Wintering Grounds of Endangered Owls Discovered in Mexico

by Geoff Holroyd, Canadian Wildlife Service

To Burrowing Owl enthusiasts, Canadian Burrowing Owl Recovery Team, Texas and Mexico owlers, NFWF, CWS managers and colleagues, and BBO executive:

Two "Canadian" Burrowing Owls have been discovered wintering in Mexico, the first time that Burrowing Owls from Canada have been discovered that far south in winter. Environment Canada biologists Geoff Holroyd and Helen Trefry flew 15,000 km in a Cessna aircraft over the Gulf coast lowlands and central Mexico. The discovery of these two owls is the culmination of a ten-year search for the winter home of "Canadian" Burrowing Owls.

During the summer of 2000, 85 transmitters were attached to Burrowing Owls in southern Saskatchewan and Alberta as part of studies of their survival and habitat use in the Canadian prairies. The transmitters have batteries that will last 12 months. An estimated 25 owls died in Canada before their migration south in September and early October. Thus the biologists searched for about 60 transmitters that were operating on 16 frequencies. Fewer frequencies were used to reduce the search time for transmitters while flying. The Cessna 172 was flown by pilot Tom Taylor of Rockport, the same pilot who conducts winter surveys of Whooping Cranes with Tom Stehn, US Fish and Wildlife Service at Aransas National Wildlife Refuge, Texas.

Two transmitter signals were located from the aircraft in late January, 2001. The biologists then returned to Mexico by vehicle to identify the owls.

One owl was in northern Veracruz state. The transmitter was attached to a juvenile owl in southern Saskatchewan by Danielle Todd, a graduate student at the University of Regina. The transmitter was found on the ground under an orange tree in a grove on the side of a small volcanic hill. The ground was littered with owl feathers, indicating that the owl had been plucked by a predator. Helen Trefry flushed Burrowing Owls twice while they searched for the transmitter. Even though they did not expect orange groves to be winter habitat for owls, the owls at least roost in groves in the daytime. To the north and east were small pastures where the owls may hunt at night, but most of the area is orange groves with small patches of banana plantations, a much different habitat than the mixed grass prairie where they nest in Saskatchewan.

The second transmitter was in northern Michoacan, just south of Guanajuato state. This transmitter was happily sitting on the back of a live owl that was banded in southern Alberta by Darcey Shyry, a graduate student at the University of Alberta. The owl was banded as an adult male. The owl was in a patch of cactus-thorn shrubland on a hillside surrounded by corn fields and pasture. Burrowing Owls were flushed nine times during the search for the owl with the transmitter. These other owls indicate that this shrubland is used by many other owls as a daytime roost. These owls were sitting on the ground at the base of thorn shrubs, This owl represents the first recovery of a Burrowing Owl from Alberta.

Michoacan is the same state where the Monarch butterflies winter. The Monarchs are further south in the coniferous forests in the mountains, while the owl was in the plains in the northern part of the state. Other Burrowing Owls have been found from Puebla and Mexico City to Guadalajara.

During the aerial surveys in central Mexico, biologists had a number of adventures. The volcano Popo erupted two hours after they had left the nearby Puebla airport, closing the airport to all traffic. A few days later the plane's engine overheated, forcing the pilot to make a precautionary landing at an abandoned rural airport. The plane was immediately surrounded by machine gun-toting Mexican troops who were stationed at the airport to wait for drug runners. The troops were very gracious but thorough in their search of the plane, before helping with temporary repairs. The team flew to Puebla to get the plane repaired before continuing their search. Near Guadalajara they were surprised by a plastic bag floating in the winds at 12,000 feet.

Only two other banded Burrowing Owls have ever been recovered in Mexico, one from Utah to Baja and the second from Oklahoma to Guadalajara. Other Burrowing Owls that were banded on the Canadian prairies have been found in Texas, Louisiana and states to the north.

In November and December, the biologists flew over south Texas and located 4 transmitters on wintering owls from Houston to McAllen on the Rio Grande border with Mexico. At least two of these owls were from Saskatchewan; the other two were from either province but were not identified on the ground.

The distance from Saskatchewan to the south Texas winter range is 2500 to 2800 km. The greatest distance was flown by the owl from Alberta that was over 3500 km from its breeding location to winter in central Mexico. The owl in Veracruz was 3400 km from its summer home.

Major funding for this study comes from Environment Canada and the National Fish and Wildlife Foundation grant to the Beaverhill Bird Observatory and in cooperation with USGS Biological Resource Division in Corpus Christi (Marc Woodin and Mary Kay Skoruppa).

An earlier study by Saskatchewan Environment and Resource Management found the owls migrated about 200 km per night. Thus the owls would take a minimum of 2-3 weeks to migrate between their summer and winter homes, if they flew that far every night. Likely they take longer, depending on spring weather and wind conditions. Gerry Batey, a biologist in Corpus Christi, determined that most of the owls that winter in that part of the Texas Gulf Coast had left by last week. Likewise another biologist, Enrique Valdez at the University of Guadalajara found that all the owls in our study area in central Mexico left in late February and early March. But many owls do not arrive on the Canadian prairies until the later half of April, indicating that they may take 6-8 weeks to migrate north.

The discovery of the wintering Burrowing Owls means that the biologists can now focus on learning more about their winter survival to determine if over-winter mortality is contributing to the decline of the owls in Canada. A preliminary study in central Mexico last winter by Holroyd, Trefry and Valdez showed that all the owls survived, but near Corpus Christi, Batey found that 15-30% of the owls died in the winter. More intensive studies are planned for the next two winters by the international team to determine if these early indications of mortality are valid.

Holroyd first traveled to Mexico in 1989 in search of the winter destination of "Canadian" Burrowing Owls. After searches in northern Mexico failed to reveal any major winter range of owls, Holroyd turned his attention to southern Texas and Valdez discovered more owls in central Mexico in winter. Fellow biologist Jason Duxbury found three transmitters on owls from Canada in southern Texas in a preliminary trial of the aerial search technique.

Students in a high school near the central Mexican study site have aided the biologist in their searches of the nearby hillside where the owls winter. The team found 21 owls on a 40 hectare hillside in February and 24 owls wintered their the previous winter. The Alberta owls with the transmitter was found about 75 km from the school. Last winter the two Canadian biologists took books and National Geographic magazines to the two room high school which does not have a library. This winter they gave the school two pair of binoculars donated by fellow biologists Loney Dickson and Brenda Dale.

Holroyd and Trefry hope to promote closer linkages between prairie towns and towns in central Mexico that share the summer and winter homes of these endangered owls.

The breeding population in Canada is declining at about 16% per year. Over 700 landowners in Alberta and Saskatchewan voluntary conserve over 50,000 hectares of native prairie as nesting habitat for the Burrowing Owls. Despite this large conservation effort that is coordinated in Alberta by Alberta Fish and Game Association, the number of owls continues to decline.

This spring, the biologists will search for the transmitters by plane in the prairies and adjacent US to determine how many owls returned to nest and how far they have wandered from last summers nest sites. Holroyd suspects that some owls are stopping in the US, further increasing the apparent decline in Canada.

The US Fish and Wildlife Service have initiated a formal review of the status of the Burrowing Owl in western North America after an international conference that was organized by Holroyd and Trefry in Utah concluded that the Burrowing Owl was declining over most of its range in western North America. In California, the Burrowing Owl conservation team believes that the owl will be wiped out within 10 years. Across the US plains the owls depend on prairie dogs for nest sites, but the decline in the prairie dogs, considered vermin by some landowners, has added to the plight of the Burrowing Owl. In Canada, the Richardson's Ground Squirrel or gopher, and the badger play the same role as the Prairie Dog in the US, providing burrows in the prairies for the owls. The owls lay their eggs and raise their young underground, a very unusual habit for owls.

Geoff Holroyd
Research Scientist
Canadian Wildlife Service, Environment Canada
Room 200, 4999-98 Ave., Edmonton, AB, T6B 2X3
phone 780-951-8689; FAX 780-495-2615
19 March 2001

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