Evaluations of morning versus evening riparian bird surveys and implications for experimental design and ecological theory

Stream ecosystems are dynamic and complex entities that often support high levels of bird community abundance and diversity.  The myriad patches (e.g., aquatic, floodplain, riparian) found in the riverscape habitat mosaic attract a unique mixture of aquatic, semi-aquatic, riparian, and upland birds, each utilizing the river corridor in a different way.  While standard morning bird surveys are widely-used across ecosystems, the temporal habitat partitioning that likely occurs in riverine ecosystems argues for the inclusion of evening surveys.  At 34 stream reaches in Vermont and Idaho, USA, we surveyed riverine bird communities using a combination of morning and evening fixed-width transect counts.  Student’s paired t-tests showed that while bird abundance was not significantly different between morning (am) and evening (pm) surveys, bird community diversity [as measured by species richness (S), Shannon’s informational index (H’), and Simpson’s dominance index (D)] was significantly different.  NMDS ordinations of bird species and time (i.e., am, pm) indicated that the structure of am bird communities was different than pm communities.  NMDS further showed that a set of species was only found in evening surveys.  The inclusion of evening counts in surveying bird communities in stream-riparian ecosystems has important experimental and ecological implications.  Experimentally, the sole use of traditional morning bird surveys may significantly underestimate diversity and misrepresent community composition of bird assemblages in riverine landscapes.  Ecologically, many of the birds detected in evening surveys represent guilds (e.g., aerial insectivores) that represent unique aquatic-terrestrial energy transfers.