Is Three a Crowd? Three Indian Ibis Species Vary Strategies to Achieve Sympatry Depending on Different Seasons, Landscapes and Habitats

Closely related sympatric species achieve sympatry using multiple strategies, which includes avoiding competition by sharing some habitats (>1 species using the same habitat at the same time; “syntopy”) while using other habitats exclusively (“allotopy”). The extent to which sympatric species use the syntopy-allotopy strategy relative to other strategies, especially with relation to changing land use and season on agricultural landscapes, is poorly documented in waterbirds. We measured year-long abundance and habitat use of three ibis species on a starkly seasonal semi-arid landscape using an a-priori design covering areas with two dominating land uses (wetlands and agriculture) in north-west India. We hypothesized that ibis abundance and allotopy would be higher in wetter areas and seasons (lower competition, higher resource availability) and that drier areas and seasons (higher competition, scarce resources) would have lower ibis abundance and increased syntopy. Ibis abundance varied by species (Red-naped > Black-headed > Glossy), season (Summer > Winter > Monsoon) and land use (wetland > agriculture). Ibises largely avoided inter-species competition (81% of observations were of single species), one species showed year-long allotopy (Red-naped Ibis used crop fields) and the other two species preferred wetland habitats. Syntopy was rare and occurred mostly in areas with more wetlands (91%) and during summer (63%). Ibises achieved sympatry by employing multiple strategies – avoiding direct competition, divergence in species' habitat use, and increased syntopy during summer. Ibis behavioural plasticity favoured their sympatry across changing landscapes and seasons, but retaining wetlands appears key to their year-long coexistence on mixed use landscapes.