When Climate Change Affects Where Birds Sing

While the effect of climate change on animal behavior is a known and visited topic, rarely do researchers delve into the effects of climate change on sub-categories of animal behavior, such as communication. In this experiment Møller (2010) studied the effects of climate change on sound through documenting changes in the height of song posts selected by songbirds. This was the first experiment to study the effects of climate change on vocalizations. Møller hypothesized that an increase in air temperature could alter sound absorption and through this, affect birdcalls. He also predicted that vocal communication could be affected by a change in plant phenology<!–[if supportFields]>XE “phenology” <![endif]–><!–[if supportFields]><![endif]–> brought on by temperature change and an increase in rainfall. Møller believed that these effects would manifest themselves in an overall change in song post height. He also believed, however, that these changes would be complicated by several variables, namely, sexual competition, susceptibility to predation, height of surrounding vegetation<!–[if supportFields]> XE “vegetation” <![endif]–><!–[if supportFields]><![endif]–>, and migration. To test these hypotheses Møller collected two sets of data on songbird post height, 20 years apart. —Emily Cole
Møller, A. P., 2010. When climate change affects where birds sing. Behavioral Ecology (Advance Access Publication).

Møller chose the village of Kraghede, Denmark to serve as the study sight. He collected data during the months of April, May, June, and July from 1986–1989 and then again in 2010. The area was 25km2 and consisted of farms, houses, open fields, hedgerows, woodlots, shrubs, and trees. The exact same area was studied during both periods of the experiment. To gather data Møller walked through the area recording no more than a single individual at each site. This was to avoid recording one bird as two. Birds don’t travel much during their breeding season so this was most likely an effective method. Møller recorded both the position of the bird on the vegetation<!–[if supportFields]> XE “vegetation” <![endif]–><!–[if supportFields]><![endif]–> and the maximum height of the vegetation using his own height as a comparison. He recorded all height values in multiples of his own height. In order to verify this method he used a hypsometer, an instrument that measures height, in addition to his visual judgment for 300 observations. The results of the two separate methods were extremely close. Møller collected mean temperature and rainfall data from April through August 1971–2010 for his study from the Danish Meterological Institute.
          Møller expected that song post height would depend on many variables, the first of which being song post habitat. He felt that species living in habitats with higher vegetation<!–[if supportFields]> XE “vegetation” <![endif]–><!–[if supportFields]><![endif]–> would necessitate higher song posts. To compensate for this he rated habitats on a three-point scale using 0 to represent grassland<!–[if supportFields]>XE “grassland” <![endif]–><!–[if supportFields]><![endif]–>, 1 to represent brush and scrub, and 2 to represent forested areas. If a species fell into both categories he assigned it the higher number.
In addition Møller predicted that the results would be affected by sparrowhawk predation, with more susceptible species showing lower song post heights. Thus he employed another numerical scale where 0 implied a species of bird was consumed as much as would be expected from it’s abundance, +1 implied that it was being consumed 10 times more frequently than would be expected from it’s abundance, and -1 implied that it was being consumed 10 times less frequently than would be expected. A higher number implied that a species was more susceptible to predation and would most likely have a lower song post height.  
Møller also hypothesized that song post height would be influenced by sexual competition within a species. He thought that species with higher levels of sexual competition would exhibit higher song post heights. He anticipated two situations in which a species would experience higher levels of sexual competition: species with growing population trends, and species in which the male and female were different colors.
Finally Møller anticipated that the results would be influenced by whether a species of birds migrated or not. He believed that migrating species were less likely to quickly adapt their phenology<!–[if supportFields]> XE “phenology” <![endif]–><!–[if supportFields]><![endif]–> to a changing environment than their residential counter-parts, and thus more likely to experience impaired environmental conditions. To account for this he recorded an estimate of each species migratory distance by averaging the northern most and southern most latitude for both the breeding and wintering locations and then subtracting the wintering mean from the breeding one.    
Møller found that many climatic aspects for April–August did indeed shift from 1971–2009. The mean temperature increased by 2 °C or almost 20% and annual precipitation increased from about 250mm to about 300mm or by about 30%. Møller concluded that such a change in temperature and rainfall would only produce negligible changes in sound absorption, if any, and thus discarded his hypothesis that this would affect song post height. However, song post height did increase on average 1.2 m, with species deemed more susceptible to sparrowhawk predation showing a smaller increase than those less preyed upon. While Møller did not find any significant changes in relation to migration, species anticipated to have higher levels of sexual competition showed a larger increase than others, as did those singing in higher vegetation<!–[if supportFields]>XE “vegetation”<![endif]–><!–[if supportFields]><![endif]–>. Møller stressed that this was not simply a case of birds singing in higher vegetation having higher options for song posts than those in lower areas, such as grassland<!–[if supportFields]> XE “grassland” <![endif]–><!–[if supportFields]><![endif]–>, because such areas often contained barns and other high posts. He found his results to be consistent with the idea that changes in plant phenology<!–[if supportFields]> XE “phenology” <![endif]–><!–[if supportFields]><![endif]–> led to this increase in height. This article is an excellent example of the broadness of variables that need to be examined when anticipating the effects of climate change. 

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