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Ruby-crowned Kinglet is just one of more than 500 species known to regularly use the Rio Grande Valley as its main migration route. Photo: William H Majoros (commons.wikimedia.org)

Rio Grande big for birds too

Posted on: 10 Jul 2012

Groundbreaking migration research has revealed that the Rio Grande Valley has 10 times more birds migrating through it than the rest of the United States.

Conducted by the Caesar Kleberg Wildlife Research Institute's (CKWRI) Dr Bart Ballard the research is based on radar data gathered over a three-year period and found that the lower Gulf of Mexico area where the Rio Grande Valley is the main route supports bird migration counts up to 10 times that of the rest of the country. This is very significant when it comes to providing habitat and planning development.

"Research using radar in North America has shown that passage rates of migratory birds range from 40-500 birds per hour per km in non-coastal areas. We are finding passage rates well over 1,000 birds per hour per km along the lower Texas Coast," remarks Ballard. "We always knew the Texas coast was important for migratory birds. We just didn't know that the number of birds migrating through this region would be so much higher than other areas in North America."

The Texas Gulf Coast and the Rio Grande Valley are already known worldwide for their bird diversity, and groundbreaking research there has made important discoveries, and this may be the most important discovery yet.

Dr Ballard has completed the first phase of a massive study addressing some basic but very critical questions about bird migration. He notes that migration is an extremely energy intensive endeavour and some birds migrate tens of thousands of miles between breeding and wintering grounds. Many birds have the capacity to store enough fat reserves to allow them to take the shortest and fastest route across the Gulf of Mexico to their breeding or wintering grounds.

However, the birds that don't have that ability or that are not equipped to fly long distances – rails for example – instead go around the Gulf. For those birds in particular, the Texas Coast serves as one of the most critical stopovers during migration. In fact it is the reserves that these birds pick up in the habitats in the Rio Grande region, particularly in the Laguna Madre, that allow them to make the journey. For some species, these reserves also influence how well they reproduce on their breeding grounds.

Despite the incredible significance of this area to the avian population, prior to this study the migration data gathered for the lower Texas Coast were largely anecdotal. For example, the literature suggests that the Gulf Coast serves as a funnel through which hundreds of millions of birds from east and west pass through en route either to their breeding grounds in the North or their wintering grounds in Central and South America.

However, it's the details about dispersal, migration timing, the altitudes at which the birds travel, how the birds use the landscape and how weather impacts on migration that the CKWRI study addresses.

Ballard, an Iowa native, has had a passion for birds all his life. "My parents always joke that they never knew me as a kid because I was always outside," Ballard says. He recalls how when he was only six or seven he recorded the sex ratio of the grackles migrating through his backyard in a little notebook which he's kept all these years, and he's always had a field guide. Working in academia has afforded him an incredible opportunity to continue what has become a lifelong passion.

"I love the university setting because there are always opportunities to keep learning. The more I learn the more I realize that there is so much about these natural systems that I don't know," says Ballard. He's been conducting bird related research along the lower Texas coast now for 18 years. This migration study, however, is one of the most intensive projects he's been involved with to date.

The first phase of the study was conducted over a three-year period inclusive of three spring migration periods and three fall migration periods. Two mobile radar units were located on two different sites along the Texas coast, one in Kleberg County, just east of Kingsville, and another on the Laguna Atascosa National Wildlife Refuge further south in Cameron County. The horizontal radar, which samples bird movements in an area that is four nautical miles in diameter, is used to gather flight dynamics, flight speed, and landscape use. The vertical radar, which is oriented on its side at a 90 degree angle, is capable of detecting migration up to 9,000 feet. It not only records altitude data but rate of passage of birds.

Ballard and his team began collecting bird migration data in the fall of 2007. The radar units ran consistently from mid-March to early June to document spring migration and again from mid-August to mid-November to assess fall migration. To say the data set is massive is an understatement. Each unit is extracting several terabytes of data during each migratory period. To date there are approximately 40 million observations just from the one vertical radar unit alone.

It took four to five months just to get the data ready to analyse from just one unit. It's a very intensive process with many different steps required. Rain and fog events had to be manually filtered out. The software was also programmed to filter out any other non-targets like insects, airplanes, bats and anything else that is not likely a bird. Studies employing radar to estimate passage rates use a standard metric for reporting the number of birds. That standard is the average number of birds per hour that pass through a 1 kilometer width of the vertical radar. This allows direct comparisons among studies.

Perhaps the most surprising results were the peak passage rates which in the autumn, in some instances, topped 5,000 birds per hour across a half-mile stretch of their radar coverage. The fall migration is expectedly more protracted because the birds are not in such a rush like they are during spring when the race is on to be among the first to the breeding grounds. For some species, so long as there is food available in northern breeding or staging areas the birds hold tight and for the most part take their time coming south though their data indicate that weather is a factor. For example, peaks in bird migration during fall appear to correspond to cold fronts.

"What we saw was that the birds were coming either just ahead or immediately after the passage of a cold front. Birds are smart," comments Ballard. "They wait until favourable winds to migrate, which is often the case after a cold front. The bigger birds, in particular, will wait till after a front and use the tailwinds to assist their travels thereby conserving lots of energy."

Another somewhat surprising result about the fall migration period was that it's more intense than spring migration. In fact, passage rate during the fall was twice that of the spring. Again this is likely due to the 'hurry up and get there mentality' associated with spring migration. Ballard speculates that during spring migration more birds travel across the Gulf of Mexico rather than around thus the reason for lower numbers appearing on their radar during the spring timeframe.

"For many migrants there is a fitness advantage to getting to the breeding grounds early," Ballard comments. "Also the birds that arrive early secure quality breeding territories and have more time to re-nest if their first nest fails," he adds.

With respect to spring migration, researchers found that in the first spring numbers peaked in mid-May. However, there wasn't any real pattern that emerged the following spring. They have discovered, however, that during spring migration birds are flying at relatively higher altitudes compared to the fall. Ballard also attributes the higher flying altitudes during the spring to the spring rush in that the birds are likely stopping less to refuel.

The researchers are in the process of teasing out more of the weather data to see if there were any distinct differences in weather patterns between the two springs. They're looking specifically at the winds aloft data, recorded every 500 feet up to 10,000 feet.

Finally their data indicates that during autumn, migration is highly diurnal while the spring migration is highly nocturnal. "Traveling at night allows birds to use their time more efficiently because they can't feed at night anyway, so they fly at night and feed during the day," Ballard points out. "Birds are smart," he reiterates. "Raptors don't migrate at night so the passerines are reducing predation risk by travelling by night and feeding by day."

As Ballard's data substantiates, the lower Texas coast is an extremely important area that connects breeding and wintering areas for many of North America's migratory birds. Furthermore, this study is critically important and timely, he stresses, because of the rapid clip at which development along the lower Texas coast is occurring. Wind energy is but one such development.

To date, there are no siting requirements that regulate placement of wind farms in Texas. While Ballard's data so far suggest that most of the birds are going to fly high enough that wind turbines will not be a problem, a fog event during spring migration causing the birds to fly lower, or a late front in which the headwinds force the trans-Gulf migrant birds inland, could be game changers. "I'm not against wind energy at all," Ballard states, "but I do think we need to be smart about where we build wind farms."

No doubt finding a way to balance growth and minimize the potentially devastating impacts on wildlife and wildlife habitat is a huge challenge. Having good science is paramount and there is more science to come. For now though, Ballard is hopeful that these data will be used to help guide future development in a way that will minimize potentially negative impacts on migratory birds and their critical habitats.

The team's results are critically important because this part of the state is being rapidly developed, and the CKWRI is hopeful that these data will help in the decision making process to ensure that there is minimal impact on the wildlife and wildlife habitat that is so critically important.