Nectar alkaloids of tree tobacco can reduce Palestine sunbird foraging performance in a colour discrimination task

Many plant species contain plant secondary metabolites (PSMs), such as alkaloids, in their tissues for protection against herbivore attack, but PSMs can also be found in floral nectar. Some pollinators have been shown to discriminate against floral nectar with PSMs and consuming PSMs may have negative fitness effects on pollinators. However, only a few studies have investigated the effects of ecologically relevant levels of PSMs on pollinator foraging performance. Here, we addressed the question of whether the natural concentrations of the alkaloids, nicotine and anabasine, found in tree tobacco, Nicotiana glauca, nectar affect foraging performance in Palestine sunbird, Nectarinia osea, pollinators that use the plant's nectar as a food source. We trained foraging sunbirds to discriminate between rewarding and nonrewarding artificial flowers based on colour. We measured sunbird foraging performance through their accuracy at distinguishing the two colours immediately after training (pretreatment), and again the following day after consuming sucrose solutions with or without alkaloids (post-treatment). We also explored other potential effects of PSM consumption by assessing bird activity level and flower visit rate. Birds that consumed alkaloids did not significantly change their activity level or flower visit rate across time (pre- and post-treatment) compared to birds that did not consume alkaloids (no significant time by treatment interaction). However, alkaloid consumption significantly decreased sunbird foraging performance in terms of their accuracy in distinguishing the rewarding colour, potentially due to reduced memory retention and/or other cognitive or physiological impairments following alkaloid consumption. We also found that sunbirds discriminated against higher, in favour of lower, ecologically relevant alkaloid concentrations in the nectar of tree tobacco and that previous exposure to alkaloids reduced overall consumption of alkaloid solutions. Reduced foraging performance due to PSM ingestion could greatly affect a pollinator's foraging efficiency, which could, in turn, affect both pollinator and plant reproductive fitness.