Cackle, cackle said the great-tailed grackle

Upon seeing a large black bird in Houston, my immediate assumption as to its species wouldn’t be a raven, or even a crow. Instead, I would immediately guess great-tailed grackle, or Quiscalus mexicanus, without even having to see it. Just today as I walked through The Rice University campus and with a quick scan spotted two females and three males, foraging in the grass near the cement. I observed one of the males catch a junebug and hold it in its beak.

My lack of surprise is largely due to the heightened presence of grackles in the southern and western United States throughout the last hundred years. According to Geoffrey LeBaron of the National Audobon Society, the huge increase in urbanization and irrigation has helped both Quiscalus and its cousin the boat-tailed grackle expand their ranges north drastically.

Despite their somewhat mundane description as a large, noisy black bird by the Cornell Lab of Ornithology, great-tailed grackles are actually easy to identify and are difficult to confuse with the much larger crow or raven. As adults, both sexes have distinctive, mildly disconcerting yellow eyes that almost look drawn.

The males are an iridescent black with blue and purple highlights, and, as their name suggests, they have very long tails that are tapered out into a fan shape. This display is sometimes shown when they are spooked, for instance when I crept too close with my camera. One of the males I saw was huddled somewhat protectively over a nut or other food item and likewise fanned its tail as well as attempting to intimidate the nearby female with a typical crooning cackles. His body feathers were puffed up a little and he hopped around, opening his wings for a moment until she flew a bit further off.

The females are smaller and brown with a darker tail and wings. Their tails are also smaller than the males’ proportionally, and they can sometimes be mistaken for juveniles, although juveniles have dark eyes.

At night, great-tailed grackles roost in trees, sometimes in large groups making a lot of noise. At a grocery store this weekend, I heard a cacophony of cackles, squawks, and soaring whistles coming from the isolated trees even though I only saw one great-tailed grackle on the ground. It is possible some of the noise came from starling or common grackles.

As I mentioned, great-tailed grackles don’t seem to mind humans, even encroaching on our sidewalks, and for the most part the presence of other birds doesn’t faze them either. Both males and females seem to amble back and forth on the periphery of human habitations foraging in most cases, but in Austin earlier this week I noticed a group of three or four birds in close proximity of each other all pecking eagerly near the base of a tree. Every time I have seen them during the day, though, they seem alert and assertive, which is only fitting of such a hugely successful invasive species.

Grackles are [understandably] considered pest birds since they are not only annoying and capable of damage to ecological balances, but they can also sometimes damage crops. Their relative, the common grackle, was partly responsible for causing an economic impact on sunflower crops in wetlands, according to Peer et al.

Despite their prevalence along the Gulf Coast and further west, I was curious to how quick and drastic their spread, and accompanying population boom, actually has been in the last century and a half. In fact, based on research and existing data compiled by Walter Wehtje, between 1880 and 2000, the great-tailed grackle’s breeding range in the USA has increased by 5530%, determined by data from published records, museum specimens, and egg collections. This doesn’t even take into account their non-breeding or migratory ranges.

Interestingly enough, Alexander F. Christensen cites historical evidence that in the 1400s, great-tailed grackles were found in the Gulf Coast lowlands and this more recent range expansion is in fact a recolonization from the Basin of Mexico. So, despite the extensive research on great-tailed grackle roosting range, is there anything else exciting about these birds?

The first thing I noticed about these birds was their sexual dimorphism, which is very obvious. I was curious about this phenomenon and other work done on other aspects of sexual dimorphism showed sex-specific energy requirements, differential energy direction throughout development and mortality rate variation.

In the late 1980s, Teather wrote three papers about intersexual differences in Quiscalus nestlings. Based on a hand-rearing experiment, males were both 1.52% larger than and needed more overall food and nutrition than females (see figure 1). However, females needed more energy per unit body mass. Because no difference was found in the amount of food brought by adults to male and female nestlings, Teather showed that males are more costly to rear, and he proposed a biased sex ratio at fledging as a result.

In line with his prediction, males suffered higher mortality during years of food shortage, showing a biased fledgling sex ratio. Another study also showed differential mortality rates in winter months (see table 1). This could be due to a number of factors, including their more noticeable plumage or greater size.

Even though great-tailed grackles are very common in Houston, most people don’t know anything about them if they can even identify them, yet now you know about their fascinating natural history as well as interesting life history traits based on sexual dimorphism!

REFERENCES

Wehtje, W. 2003. The range expansion of the great-tailed grackle (Quiscalus mexicanus Gmelin) in North America since 1880. Journal of Biogeography., 30: 1593-1607. doi:10.1046/j.1365-2699.2003.00970.x

Christensen, A. F. 2000. The Fifteenth- and Twentieth-Century Colonization of the Basin of Mexico by the Great-Tailed Grackle (Quiscalus mexicanus). Global Ecology and Biogeography., 9: 415-420. doi:10.1046/j.1365-2699.2000.00190.x

These articles describe the colonization of the Gulf Coast and other parts of North America by the great-tailed grackle and evidence of a previous migration from North America to the Basin of Mexico in the 15th century, respectively.

Teather, K. L. 1987. Intersexual Differences in Food Consumption by Hand-Reared Great-Tailed Grackle (Quiscalus mexicanus) Nestlings. The Auk., 104: 635-639.

Teather, K. L. and Weatherhead, P. J. 1988. Sex-Specific Energy Requirements of Great-Tailed Grackle (Quiscalus mexicanus) Nestlings. Journal of Animal Ecology. 57: 659-668. doi:10.2307/4931

Selander. R.K. 1965. On Mating Systems and Sexual Selection. The American Naturalist., 99: 129-141. doi:10.1086/282360

These describes sexual dimorphism in Quiscalus mexicanus and differential energy requirements and resultant sex ratio and mortality rate.

Peer, B.D. 2003. Impact of Blackbird Damage to Sunflower Bioenergetic and Economic Models. Ecological Applications., 13:248-256.

This describes the harm done to crop plants by grackles and other blackbirds.

FIGURES

Photograph 1. Male great-tailed grackle. Photograph taken by Patrick Coin

Photograph 2. Female great-tailed grackle. Photo by Michael “Mike” L. Baird

Graph 1. Graph of the common distribution of Quiscalus mexicanus based on breeding patterns and migration routes. Range map information from Ridgely, R.S., Allnutt, T.F., Brooks, T., McNicol, D.K., Mehlman, D.W., Young, B.E., and Zook, J.R. 2003.


Figure 1. Average amount of food consumer per day by male and female nestlings for six days once aged 6 days old. Asterisks based on t-tests. (* 0.1 > P > 0.05, ** 0.05 > P > 0.01, *** P < 0.01) (Teather, 1987)

Table 1. Sex ratios in Quiscalus mexicanus and Quiscalus major during 1960-61. (Selander, 1965)

Advertisements
This entry was posted in grackle. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s