Saturday, 8 October 2011

Anthropogenic landscape modifications mislead settling migratory birds

The phrase "anthropogenic habitat change" is usually associated with fairly obvious alterations to the environment, such as surface mining or the introduction of buildings and roads. However, anthropogenic modifications can also be more subtle and, as a result, potentially more harmful. For instance, many animals--particularly those that migrate to summer breeding grounds--use environmental cues to assess how good a habitat is before they settle there. They might look at the structure or stage of vegetation growth, the availability of food and shelter, or even the presence of nearby conspecifics. Generally, these cues are useful because they indicate that a habitat will be favorable for some aspect of an animal's fitness--including its own survival, longevity, or reproductive output, or those of its young.

In anthropogenic environments, however, habitat selection cues may no longer be accurate indicators of these fitness benefits. This can result from an uncoupling of cues and habitat quality, the presence of new or variable cues that make the habitat look attractive even though it is not high quality, or a combination of the two. Under these circumstances, the habitats can be thought of as "ecological traps." Unsurprisingly, these can be very detrimental for the widespread and long-term success of species' populations.

 (A spruce plantation--high-quality breeding habitat or ecological trap?)

A new example of "ecological traps" has recently been described by collaborators from the Catholic University of Louvain and the Gabriel Lippman Public Research Centre. The researchers examined reproductive success among red-backed shrikes (Lanius collurio) breeding in the south of Belgium at farmland and woodland locations. Although it seems counterintuitive, the agricultural areas are actually the more "traditional" habitats for these birds, and only in the past several decades have the shrikes been settling in woodlands--specifically, areas of spruce plantations where trees have recently been felled. These are thought to mimic the birds' ancestral (pre-agricultural-field) habitats--open areas that periodically emerged in forests after fires or tree collapse.

(A red-backed shrike, Lanius collurio)

Preferable habitats are generally those that are settled earlier in the season and/or are occupied by higher-quality males;on the other hand, good habitats are those that have a higher proportion of successful nests, larger broods, and/or chicks in better condition. In order to investigate which habitats the Belgian shrikes prefer, and whether these same habitats are actually high quality, the researchers measured a number of variables for each active nesting territory: how early in the season it was settled by a male; dominance and quality of the resident male (assessed by measuring wing length and eye-stripe size); productivity of the resulting nest (assessed by measuring whether it produced at least one fledgling); brood size; and body condition of the nestlings produced.

(Farm in south Belgium. Shrikes hunt over open fields but nest in nearby patches of trees. They use thin, pointy objects--such as the barbs on barbed wire, or thorns/branches on trees--to impale their prey; collections of prey are known as "larders.")

The shrikes clearly preferred the woodland sites. These areas were settled an average of 3 days earlier than farmland sites, and better males (those with longer wings and larger eye-stripes) selected spruce plantation territories over agricultural areas. However, reproductive outcomes were consistently better at farmland sites. In agricultural areas, nest success was higher, an average of 0.5 more chicks were produced per nest, and nestlings were in better condition.

All of these findings suggest that spruce plantations are ecological traps. Furthermore, these patterns were consistent across all 3 years of the study, indicating that the shrikes' preferences were consistently maladaptive across time, rather than being beneficial in some years and less useful in others. It is not entirely clear what the shrikes are looking at when they decide that the woodland habitats are preferential, but it is likely that they are responding to an innate instinct to settle in these areas that so closely resemble their ancestral breeding grounds. Likewise, the researchers also aren't yet sure why plantations are physically similar to historical nesting habitats, but are poorer-quality breeding sites; perhaps nearby reforestation or logging efforts disrupt the birds' activities, or the habitat has a different vegetation structure or unfavorable microclimate conditions. Differences in breeding success between farmland and woodland sites are likely driven by variations in the amount of edge habitat and availability of sheltered sites that can be used for nesting and hiding from predators.

The major question raised by this research is whether woodland sites are not just ecological traps, but also "sinks"--sites where the poor breeding conditions end up having a negative impact on a species' entire population. In order to determine whether this is the case, it will be necessary to investigate other components of fitness that may make these anthropogenically modified habitats more worthwhile--for instance, long-term survival rates of chicks and adults. It is also important to understand the large-scale movements of the shrikes, since migration between different types of site from one breeding season to the next may end up "rescuing" both individual birds and entire populations from the negative reproductive effects of woodland habitats.

If management efforts are required to preserve shrike populations, future studies will need to better understand the behavioral processes that drive habitat selection. In this study, the authors found that the presence of conspecifics had a small positive effect on settlement patterns, such that the shrikes were interested in living in habitats where other birds had already taken up territories--possibly because newly-arriving birds interpret the presence of other individuals as a sign of a habitat's quality. If this is the case, managers could try to attract shrikes to more suitable habitats by broadcasting their vocalizations or setting out stuffed birds. It has also been suggested that incoming migrants may look for larders--areas where other shrikes store their food--as an indication that there are plenty of resources in the area. Thus, installation of artificial larders in agricultural areas could convince the birds to nest there rather than wasting their reproductive energy at nearby spruce plantations.

Hollander, F.A., Van Dyck, H., San Martin, G., Titeux, N. 2011. Maladaptive habitat selection of a migratory passerine bird in a human-modified landscape. PLoS ONE 6(9):e25703.

Thanks to the following websites for providing the images used in this post:

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