Devising A Catchability Index Incorporating Weather to Re-Evaluate Population Trends of Migratory Warblers in Western Kansas
Authors:   , Chen Constance 
Published: February 2023

Studies in recent decades, including some based on mist-net counts, have suggested that populations of certain Neotropical migrants have been declining. Many of these studies utilize total birds per net hour as an index of actual population levels. However, other factors besides banding effort, such as weather, should be considered when determining changes in bird populations. In an attempt to calculate a standardized index of Catchability, I analyzed a series of weather factors and warbler captures to assess if changes in weather could bias interpretations of population trends of Neotropical-Nearctic migrant warblers in western Kansas during the fall migratory seasons of 1966 through 2000. Warbler captures varied throughout the 1966-1998 banding seasons, revealing the highest captures in the first six years of that time interval, from 1966 to 1972. Lower numbers and slight fluctuations in captures characterized the next ten years, from 1973 to 1983. From 1984 to 1991, numbers remained lower than in previous years. The last seven years of the study, 1992 to 1998, have shown fluctuations in numbers; however, they appear to be increasing. When recent banding seasons of 1998 through 2000 were examined using four intervals of the banding season as groups, certain patterns between weather and captures of warblers were observed. High captures appeared to correspond to total seasonal precipitation, although no within-year patterns were observed at the interval level. When examined at a daily scale, changes in temperature, both drops and increases, appeared to be associated with a higher capture success, although no statistically significant relationship was found through analyses. Minimum temperatures did not share any patterns with warbler captures, but high variability in minimum temperatures in Interval III, Julian days 269-286, of each year did seem to occur with higher warbler captures. Daily barometric pressure lows and ranges did not share any patterns with warbler captures when examined at the interval level. When the effects of both Julian days and significant temperature drops were examined, high warbler captures occurred in Interval III of the banding season, but not all days with high temperature drops within that time period resulted in high warbler captures. Multiple regression analyses of 1998-1999 data resulted in a significant relationship among warbler captures and weather variables tested, explaining 27.8% of the variation. The variables of Julian day, precipitation, and temperature contributed most significantly to this variation in warbler captures. A multiple regression of 1998-2000 data, excluding barometric pressure variables resulted in a significant equation, explaining only 12.8% of the variation. Precipitation and minimum temperature differential had significant coefficients. Discriminant function analyses resulted in inadequate separation of zero capture, medium capture, and high capture groups using the same weather variables. Data are incomplete for the 2000 banding season. Once more data are received, further analyses will be conducted to examine the influence of important weather variables on warbler captures. Perhaps, regional weather patterns or climatic changes may need to be examined in addition to local weather. A Catchability index may be calculated using a multiple regression model to standardize annual captures according to weather variables and Julian days. It may then be used to assess actual population changes as well as predict warbler captures according to weather conditions.