been raised about the potential for, and instances of, interbreeding between the two species and the viability of the hybrids. While the use of human-made nesting sites has allowed the cave swallow to greatly increase its range across North America, it may come with its own risks. Culvert and bridge nesting sites have an increased risk of flooding the nest area, a more variable thermal regime during the breeding season, an increased instance of sharing the colony site with other species, an increased risk of nest predation, and fewer available nesting materials. Caves and sinkholes, however, are not at great risk of flooding, have a less variable thermal regime, lower instance of sharing the site with other species, lower risk of predation, and easily available nesting materials. All this being said, cave swallows nesting at the culvert site laid more eggs, had more eggs hatch, and had more nestlings survive than those nesting at Dunbar Cave in 1974, which could explain why the population is growing so rapidly while using a riskier nest site. Incubation behaviour and clutch characteristics Cave swallow eggs are elliptical ovate in shape and typically white in colour with some variation of fine spotting of light to dark brown or even lilac and dull purple. Clutches of three to five eggs are typical and both sexes are thought to contribute to the incubation of the clutch. Only the female is observed to develop a brood patch. Cave swallows are altricial when they hatch; they are blind and incapable of sustaining their own body heat. They remain in the nest until they are capable of flying, which is approximately 20 to 22 days after hatching. Both parents will feed the nestlings small insects throughout the day and will defend the nest if threatened by swooping and loudly calling at the threat. References External links http://www.allaboutbirds.org/guide/Cave_Swallow/id http://www.sibleyguides.com/bird-info/cave-swallow/ http://xeno-canto.org/recording.php?XC=34112 http://xeno-canto.org/recording.php?XC=27723 http://ibc.lynxeds.com/species/cave-swallow-petrochelidon-fulva Cave swallow videos] on the Internet Bird Collection] Cave swallow photo gallery VIREO cave swallow cave swallow Birds of the Rio Grande valleys Birds of the Yucatán Peninsula Birds of Mexico Birds of the Caribbean Birds of Cuba Birds of the Dominican Republic Birds of Puerto Rico Birds of Jamaica cave swallow cave swallow cave swallow