Reduced parental care, compensatory behaviour and reproductive costs of thick-billed murres equipped with data loggers

Theoretical models predict that in species with obligate biparental care, individuals will partially compensate for decreased parental effort by their partners as a stable evolutionary strategy. Full compensation may occur when breeding success is an accelerating function of parental effort, especially in long-lived bird species. We experimentally examined the effect of time-depth recorders (TDRs) on body mass and parental behaviour of thick-billed murres, Uria lomvia, and evaluated the effect of TDRs as handicaps to test whether individuals compensate for decreased partner effort and whether compensation is sex biased. Compared with control birds, TDR-equipped birds had reduced body mass, offspring attendance, number of foraging trips and feeding rates, and males had increased foraging trip duration. in general, males lost mass at a higher rate and made longer foraging trips than females. Partners of TDR-equipped birds compensated for their mates' reduced parental effort by increasing offspring attendance and by increasing chick provisioning above average rates of control birds. Although partners of TDR-equipped birds fully compensated for their mates' reduced offspring attendance (i.e. chicks were never observed to be left unattended), total provisioning rates of pairs with a TDR-equipped bird ((X) over bar +/-SE=4.38+/-0.26 meals/day) were significantly lower than those of control pairs (5.74+/-0.31 meals/day). However, fledgling success of control and TDR-equipped birds did not differ between reproductive seasons. TDR-equipped birds also had a significantly lower rate of return to breed than their non-TDR-equipped partners or control birds, and those that did return the following season were more likely to change mates (32%) compared with controls (0%). Taken together, our results underline the need to quantify the effects of monitoring equipment used to measure seabirds' activities, and indicate the ability of female and male thick-billed murres to compensate for reduced partner effort due to handicapping-Compensator behaviour seems to be a necessary response of this single-brooded species to ensure current breeding success; however, it may affect the stability of pair bonds in some individuals. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.