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1998: Ecology and Conservation of Altamira Oriole in the Lower Rio Grande Valley of Texas

The Altamira Oriole (Icterus gularis) is one of several tropical birds whose U.S. range is limited to southern Texas, and which has declined in recent years in former strongholds. The Altamira's Santa Ana National Wildlife Refuge population has declined from 15-20 nesting pairs in the 1970s to 4-5 pairs during the 1990s. The 20-25 pairs which nest along the Rio Grande in the Falcon riparian woodland below Falcon Dam represent the largest known population in the U.S. Reasons for the declines at Santa Ana and general rarity of the Altamira in the Lower Rio Grande Valley (LRGV) may be related to two main factors:

1) Habitat deterioration, due to droughts and lack of flooding: Since the 1950s, Falcon Dam (and others upstream) have prevented the Rio Grande from flooding in the LRGV. Death of large trees has been noticed since then, particularly during drought periods. Although Altamira Orioles forage in thorn-scrub, they require relatively tall trees emerging from the forest canopy, or at the forest edge, as nest sites. Texas ebonies and tepehuajes, formerly preferred as nest sites at Santa Ana, have died or remained stunted in many areas, due to the combined effects of freezes and droughts.

2) Bronzed Cowbirds have increased greatly in the LRGV since the late 1970s. This brood parasite may have driven other orioles from the area, and we have determined that Bronzed Cowbirds also affect Altamira Oriole nesting success. We have confirmed that Bronzed Cowbirds can successfully enter Altamira Oriole nests, by waiting until the orioles are away from the nest site. Once inside, cowbirds may lay their own eggs and also puncture Altamira eggs. This may cause abandonment by the Altamiras, but in some cases the Altamiras end up raising Bronzed Cowbird fledglings (noted on 4 different occasions).

Although our data analysis is not yet complete, since the field season has just ended in August 1999, I can make some tentative conclusions and recommendations:

Pairs which have been successful in raising their own offspring tend to be 1) full adults, whose experience may enable them to forage and defend their nests more successfully, and 2) birds nesting adjacent to wetland or dense thorn-scrub, and 3) those not nesting over known cowbird foraging areas, such as roadsides, campgrounds, and private yards. In addition, some pairs of Altamiras seem to spend more time around or in the nest, and are thus more likely to keep cowbirds from entering.

Cowbird control may be eventually needed, but habitat restoration and improvement techniques should first be continued and expanded. Revegetation of abandoned farmlands should be continued, and habitat restoration within major parks and refuges should be expanded (by controlled flooding).

We greatly appreciate and wish to thank the American Birding Association for providing the funds needed to purchase accurate nest-checking equipment and associated material, and for supporting the summer field work of a graduate student and myself during the last two field seasons. We also gratefully acknowledge the support of the National Fish and Wildlife Foundation and Swarovski Optik.

Tim Brush
Department of Biology
University of Texas-Pan American
September, 1999