| The northern Swallow-tailed Kite, Elanoides forficatus forficatus, a Neotropical migrant raptor of conservation concern, has failed to recover its former abundance and breeding range following widespread deforestation of breeding habitat and human persecution beginning in the late 19 th Century. The Louisiana-Mississippi subpopulation studied here appears to be limited by intraguild predation involving other species of raptors, especially Great Horned Owls (Bubo virginianus). Predation was documented using a multiple-methods approach that minimized sources of bias. Intraguild predation impacted the subpopulation in multiple ways: Raptors, particularly Great Horned Owls, killed kites of all ages, but especially adult females attending nests, possibly causing breeding-aged female limitation. During monitoring of 290 nests, recently fledged young, and radio-tagged birds (90 fledglings, 13 adults), intraguild predation was the leading cause of mortality, accounting for 50.5-56.8%. Considering mortality attributable to predators on or near nests, the Great Horned Owl was responsible for 50.5-98.1%. Raptor predation was also the leading cause (44.8%) of nest failure (N = 87 failed nests), the remainder attributable to weather and other factors. Intraguild predation was the key-factors (greatest impact on mortality of nest contents), explaining most variation in annual productivity, although weather and unknown causes were nearly as important. Productivity also declined at a rate of 0.057 young per year (1995-2005, 305 nests), and the annual frequency of nest predation contributed substantially to this decline (r = -0.728). Considering radio-tagged fledglings, 12.2% were depredated by raptors (at least 54.5% by owls) prior to first migration. Predation of adult kites attending nests during the 60-day exposure period for this species contributed between 7.87-9.62% to annual adult mortality. Raptor predation was the probable explanation for 81.8% of 22 instances of nesting neighborhood disappearance. The present study identifies the Great Horned Owl as a keystone predator of kites and many other birds.; Intraguild predation alone is not sufficient to explain the slowness of the kite's recovery. Additional factors implicated include (1) delayed age of first reproduction, perhaps as late as five years of age; (2) social dominance by adults slowing recruitment by pre-breeders failing to obtain a mate or territory; and (3) conspecific attraction potentially inhibiting re-colonization of formerly inhabited geographic areas. |
