Nest survival of tricolored blackbirds in California’s San Joaquin Valley

The tricolored blackbird (Agelaius tricolor) is considered a Bird of Conservation concern by the U.S. Fish and Wildlife Service and a Bird Species of Special Concern by the State of California due to dramatic and on-going population declines likely caused by the loss of wetland nesting habitat. Over the past few decades an increasing proportion of the population has nested in silage fields, prompting the U.S. Fish and Wildlife Service to pay farmers to delay harvesting some of these fields in an effort to increase nesting productivity. The relative nesting success of tricolored blackbirds breeding in silage fields is unknown, however. If birds experience lower nest survival in silage fields than in wetland vegetation, payments to farmers may be a poor conservation strategy. My objectives were to estimate the daily survival rate (DSR) of nests in wetlands and in silage fields and to identify habitat covariates that influence nest survival. During 2011–2012, I monitored 1,443 tricolored blackbird nests in 14 colonies using small temperature data loggers. I also counted the number of young 5 days old in 579 nests to estimate the number of young produced per nest. I modeled the DSR of 1,252 nests using program RMark with nesting site, habitat type, nest initiation date, nest height, water depth, number of active nests within 1 meter, colony population size, and year as covariates. Nest height and nest density were positively associated with DSR while colony size was negatively associated with DSR. DSR was higher in 2012 than in 2011 and there was a quadratic relationship between DSR and day of the breeding season due to low nest survival mid-season. Nest survival varied greatly from site to site but was not explained by habitat type. Post hoc analyses indicated that larger population sizes had a negative effect on DSR in 2011 but a positive effect in 2012 and that the presence of white-faced ibis (Plegadis chihi) lowered tricolored blackbird nest survival. Number of young produced per nest varied by site, was lowest in colonies of 1,000 – 5,000 birds, and was higher in 2011 than 2012. Managers of habitats used by tricolored blackbirds should continue to protect colonies in silage fields and consider management approaches that increase the height of nesting substrate and discourage white-faced ibis from nesting in association with tricolored blackbirds. In addition, small colonies that begin nesting late in the season can be productive and should not be dismissed as unimportant.