Abstract
BackgroundGrit is used by birds mainly for grinding hard food items but can also serve a nutritional role as a source of minerals. Ingestion of grit by birds has been documented primarily in species that feed on seeds and invertebrates. Although feeding mainly on nectar and small arthropods, hummingbirds also ingest grit, but why they do so is unclear. We quantified the number of grit particles in the stomachs of six species of hummingbirds during an annual cycle in a seasonal ecosystem of West Mexico.MethodsWe compared the number of grit particles in the stomachs of different hummingbird species (Mexican Violetear Colibri thalassinus, Amethyst-throated Mountaingem Lampornis amethystinus, White-eared Hummingbird Basilinna leucotis, Rivoli’s Hummingbird Eugenes fulgens, Broad-tailed Hummingbird Selasphorus platycercus, and Rufous Hummingbird S. rufus), and between sex and age categories during the different seasons of a year. To determine if grit was used to grind ingested arthropods, we examined the relationships between the number of grit particles, the biomass of arthropods ingested, and their chitin content.ResultsAlthough species did not differ in the number of grit particles in their stomachs, we found that grit was mostly ingested by female individuals, with only one male of one species (Mexican Violetear) presenting grit in its stomach. We also found that female hummingbirds had grit in their stomachs during the rainy and the cold-dry season (June–February) but not during the warm-dry season (March–May). Our analyses revealed no relationship between the number of grit particles and the amount of ingested arthropods and arthropod chitin content. However, high grit consumption was related to wasp ingestion on Mexican Violetears.ConclusionsOur results indicate that grit is used mainly by female hummingbirds. The seasonal variation in the ingestion of grit by female individuals suggests that it can be used to meet mineral requirements related to breeding; however, this topic needs further exploration. Additionally, the use of grit was proportionally higher in juvenile individuals, suggesting it is used for grinding arthropods during a period of fast development.
Highlights
Grit is used by birds mainly for grinding hard food items but can serve a nutritional role as a source of minerals
We found that 70% of the grit particles did not break when pressed with the tip of the tweezers, whereas the other 30% broke after the first or second pressing
This result suggests that ingested grit could have played a role as a supplement of some micronutrients, such as calcium needed for eggshell production
Summary
Grit is used by birds mainly for grinding hard food items but can serve a nutritional role as a source of minerals. Many different species of birds ingest grit, few research papers have documented the use of grit by hummingbirds In these reports, different species of hummingbirds ingested materials such as wood ashes, lime dust, sand particles, and small rocks of different geological origins (Haverschmidt 1952; Verbeek 1971; des Lauriers 1994; Adam and des Lauriers 1998; Graves 2007; Estades et al 2008; Hickman et al 2012; See Additional file 1: Table S1 for details). Female hummingbirds consume more, and higher quality arthropods (e.g. spiders) during the breeding season to provide protein for egg production and to provision nestlings (Hainsworth 1977; Montgomerie and Redsell 1980; Chavez-Ramirez and Down 1992; Stiles 1995; Murphy 1996; Rico-Guevara 2008), so grit could be used to aid in the mechanical digestion of these arthropods (Ziswiler and Farner 1972; Brown 1976; Barrentine 1980; Bishton 1986; Gionfriddo and Best 1999)
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