Has Anthropogenic Disturbance Set A "trap" For Northern Cardinals?
    By Caitlin Kight | March 3rd 2012 09:37 AM | 8 comments | Print | E-mail | Track Comments
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    An "evolutionary trap" is a situation in which changing environmental conditions render species' usually beneficial behaviors unhelpful or even dangerous. Researchers have suggested that these traps may occur with some frequency in anthropogenic environments, which would explain the disappearance or decline of many species from these habitats. 

    (Male northern cardinal, Cardinalis cardinalis)

    Northern cardinals (Cardinalis cardinalis) are one of the latest species proposed to have fallen in to such a trap. These bright red birds achieve their coloration by consuming carotenoid-rich foods. Those that forage most successfully are the most colorful, and this signal of feeding prowess is used to advertise quality and good condition both to potential mates and rivals; the brightest males are typically those who produce the most, or best, young.

    In human-disturbed areas, a prominent source of carotenoids is fruiting Amur honeysuckle (Lonicera maackii) plants, which provide plenty of pigment but not as much nutrition as many alternative species. Thus, in urban areas full of this exotic vine, there may be a disconnect between color and condition, which could make it difficult for females to choose those males that will help them produce the most young. 

    (Amur honeysuckle, Lonicera maackii)

    A further complication is the fact that cardinals appear to prefer breeding habitats with a high density of the early-leafing honeysuckle. By securing these nesting sites, males can get a head start on their rivals, wooing the best females and utilizing food resources before the other birds begin their own reproductive efforts. Unfortunately, honeysuckle nests are at an increased risk of predation--partly because the vines just don't provide very good cover, and partly because they are the only available nesting habitat early in the season, making it easy for predators to figure out where to look for prey. Brighter males are more likely to win contests for possession of these breeding areas, which introduces another deviation from the standard relationship between color and reproductive success.

    In order to assess whether urban habitats--specifically, those dominated by the invasive Amur honeysuckle--act as an evolutionary trap to breeding northern cardinals, a team of researchers from the Ohio State University recently quantified the complex relationships between feather color, habitat characteristics, and annual reproductive success. At each of 14 study sites along an urban-to-rural gradient, the researchers collected feather samples from adult birds, located and monitored active nests, and quantified habitat traits--in particular, the density of understory vegetation and the presence of exotic shrubs. Because breeding adults were marked with unique leg bands, the researchers could follow individual birds across the entire nesting season in order to calculate their annual productivity, or total number of fledglings produced. 

    (Female northern cardinal)

    In rural areas, bright males acquired nesting territories with high levels of exotic woody vegetation--a habitat category dominated by Amur honeysuckle. These same males bred earlier in the season and had lower annual productivity, fledging approximately 20% fewer young over the course of the breeding season. In urban habitats, on the other hand, brightness was not associated with nest site structure, the earliest breeders had duller plumage, and there was no relationship between brightness and productivity.

    In other words, urban areas do not appear to act as evolutionary traps for northern cardinals; surprisingly, it is rural areas that seem to be more dangerous. In this case, though, the drawback of rural environments is still related to anthropogenic disturbance: the introduction of invasive species. Honeysuckle plants attract the brightest males in the best condition, but breeders in these substrates produce the fewest young. Unaware of this disconnect between color and quality, females continue selecting their mates using a now inaccurate checklist of sexy attributes, and ultimately suffer reductions in breeding success. The absence of this same scenario in urban areas is probably related to the fact that the overabundance of resources (carotenoids and breeding substrates alike) increases equality among males. As a result, females may base their mate choice on other (potentially more accurate) signals of quality, such as males' songs or the outcomes of physical fights between males. 

    (Fledgling northern cardinal--identifiable thanks to its brown bill)

    Many factors will influence whether these urban-rural differences will have a long-term impact on cardinal populations. One important factor is whether, and to what extent, birds move between habitat types; this also affects gene flow. Another issue is "temporal heterogeneity," or how consistent these ecological conditions are over time. If honeysuckle plants are quickly replaced by other species in rural areas, they may not have too much of a cumulative effect on breeding birds. As is so often the case with ecological research, more data will be required to explore these possibilities.


    For supplementary photos associated with this post, visit the Anthrophysis pin board at Pinterest.

    Rodewald, A.D., Shustack, D.P., and Jones, T.M. 2011. Dynamic selective environments and evolutionary traps in human-dominated landscapes. Ecology 92(9): 1781-1788.

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


    I looked up the Amur Honeysuckle on Wikipedia.  Here are the berries of this offender:

    It seems that there’s a posse out after this plant, as Wikipedia states:
    Spread of this plant is illegal or controlled in some areas of the United States due to its well documented invasive character. It is listed as a "invasive, banned" species in Connecticut, "prohibited" in Massachusetts, and a "Class B noxious weed" in Vermont. It is also officially listed as an invasive species by government agencies in Wisconsin and Tennessee.

    This plant is adaptable and successful in a wide range of conditions. In the United States, Amur honeysuckle was once planted to control erosion, and as hedges. It spread quickly as birds eat the fruit and disperse the seeds, and was soon naturalized. Notably, in deciduous forest understories of the eastern United States it forms dense growths with thick canopies that shade out native shrubs, young trees, and wild flowers. Uncontrolled, these growths create a near monoculture of Amur Honeysuckle. This species poses a serious threat not only to the diversity of the ecosystems which they invade but also to forest regeneration itself, as the plant is known for reducing the growth and diversity of native seedlings.  Moreover studies have shown that plant is responsible for having a negative impact on birds, and tadpoles.

    This is a map showing the Amur river, along with its name in written in Manchurian (meaning Black River).   One might well expect a plant from that part of the world to be as tough as old boots.

    (Nice to see that Science 2.0 copes well with the Manchurian characters in the filename.)

    Chinese adds one character and calls it the Black Dragon River, giving its name to China’s coldest province, Heilongjiang.  So get out your flame-thrower, here comes The Plant from Black Dragon River!

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    It's a shame honeysuckle is an invasive, because it is probably my all-time favorite smell. There is nothing quite like a warm summer night with the smell of honeysuckle blossoms hanging in the humid air. Now I feel guilty every time I start to enjoy it!
    NSF Postdoctoral Researcher, University of Exeter--Tremough Campus, UK. Personal website:
    The Amur Honeysuckle and the Japanese Honeysuckle are the only ones I know that are such thugs.
    Our native honeysuckle is, on the contrary, a very bird-friendly species, while Lonicera fragrantissima (native to China) is a boon to bees in the early part of the year when warm spells bring them out.  This photograph is from the front of our garden:

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    I didn't even realize there was a native honeysuckle--I think I have only ever been exposed to the "thug" variety. That one is certainly very beautiful, and the bee looks quite happy to visit!
    NSF Postdoctoral Researcher, University of Exeter--Tremough Campus, UK. Personal website:
    This one (Lonicera periclymenum) is our main native British honeysuckle, a vigorous climber, and we also have that in our garden.  The one I pictured was the Asian fragrantissima, a sturdy shrub, but it doesn’t seem to be getting out of hand.

    The birds (particularly blackbirds) like the berries of our periclymenum, but perhaps they are more nutritious.

    This is a non-native Sorbus in our garden.  In late autumn it is visited by blackbirds, but a few weeks ago some migratory redwings came and cleared all the remaining berries.

    Robert H. Olley / Quondam Physics Department / University of Reading / England

    FYI, the two links [ and] in your signature line are borked (they have a semi-colon and foward slash at the end for some reason.) Only works... atleast for me.

    Thanks very much--I will have to fix that!
    NSF Postdoctoral Researcher, University of Exeter--Tremough Campus, UK. Personal website:
    Just had a thought:

    Given the geographical location of Germany, wasn’t Pope Benedict XVI previously a Northern Cardinal?
    Robert H. Olley / Quondam Physics Department / University of Reading / England