These type of shot were formerly commonly used as a line weight in angling. They are no longer solely manufactured from lead but instead are often made from softer materials that can be easily pressed onto the fishing line instead of being closed in a crimp using pliers, as was once common. The heads of some dead blow hammers are filled with shot to minimize rebound off the struck surface. Shot belt: some scuba diving weight belts contain pouches filled with lead shot. speaker stands can be filled with lead shot for additional acoustic decoupling, as well as stability. Model rocketry: to add weight to the nose of the rocket, increasing the stability factor. Due to its heat capacity and low thermal conductivity at low temperatures, lead shot has been used as a suitable material for a regenerator in Stirling engines and thermoacoustic cryocoolers. Due to lead's high density, it is used to attenuate radiation, especially X-rays and gamma rays. Lead shot may be enclosed in a bag and then be placed around a point source for radiation protection or for radiation shielding. Bird lead poisoning Lead shot-related waterfowl poisonings were first documented in the US in the 1880s; by 1919, the spent lead pellets from waterfowl hunting was positively identified as a major source of deaths of bottom-feeding waterfowl. Once ingested, stomach acids and mechanical action cause the lead to break down and be absorbed into the body and bloodstream, resulting in death. "If a bird swallows only one pellet, it usually survives, although its immune system and fertility are likely to be affected. Even low concentrations of lead have a negative impact on energy storage, which affects the ability to prepare for migration." Upland game birds such as mourning doves, ring-necked pheasants, wild turkey, northern bobwhite quail and chukars can also ingest lead and thus be poisoned when they feed on seeds. Lead from spent ammunition also impacts scavenging bird species such as vultures, ravens, eagles and other birds of prey. Foraging studies of the endangered Californian condor have shown that avian scavengers consume lead fragments in gut piles left in the field from harvested big game animals, as well as by the consumption of small game, or "pest animal," carcasses that have been shot with lead-core ammo, but not retrieved. Not all lead exposure in these circumstances leads to immediate mortality, but multiple sub-lethal exposures result in