  Magnetic field dependent modulated microwave absorption (MMWA) measurements
have been carried out to investigate vortex pinning effects in single crystals
of the iron-based high-Tc superconductor Ba(Fe_{1-x}Co_x)_2As_2 with three
different cobalt doping levels of x = 0.07, 0.09, and 0.11. The dependence of
the MMWA hysteresis loops on temperature, magnetic field, and Co-concentration
have been measured and analyzed using a theoretical model of microwave
absorption in superconductors. The analysis reveals that in an underdoped
crystal (x = 0.07) the so called \dtc-pinning due to magnetically ordered
regions defines the temperature dependence of the critical current density,
while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the
pinning is governed by structural imperfections due to inhomogeneous
distribution of cobalt dopant and has the so called \delta-l character.
