicles and speed of vehicles are two factors that greatly impact road noise. One way to decrease this is restricting louder vehicles such as motorcycles and semi-trailers entirely, at certain times of day such as the morning, or at certain times of year such as spring to fall for migration and breeding seasons. A 2012 study found that restricting heavy truck volume was necessary for spoonbill conservation in a wildlife reserve in China.¹²
Another way to decrease road noise is to reduce the speed limit. Again this can be in general, at certain times of day, or at certain times of year. This would also have the effect of reducing bird fatalities due to collisions with vehicles. The speed of traffic greatly affects the noise produced. This is shown in a University of Alberta study that found burrowing owls avoided any roads with a speed limit greater than 80 km/h so that they were able to effectively detect and react to predators.¹³ Research from Florida showed that by reducing speeds and by changing the type of pavement on the road greatly reduced the noise generated.¹⁴ Another study mentioned that both speed and flow control of traffic could be an effective measure to reduce anthropogenic noise.¹⁵
An important note is that the reduction of speed limits does not need to mean a reduction in traffic flow. A 2017 study found that a 40 km/h speed limit provided greater traffic flow than either 60 km/h or 80 km/h speed limits.¹⁶
Buffer Zone Between Development and Habitat
If we are trying to protect an area it is crucial that the protection starts well away from the natural area to allow for a buffer zone. There is more to protecting an area than just stopping development within it. To effectively protect a natural area consideration needs to be put into the noise created by development and traffic near the site as well. A 1994 study about willow warblers found that inside of 200 m of the roadway, male output (recruitment combined with productivity) was 40% less than further from the roadway.¹⁷ As well, a meta-analysis of 9 studies found that roads with 50 000 vehicles per day impacted grassland bird species from 75 m to 930 m and woodland species from 60 m to 810 m.¹⁸
Increasing Suitable Habitat in Residential Areas for Noise Sensitive Birds
In some cases, urban birds that are sensitive to noise can use residential areas as a possible refuge¹⁹ as these areas tend to already have measures put in place to reduce noise since noise is undesirable in these areas. It is important for us to recognise this potential use and make this habitat as desirable to birds as possible. We could accomplish this through planting a variety of plants including understory such as shrubs in our residential areas and parks being sure to select native varieties that attract considerably more insects which many birds use as a critical food source.
Closing
Birds have evolved to use amazing songs to communicate. These songs are not only a pleasure to hear but are critical for the birds themselves. With birds under threat from so many angles, it is important that we take steps to minimize our own impacts on them. Reducing the impacts of anthropogenic noise is one way we can do this.
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